greenplumn async 源码
greenplumn async 代码
文件路径：/src/backend/commands/async.c
/*-------------------------------------------------------------------------
*
* async.c
* Asynchronous notification: NOTIFY, LISTEN, UNLISTEN
*
* Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/commands/async.c
*
*-------------------------------------------------------------------------
*/
/*-------------------------------------------------------------------------
* Async Notification Model as of 9.0:
*
* 1. Multiple backends on same machine. Multiple backends listening on
* several channels. (Channels are also called "conditions" in other
* parts of the code.)
*
* 2. There is one central queue in disk-based storage (directory pg_notify/),
* with actively-used pages mapped into shared memory by the slru.c module.
* All notification messages are placed in the queue and later read out
* by listening backends.
*
* There is no central knowledge of which backend listens on which channel;
* every backend has its own list of interesting channels.
*
* Although there is only one queue, notifications are treated as being
* database-local; this is done by including the sender's database OID
* in each notification message. Listening backends ignore messages
* that don't match their database OID. This is important because it
* ensures senders and receivers have the same database encoding and won't
* misinterpret non-ASCII text in the channel name or payload string.
*
* Since notifications are not expected to survive database crashes,
* we can simply clean out the pg_notify data at any reboot, and there
* is no need for WAL support or fsync'ing.
*
* 3. Every backend that is listening on at least one channel registers by
* entering its PID into the array in AsyncQueueControl. It then scans all
* incoming notifications in the central queue and first compares the
* database OID of the notification with its own database OID and then
* compares the notified channel with the list of channels that it listens
* to. In case there is a match it delivers the notification event to its
* frontend. Non-matching events are simply skipped.
*
* 4. The NOTIFY statement (routine Async_Notify) stores the notification in
* a backend-local list which will not be processed until transaction end.
*
* Duplicate notifications from the same transaction are sent out as one
* notification only. This is done to save work when for example a trigger
* on a 2 million row table fires a notification for each row that has been
* changed. If the application needs to receive every single notification
* that has been sent, it can easily add some unique string into the extra
* payload parameter.
*
* When the transaction is ready to commit, PreCommit_Notify() adds the
* pending notifications to the head of the queue. The head pointer of the
* queue always points to the next free position and a position is just a
* page number and the offset in that page. This is done before marking the
* transaction as committed in clog. If we run into problems writing the
* notifications, we can still call elog(ERROR, ...) and the transaction
* will roll back.
*
* Once we have put all of the notifications into the queue, we return to
* CommitTransaction() which will then do the actual transaction commit.
*
* After commit we are called another time (AtCommit_Notify()). Here we
* make the actual updates to the effective listen state (listenChannels).
*
* Finally, after we are out of the transaction altogether, we check if
* we need to signal listening backends. In SignalBackends() we scan the
* list of listening backends and send a PROCSIG_NOTIFY_INTERRUPT signal
* to every listening backend (we don't know which backend is listening on
* which channel so we must signal them all). We can exclude backends that
* are already up to date, though. We don't bother with a self-signal
* either, but just process the queue directly.
*
* 5. Upon receipt of a PROCSIG_NOTIFY_INTERRUPT signal, the signal handler
* sets the process's latch, which triggers the event to be processed
* immediately if this backend is idle (i.e., it is waiting for a frontend
* command and is not within a transaction block. C.f.
* ProcessClientReadInterrupt()). Otherwise the handler may only set a
* flag, which will cause the processing to occur just before we next go
* idle.
*
* Inbound-notify processing consists of reading all of the notifications
* that have arrived since scanning last time. We read every notification
* until we reach either a notification from an uncommitted transaction or
* the head pointer's position. Then we check if we were the laziest
* backend: if our pointer is set to the same position as the global tail
* pointer is set, then we move the global tail pointer ahead to where the
* second-laziest backend is (in general, we take the MIN of the current
* head position and all active backends' new tail pointers). Whenever we
* move the global tail pointer we also truncate now-unused pages (i.e.,
* delete files in pg_notify/ that are no longer used).
*
* An application that listens on the same channel it notifies will get
* NOTIFY messages for its own NOTIFYs. These can be ignored, if not useful,
* by comparing be_pid in the NOTIFY message to the application's own backend's
* PID. (As of FE/BE protocol 2.0, the backend's PID is provided to the
* frontend during startup.) The above design guarantees that notifies from
* other backends will never be missed by ignoring self-notifies.
*
* The amount of shared memory used for notify management (NUM_ASYNC_BUFFERS)
* can be varied without affecting anything but performance. The maximum
* amount of notification data that can be queued at one time is determined
* by slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <limits.h>
#include <unistd.h>
#include <signal.h>
#include "access/parallel.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/pg_database.h"
#include "commands/async.h"
#include "common/hashfn.h"
#include "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/sinval.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
/*
* Maximum size of a NOTIFY payload, including terminating NULL. This
* must be kept small enough so that a notification message fits on one
* SLRU page. The magic fudge factor here is noncritical as long as it's
* more than AsyncQueueEntryEmptySize --- we make it significantly bigger
* than that, so changes in that data structure won't affect user-visible
* restrictions.
*/
#define NOTIFY_PAYLOAD_MAX_LENGTH (BLCKSZ - NAMEDATALEN - 128)
/*
* Struct representing an entry in the global notify queue
*
* This struct declaration has the maximal length, but in a real queue entry
* the data area is only big enough for the actual channel and payload strings
* (each null-terminated). AsyncQueueEntryEmptySize is the minimum possible
* entry size, if both channel and payload strings are empty (but note it
* doesn't include alignment padding).
*
* The "length" field should always be rounded up to the next QUEUEALIGN
* multiple so that all fields are properly aligned.
*/
typedef struct AsyncQueueEntry
{
int length; /* total allocated length of entry */
Oid dboid; /* sender's database OID */
TransactionId xid; /* sender's XID */
int32 srcPid; /* sender's PID */
char data[NAMEDATALEN + NOTIFY_PAYLOAD_MAX_LENGTH];
} AsyncQueueEntry;
/* Currently, no field of AsyncQueueEntry requires more than int alignment */
#define QUEUEALIGN(len) INTALIGN(len)
#define AsyncQueueEntryEmptySize (offsetof(AsyncQueueEntry, data) + 2)
/*
* Struct describing a queue position, and assorted macros for working with it
*/
typedef struct QueuePosition
{
int page; /* SLRU page number */
int offset; /* byte offset within page */
} QueuePosition;
#define QUEUE_POS_PAGE(x) ((x).page)
#define QUEUE_POS_OFFSET(x) ((x).offset)
#define SET_QUEUE_POS(x,y,z) \
do { \
(x).page = (y); \
(x).offset = (z); \
} while (0)
#define QUEUE_POS_EQUAL(x,y) \
((x).page == (y).page && (x).offset == (y).offset)
/* choose logically smaller QueuePosition */
#define QUEUE_POS_MIN(x,y) \
(asyncQueuePagePrecedes((x).page, (y).page) ? (x) : \
(x).page != (y).page ? (y) : \
(x).offset < (y).offset ? (x) : (y))
/* choose logically larger QueuePosition */
#define QUEUE_POS_MAX(x,y) \
(asyncQueuePagePrecedes((x).page, (y).page) ? (y) : \
(x).page != (y).page ? (x) : \
(x).offset > (y).offset ? (x) : (y))
/*
* Struct describing a listening backend's status
*/
typedef struct QueueBackendStatus
{
int32 pid; /* either a PID or InvalidPid */
Oid dboid; /* backend's database OID, or InvalidOid */
QueuePosition pos; /* backend has read queue up to here */
} QueueBackendStatus;
/*
* Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
*
* The AsyncQueueControl structure is protected by the AsyncQueueLock.
*
* When holding the lock in SHARED mode, backends may only inspect their own
* entries as well as the head and tail pointers. Consequently we can allow a
* backend to update its own record while holding only SHARED lock (since no
* other backend will inspect it).
*
* When holding the lock in EXCLUSIVE mode, backends can inspect the entries
* of other backends and also change the head and tail pointers.
*
* AsyncCtlLock is used as the control lock for the pg_notify SLRU buffers.
* In order to avoid deadlocks, whenever we need both locks, we always first
* get AsyncQueueLock and then AsyncCtlLock.
*
* Each backend uses the backend[] array entry with index equal to its
* BackendId (which can range from 1 to MaxBackends). We rely on this to make
* SendProcSignal fast.
*/
typedef struct AsyncQueueControl
{
QueuePosition head; /* head points to the next free location */
QueuePosition tail; /* the global tail is equivalent to the pos of
* the "slowest" backend */
TimestampTz lastQueueFillWarn; /* time of last queue-full msg */
QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
/* backend[0] is not used; used entries are from [1] to [MaxBackends] */
} AsyncQueueControl;
static AsyncQueueControl *asyncQueueControl;
#define QUEUE_HEAD (asyncQueueControl->head)
#define QUEUE_TAIL (asyncQueueControl->tail)
#define QUEUE_BACKEND_PID(i) (asyncQueueControl->backend[i].pid)
#define QUEUE_BACKEND_DBOID(i) (asyncQueueControl->backend[i].dboid)
#define QUEUE_BACKEND_POS(i) (asyncQueueControl->backend[i].pos)
/*
* The SLRU buffer area through which we access the notification queue
*/
static SlruCtlData AsyncCtlData;
#define AsyncCtl (&AsyncCtlData)
#define QUEUE_PAGESIZE BLCKSZ
#define QUEUE_FULL_WARN_INTERVAL 5000 /* warn at most once every 5s */
/*
* slru.c currently assumes that all filenames are four characters of hex
* digits. That means that we can use segments 0000 through FFFF.
* Each segment contains SLRU_PAGES_PER_SEGMENT pages which gives us
* the pages from 0 to SLRU_PAGES_PER_SEGMENT * 0x10000 - 1.
*
* It's of course possible to enhance slru.c, but this gives us so much
* space already that it doesn't seem worth the trouble.
*
* The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
* pages, because more than that would confuse slru.c into thinking there
* was a wraparound condition. With the default BLCKSZ this means there
* can be up to 8GB of queued-and-not-read data.
*
* Note: it's possible to redefine QUEUE_MAX_PAGE with a smaller multiple of
* SLRU_PAGES_PER_SEGMENT, for easier testing of queue-full behaviour.
*/
#define QUEUE_MAX_PAGE (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)
/*
* listenChannels identifies the channels we are actually listening to
* (ie, have committed a LISTEN on). It is a simple list of channel names,
* allocated in TopMemoryContext.
*/
static List *listenChannels = NIL; /* list of C strings */
/*
* State for pending LISTEN/UNLISTEN actions consists of an ordered list of
* all actions requested in the current transaction. As explained above,
* we don't actually change listenChannels until we reach transaction commit.
*
* The list is kept in CurTransactionContext. In subtransactions, each
* subtransaction has its own list in its own CurTransactionContext, but
* successful subtransactions attach their lists to their parent's list.
* Failed subtransactions simply discard their lists.
*/
typedef enum
{
LISTEN_LISTEN,
LISTEN_UNLISTEN,
LISTEN_UNLISTEN_ALL
} ListenActionKind;
typedef struct
{
ListenActionKind action;
char channel[FLEXIBLE_ARRAY_MEMBER]; /* nul-terminated string */
} ListenAction;
static List *pendingActions = NIL; /* list of ListenAction */
static List *upperPendingActions = NIL; /* list of upper-xact lists */
/*
* State for outbound notifies consists of a list of all channels+payloads
* NOTIFYed in the current transaction. We do not actually perform a NOTIFY
* until and unless the transaction commits. pendingNotifies is NIL if no
* NOTIFYs have been done in the current transaction.
*
* The list is kept in CurTransactionContext. In subtransactions, each
* subtransaction has its own list in its own CurTransactionContext, but
* successful subtransactions attach their lists to their parent's list.
* Failed subtransactions simply discard their lists.
*
* Note: the action and notify lists do not interact within a transaction.
* In particular, if a transaction does NOTIFY and then LISTEN on the same
* condition name, it will get a self-notify at commit. This is a bit odd
* but is consistent with our historical behavior.
*/
typedef struct Notification
{
char *channel; /* channel name */
char *payload; /* payload string (can be empty) */
} Notification;
static List *pendingNotifies = NIL; /* list of Notifications */
static List *upperPendingNotifies = NIL; /* list of upper-xact lists */
/*
* Inbound notifications are initially processed by HandleNotifyInterrupt(),
* called from inside a signal handler. That just sets the
* notifyInterruptPending flag and sets the process
* latch. ProcessNotifyInterrupt() will then be called whenever it's safe to
* actually deal with the interrupt.
*/
volatile sig_atomic_t notifyInterruptPending = false;
/* True if we've registered an on_shmem_exit cleanup */
static bool unlistenExitRegistered = false;
/* True if we're currently registered as a listener in asyncQueueControl */
static bool amRegisteredListener = false;
/* has this backend sent notifications in the current transaction? */
static bool backendHasSentNotifications = false;
/* GUC parameter */
bool Trace_notify = false;
/* local function prototypes */
static bool asyncQueuePagePrecedes(int p, int q);
static void queue_listen(ListenActionKind action, const char *channel);
static void Async_UnlistenOnExit(int code, Datum arg);
static void Exec_ListenPreCommit(void);
static void Exec_ListenCommit(const char *channel);
static void Exec_UnlistenCommit(const char *channel);
static void Exec_UnlistenAllCommit(void);
static bool IsListeningOn(const char *channel);
static void asyncQueueUnregister(void);
static bool asyncQueueIsFull(void);
static bool asyncQueueAdvance(volatile QueuePosition *position, int entryLength);
static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
static double asyncQueueUsage(void);
static void asyncQueueFillWarning(void);
static bool SignalBackends(void);
static void asyncQueueReadAllNotifications(void);
static bool asyncQueueProcessPageEntries(volatile QueuePosition *current,
QueuePosition stop,
char *page_buffer,
Snapshot snapshot);
static void asyncQueueAdvanceTail(void);
static void ProcessIncomingNotify(void);
static bool AsyncExistsPendingNotify(const char *channel, const char *payload);
static void ClearPendingActionsAndNotifies(void);
/*
* We will work on the page range of 0..QUEUE_MAX_PAGE.
*/
static bool
asyncQueuePagePrecedes(int p, int q)
{
int diff;
/*
* We have to compare modulo (QUEUE_MAX_PAGE+1)/2. Both inputs should be
* in the range 0..QUEUE_MAX_PAGE.
*/
Assert(p >= 0 && p <= QUEUE_MAX_PAGE);
Assert(q >= 0 && q <= QUEUE_MAX_PAGE);
diff = p - q;
if (diff >= ((QUEUE_MAX_PAGE + 1) / 2))
diff -= QUEUE_MAX_PAGE + 1;
else if (diff < -((QUEUE_MAX_PAGE + 1) / 2))
diff += QUEUE_MAX_PAGE + 1;
return diff < 0;
}
/*
* Report space needed for our shared memory area
*/
Size
AsyncShmemSize(void)
{
Size size;
/* This had better match AsyncShmemInit */
size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
size = add_size(size, offsetof(AsyncQueueControl, backend));
size = add_size(size, SimpleLruShmemSize(NUM_ASYNC_BUFFERS, 0));
return size;
}
/*
* Initialize our shared memory area
*/
void
AsyncShmemInit(void)
{
bool found;
int slotno;
Size size;
/*
* Create or attach to the AsyncQueueControl structure.
*
* The used entries in the backend[] array run from 1 to MaxBackends; the
* zero'th entry is unused but must be allocated.
*/
size = mul_size(MaxBackends + 1, sizeof(QueueBackendStatus));
size = add_size(size, offsetof(AsyncQueueControl, backend));
asyncQueueControl = (AsyncQueueControl *)
ShmemInitStruct("Async Queue Control", size, &found);
if (!found)
{
/* First time through, so initialize it */
int i;
SET_QUEUE_POS(QUEUE_HEAD, 0, 0);
SET_QUEUE_POS(QUEUE_TAIL, 0, 0);
asyncQueueControl->lastQueueFillWarn = 0;
/* zero'th entry won't be used, but let's initialize it anyway */
for (i = 0; i <= MaxBackends; i++)
{
QUEUE_BACKEND_PID(i) = InvalidPid;
QUEUE_BACKEND_DBOID(i) = InvalidOid;
SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
}
}
/*
* Set up SLRU management of the pg_notify data.
*/
AsyncCtl->PagePrecedes = asyncQueuePagePrecedes;
SimpleLruInit(AsyncCtl, "async", NUM_ASYNC_BUFFERS, 0,
AsyncCtlLock, "pg_notify", LWTRANCHE_ASYNC_BUFFERS);
/* Override default assumption that writes should be fsync'd */
AsyncCtl->do_fsync = false;
if (!found)
{
/*
* During start or reboot, clean out the pg_notify directory.
*/
(void) SlruScanDirectory(AsyncCtl, SlruScanDirCbDeleteAll, NULL);
/* Now initialize page zero to empty */
LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(QUEUE_HEAD));
/* This write is just to verify that pg_notify/ is writable */
SimpleLruWritePage(AsyncCtl, slotno);
LWLockRelease(AsyncCtlLock);
}
}
/*
* pg_notify -
* SQL function to send a notification event
*/
Datum
pg_notify(PG_FUNCTION_ARGS)
{
const char *channel;
const char *payload;
if (PG_ARGISNULL(0))
channel = "";
else
channel = text_to_cstring(PG_GETARG_TEXT_PP(0));
if (PG_ARGISNULL(1))
payload = "";
else
payload = text_to_cstring(PG_GETARG_TEXT_PP(1));
/* For NOTIFY as a statement, this is checked in ProcessUtility */
PreventCommandDuringRecovery("NOTIFY");
Async_Notify(channel, payload);
PG_RETURN_VOID();
}
/*
* Async_Notify
*
* This is executed by the SQL notify command.
*
* Adds the message to the list of pending notifies.
* Actual notification happens during transaction commit.
* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
*/
void
Async_Notify(const char *channel, const char *payload)
{
Notification *n;
MemoryContext oldcontext;
if (IsParallelWorker())
elog(ERROR, "cannot send notifications from a parallel worker");
if (Trace_notify)
elog(DEBUG1, "Async_Notify(%s)", channel);
/* a channel name must be specified */
if (!channel || !strlen(channel))
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("channel name cannot be empty")));
if (strlen(channel) >= NAMEDATALEN)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("channel name too long")));
if (payload)
{
if (strlen(payload) >= NOTIFY_PAYLOAD_MAX_LENGTH)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("payload string too long")));
}
/* no point in making duplicate entries in the list ... */
if (AsyncExistsPendingNotify(channel, payload))
return;
/*
* The notification list needs to live until end of transaction, so store
* it in the transaction context.
*/
oldcontext = MemoryContextSwitchTo(CurTransactionContext);
n = (Notification *) palloc(sizeof(Notification));
n->channel = pstrdup(channel);
if (payload)
n->payload = pstrdup(payload);
else
n->payload = "";
/*
* We want to preserve the order so we need to append every notification.
* See comments at AsyncExistsPendingNotify().
*/
pendingNotifies = lappend(pendingNotifies, n);
MemoryContextSwitchTo(oldcontext);
}
/*
* queue_listen
* Common code for listen, unlisten, unlisten all commands.
*
* Adds the request to the list of pending actions.
* Actual update of the listenChannels list happens during transaction
* commit.
*/
static void
queue_listen(ListenActionKind action, const char *channel)
{
MemoryContext oldcontext;
ListenAction *actrec;
/*
* Unlike Async_Notify, we don't try to collapse out duplicates. It would
* be too complicated to ensure we get the right interactions of
* conflicting LISTEN/UNLISTEN/UNLISTEN_ALL, and it's unlikely that there
* would be any performance benefit anyway in sane applications.
*/
oldcontext = MemoryContextSwitchTo(CurTransactionContext);
/* space for terminating null is included in sizeof(ListenAction) */
actrec = (ListenAction *) palloc(offsetof(ListenAction, channel) +
strlen(channel) + 1);
actrec->action = action;
strcpy(actrec->channel, channel);
pendingActions = lappend(pendingActions, actrec);
MemoryContextSwitchTo(oldcontext);
}
/*
* Async_Listen
*
* This is executed by the SQL listen command.
*/
void
Async_Listen(const char *channel)
{
if (Trace_notify)
elog(DEBUG1, "Async_Listen(%s,%d)", channel, MyProcPid);
queue_listen(LISTEN_LISTEN, channel);
}
/*
* Async_Unlisten
*
* This is executed by the SQL unlisten command.
*/
void
Async_Unlisten(const char *channel)
{
if (Trace_notify)
elog(DEBUG1, "Async_Unlisten(%s,%d)", channel, MyProcPid);
/* If we couldn't possibly be listening, no need to queue anything */
if (pendingActions == NIL && !unlistenExitRegistered)
return;
queue_listen(LISTEN_UNLISTEN, channel);
}
/*
* Async_UnlistenAll
*
* This is invoked by UNLISTEN * command, and also at backend exit.
*/
void
Async_UnlistenAll(void)
{
if (Trace_notify)
elog(DEBUG1, "Async_UnlistenAll(%d)", MyProcPid);
/* If we couldn't possibly be listening, no need to queue anything */
if (pendingActions == NIL && !unlistenExitRegistered)
return;
queue_listen(LISTEN_UNLISTEN_ALL, "");
}
/*
* SQL function: return a set of the channel names this backend is actively
* listening to.
*
* Note: this coding relies on the fact that the listenChannels list cannot
* change within a transaction.
*/
Datum
pg_listening_channels(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
ListCell **lcp;
/* stuff done only on the first call of the function */
if (SRF_IS_FIRSTCALL())
{
MemoryContext oldcontext;
/* create a function context for cross-call persistence */
funcctx = SRF_FIRSTCALL_INIT();
/* switch to memory context appropriate for multiple function calls */
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
/* allocate memory for user context */
lcp = (ListCell **) palloc(sizeof(ListCell *));
*lcp = list_head(listenChannels);
funcctx->user_fctx = (void *) lcp;
MemoryContextSwitchTo(oldcontext);
}
/* stuff done on every call of the function */
funcctx = SRF_PERCALL_SETUP();
lcp = (ListCell **) funcctx->user_fctx;
while (*lcp != NULL)
{
char *channel = (char *) lfirst(*lcp);
*lcp = lnext(*lcp);
SRF_RETURN_NEXT(funcctx, CStringGetTextDatum(channel));
}
SRF_RETURN_DONE(funcctx);
}
/*
* Async_UnlistenOnExit
*
* This is executed at backend exit if we have done any LISTENs in this
* backend. It might not be necessary anymore, if the user UNLISTENed
* everything, but we don't try to detect that case.
*/
static void
Async_UnlistenOnExit(int code, Datum arg)
{
Exec_UnlistenAllCommit();
asyncQueueUnregister();
}
/*
* AtPrepare_Notify
*
* This is called at the prepare phase of a two-phase
* transaction. Save the state for possible commit later.
*/
void
AtPrepare_Notify(void)
{
/* It's not allowed to have any pending LISTEN/UNLISTEN/NOTIFY actions */
if (pendingActions || pendingNotifies)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("cannot PREPARE a transaction that has executed LISTEN, UNLISTEN, or NOTIFY")));
}
/*
* PreCommit_Notify
*
* This is called at transaction commit, before actually committing to
* clog.
*
* If there are pending LISTEN actions, make sure we are listed in the
* shared-memory listener array. This must happen before commit to
* ensure we don't miss any notifies from transactions that commit
* just after ours.
*
* If there are outbound notify requests in the pendingNotifies list,
* add them to the global queue. We do that before commit so that
* we can still throw error if we run out of queue space.
*/
void
PreCommit_Notify(void)
{
ListCell *p;
if (pendingActions == NIL && pendingNotifies == NIL)
return; /* no relevant statements in this xact */
if (Trace_notify)
elog(DEBUG1, "PreCommit_Notify");
/* Preflight for any pending listen/unlisten actions */
foreach(p, pendingActions)
{
ListenAction *actrec = (ListenAction *) lfirst(p);
switch (actrec->action)
{
case LISTEN_LISTEN:
Exec_ListenPreCommit();
break;
case LISTEN_UNLISTEN:
/* there is no Exec_UnlistenPreCommit() */
break;
case LISTEN_UNLISTEN_ALL:
/* there is no Exec_UnlistenAllPreCommit() */
break;
}
}
/* Queue any pending notifies (must happen after the above) */
if (pendingNotifies)
{
ListCell *nextNotify;
/*
* Make sure that we have an XID assigned to the current transaction.
* GetCurrentTransactionId is cheap if we already have an XID, but not
* so cheap if we don't, and we'd prefer not to do that work while
* holding AsyncQueueLock.
*/
(void) GetCurrentTransactionId();
/*
* Serialize writers by acquiring a special lock that we hold till
* after commit. This ensures that queue entries appear in commit
* order, and in particular that there are never uncommitted queue
* entries ahead of committed ones, so an uncommitted transaction
* can't block delivery of deliverable notifications.
*
* We use a heavyweight lock so that it'll automatically be released
* after either commit or abort. This also allows deadlocks to be
* detected, though really a deadlock shouldn't be possible here.
*
* The lock is on "database 0", which is pretty ugly but it doesn't
* seem worth inventing a special locktag category just for this.
* (Historical note: before PG 9.0, a similar lock on "database 0" was
* used by the flatfiles mechanism.)
*/
LockSharedObject(DatabaseRelationId, InvalidOid, 0,
AccessExclusiveLock);
/* Now push the notifications into the queue */
backendHasSentNotifications = true;
nextNotify = list_head(pendingNotifies);
while (nextNotify != NULL)
{
/*
* Add the pending notifications to the queue. We acquire and
* release AsyncQueueLock once per page, which might be overkill
* but it does allow readers to get in while we're doing this.
*
* A full queue is very uncommon and should really not happen,
* given that we have so much space available in the SLRU pages.
* Nevertheless we need to deal with this possibility. Note that
* when we get here we are in the process of committing our
* transaction, but we have not yet committed to clog, so at this
* point in time we can still roll the transaction back.
*/
LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
asyncQueueFillWarning();
if (asyncQueueIsFull())
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("too many notifications in the NOTIFY queue")));
nextNotify = asyncQueueAddEntries(nextNotify);
LWLockRelease(AsyncQueueLock);
}
}
}
/*
* AtCommit_Notify
*
* This is called at transaction commit, after committing to clog.
*
* Update listenChannels and clear transaction-local state.
*/
void
AtCommit_Notify(void)
{
ListCell *p;
/*
* Allow transactions that have not executed LISTEN/UNLISTEN/NOTIFY to
* return as soon as possible
*/
if (!pendingActions && !pendingNotifies)
return;
if (Trace_notify)
elog(DEBUG1, "AtCommit_Notify");
/* Perform any pending listen/unlisten actions */
foreach(p, pendingActions)
{
ListenAction *actrec = (ListenAction *) lfirst(p);
switch (actrec->action)
{
case LISTEN_LISTEN:
Exec_ListenCommit(actrec->channel);
break;
case LISTEN_UNLISTEN:
Exec_UnlistenCommit(actrec->channel);
break;
case LISTEN_UNLISTEN_ALL:
Exec_UnlistenAllCommit();
break;
}
}
/* If no longer listening to anything, get out of listener array */
if (amRegisteredListener && listenChannels == NIL)
asyncQueueUnregister();
/* And clean up */
ClearPendingActionsAndNotifies();
}
/*
* Exec_ListenPreCommit --- subroutine for PreCommit_Notify
*
* This function must make sure we are ready to catch any incoming messages.
*/
static void
Exec_ListenPreCommit(void)
{
QueuePosition head;
QueuePosition max;
int i;
/*
* Nothing to do if we are already listening to something, nor if we
* already ran this routine in this transaction.
*/
if (amRegisteredListener)
return;
if (Trace_notify)
elog(DEBUG1, "Exec_ListenPreCommit(%d)", MyProcPid);
/*
* Before registering, make sure we will unlisten before dying. (Note:
* this action does not get undone if we abort later.)
*/
if (!unlistenExitRegistered)
{
before_shmem_exit(Async_UnlistenOnExit, 0);
unlistenExitRegistered = true;
}
/*
* This is our first LISTEN, so establish our pointer.
*
* We set our pointer to the global tail pointer and then move it forward
* over already-committed notifications. This ensures we cannot miss any
* not-yet-committed notifications. We might get a few more but that
* doesn't hurt.
*
* In some scenarios there might be a lot of committed notifications that
* have not yet been pruned away (because some backend is being lazy about
* reading them). To reduce our startup time, we can look at other
* backends and adopt the maximum "pos" pointer of any backend that's in
* our database; any notifications it's already advanced over are surely
* committed and need not be re-examined by us. (We must consider only
* backends connected to our DB, because others will not have bothered to
* check committed-ness of notifications in our DB.) But we only bother
* with that if there's more than a page worth of notifications
* outstanding, otherwise scanning all the other backends isn't worth it.
*
* We need exclusive lock here so we can look at other backends' entries.
*/
LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
head = QUEUE_HEAD;
max = QUEUE_TAIL;
if (QUEUE_POS_PAGE(max) != QUEUE_POS_PAGE(head))
{
for (i = 1; i <= MaxBackends; i++)
{
if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
max = QUEUE_POS_MAX(max, QUEUE_BACKEND_POS(i));
}
}
QUEUE_BACKEND_POS(MyBackendId) = max;
QUEUE_BACKEND_PID(MyBackendId) = MyProcPid;
QUEUE_BACKEND_DBOID(MyBackendId) = MyDatabaseId;
LWLockRelease(AsyncQueueLock);
/* Now we are listed in the global array, so remember we're listening */
amRegisteredListener = true;
/*
* Try to move our pointer forward as far as possible. This will skip over
* already-committed notifications. Still, we could get notifications that
* have already committed before we started to LISTEN.
*
* Note that we are not yet listening on anything, so we won't deliver any
* notification to the frontend. Also, although our transaction might
* have executed NOTIFY, those message(s) aren't queued yet so we can't
* see them in the queue.
*
* This will also advance the global tail pointer if possible.
*/
if (!QUEUE_POS_EQUAL(max, head))
asyncQueueReadAllNotifications();
}
/*
* Exec_ListenCommit --- subroutine for AtCommit_Notify
*
* Add the channel to the list of channels we are listening on.
*/
static void
Exec_ListenCommit(const char *channel)
{
MemoryContext oldcontext;
/* Do nothing if we are already listening on this channel */
if (IsListeningOn(channel))
return;
/*
* Add the new channel name to listenChannels.
*
* XXX It is theoretically possible to get an out-of-memory failure here,
* which would be bad because we already committed. For the moment it
* doesn't seem worth trying to guard against that, but maybe improve this
* later.
*/
oldcontext = MemoryContextSwitchTo(TopMemoryContext);
listenChannels = lappend(listenChannels, pstrdup(channel));
MemoryContextSwitchTo(oldcontext);
}
/*
* Exec_UnlistenCommit --- subroutine for AtCommit_Notify
*
* Remove the specified channel name from listenChannels.
*/
static void
Exec_UnlistenCommit(const char *channel)
{
ListCell *q;
ListCell *prev;
if (Trace_notify)
elog(DEBUG1, "Exec_UnlistenCommit(%s,%d)", channel, MyProcPid);
prev = NULL;
foreach(q, listenChannels)
{
char *lchan = (char *) lfirst(q);
if (strcmp(lchan, channel) == 0)
{
listenChannels = list_delete_cell(listenChannels, q, prev);
pfree(lchan);
break;
}
prev = q;
}
/*
* We do not complain about unlistening something not being listened;
* should we?
*/
}
/*
* Exec_UnlistenAllCommit --- subroutine for AtCommit_Notify
*
* Unlisten on all channels for this backend.
*/
static void
Exec_UnlistenAllCommit(void)
{
if (Trace_notify)
elog(DEBUG1, "Exec_UnlistenAllCommit(%d)", MyProcPid);
list_free_deep(listenChannels);
listenChannels = NIL;
}
/*
* ProcessCompletedNotifies --- send out signals and self-notifies
*
* This is called from postgres.c just before going idle at the completion
* of a transaction. If we issued any notifications in the just-completed
* transaction, send signals to other backends to process them, and also
* process the queue ourselves to send messages to our own frontend.
*
* The reason that this is not done in AtCommit_Notify is that there is
* a nonzero chance of errors here (for example, encoding conversion errors
* while trying to format messages to our frontend). An error during
* AtCommit_Notify would be a PANIC condition. The timing is also arranged
* to ensure that a transaction's self-notifies are delivered to the frontend
* before it gets the terminating ReadyForQuery message.
*
* Note that we send signals and process the queue even if the transaction
* eventually aborted. This is because we need to clean out whatever got
* added to the queue.
*
* NOTE: we are outside of any transaction here.
*/
void
ProcessCompletedNotifies(void)
{
MemoryContext caller_context;
bool signalled;
/* Nothing to do if we didn't send any notifications */
if (!backendHasSentNotifications)
return;
/*
* We reset the flag immediately; otherwise, if any sort of error occurs
* below, we'd be locked up in an infinite loop, because control will come
* right back here after error cleanup.
*/
backendHasSentNotifications = false;
/*
* We must preserve the caller's memory context (probably MessageContext)
* across the transaction we do here.
*/
caller_context = CurrentMemoryContext;
if (Trace_notify)
elog(DEBUG1, "ProcessCompletedNotifies");
/*
* We must run asyncQueueReadAllNotifications inside a transaction, else
* bad things happen if it gets an error.
*/
StartTransactionCommand();
/* Send signals to other backends */
signalled = SignalBackends();
if (listenChannels != NIL)
{
/* Read the queue ourselves, and send relevant stuff to the frontend */
asyncQueueReadAllNotifications();
}
else if (!signalled)
{
/*
* If we found no other listening backends, and we aren't listening
* ourselves, then we must execute asyncQueueAdvanceTail to flush the
* queue, because ain't nobody else gonna do it. This prevents queue
* overflow when we're sending useless notifies to nobody. (A new
* listener could have joined since we looked, but if so this is
* harmless.)
*/
asyncQueueAdvanceTail();
}
CommitTransactionCommand();
MemoryContextSwitchTo(caller_context);
/* We don't need pq_flush() here since postgres.c will do one shortly */
}
/*
* Test whether we are actively listening on the given channel name.
*
* Note: this function is executed for every notification found in the queue.
* Perhaps it is worth further optimization, eg convert the list to a sorted
* array so we can binary-search it. In practice the list is likely to be
* fairly short, though.
*/
static bool
IsListeningOn(const char *channel)
{
ListCell *p;
foreach(p, listenChannels)
{
char *lchan = (char *) lfirst(p);
if (strcmp(lchan, channel) == 0)
return true;
}
return false;
}
/*
* Remove our entry from the listeners array when we are no longer listening
* on any channel. NB: must not fail if we're already not listening.
*/
static void
asyncQueueUnregister(void)
{
bool advanceTail;
Assert(listenChannels == NIL); /* else caller error */
if (!amRegisteredListener) /* nothing to do */
return;
LWLockAcquire(AsyncQueueLock, LW_SHARED);
/* check if entry is valid and oldest ... */
advanceTail = (MyProcPid == QUEUE_BACKEND_PID(MyBackendId)) &&
QUEUE_POS_EQUAL(QUEUE_BACKEND_POS(MyBackendId), QUEUE_TAIL);
/* ... then mark it invalid */
QUEUE_BACKEND_PID(MyBackendId) = InvalidPid;
QUEUE_BACKEND_DBOID(MyBackendId) = InvalidOid;
LWLockRelease(AsyncQueueLock);
/* mark ourselves as no longer listed in the global array */
amRegisteredListener = false;
/* If we were the laziest backend, try to advance the tail pointer */
if (advanceTail)
asyncQueueAdvanceTail();
}
/*
* Test whether there is room to insert more notification messages.
*
* Caller must hold at least shared AsyncQueueLock.
*/
static bool
asyncQueueIsFull(void)
{
int nexthead;
int boundary;
/*
* The queue is full if creating a new head page would create a page that
* logically precedes the current global tail pointer, ie, the head
* pointer would wrap around compared to the tail. We cannot create such
* a head page for fear of confusing slru.c. For safety we round the tail
* pointer back to a segment boundary (compare the truncation logic in
* asyncQueueAdvanceTail).
*
* Note that this test is *not* dependent on how much space there is on
* the current head page. This is necessary because asyncQueueAddEntries
* might try to create the next head page in any case.
*/
nexthead = QUEUE_POS_PAGE(QUEUE_HEAD) + 1;
if (nexthead > QUEUE_MAX_PAGE)
nexthead = 0; /* wrap around */
boundary = QUEUE_POS_PAGE(QUEUE_TAIL);
boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
return asyncQueuePagePrecedes(nexthead, boundary);
}
/*
* Advance the QueuePosition to the next entry, assuming that the current
* entry is of length entryLength. If we jump to a new page the function
* returns true, else false.
*/
static bool
asyncQueueAdvance(volatile QueuePosition *position, int entryLength)
{
int pageno = QUEUE_POS_PAGE(*position);
int offset = QUEUE_POS_OFFSET(*position);
bool pageJump = false;
/*
* Move to the next writing position: First jump over what we have just
* written or read.
*/
offset += entryLength;
Assert(offset <= QUEUE_PAGESIZE);
/*
* In a second step check if another entry can possibly be written to the
* page. If so, stay here, we have reached the next position. If not, then
* we need to move on to the next page.
*/
if (offset + QUEUEALIGN(AsyncQueueEntryEmptySize) > QUEUE_PAGESIZE)
{
pageno++;
if (pageno > QUEUE_MAX_PAGE)
pageno = 0; /* wrap around */
offset = 0;
pageJump = true;
}
SET_QUEUE_POS(*position, pageno, offset);
return pageJump;
}
/*
* Fill the AsyncQueueEntry at *qe with an outbound notification message.
*/
static void
asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe)
{
size_t channellen = strlen(n->channel);
size_t payloadlen = strlen(n->payload);
int entryLength;
Assert(channellen < NAMEDATALEN);
Assert(payloadlen < NOTIFY_PAYLOAD_MAX_LENGTH);
/* The terminators are already included in AsyncQueueEntryEmptySize */
entryLength = AsyncQueueEntryEmptySize + payloadlen + channellen;
entryLength = QUEUEALIGN(entryLength);
qe->length = entryLength;
qe->dboid = MyDatabaseId;
qe->xid = GetCurrentTransactionId();
qe->srcPid = MyProcPid;
memcpy(qe->data, n->channel, channellen + 1);
memcpy(qe->data + channellen + 1, n->payload, payloadlen + 1);
}
/*
* Add pending notifications to the queue.
*
* We go page by page here, i.e. we stop once we have to go to a new page but
* we will be called again and then fill that next page. If an entry does not
* fit into the current page, we write a dummy entry with an InvalidOid as the
* database OID in order to fill the page. So every page is always used up to
* the last byte which simplifies reading the page later.
*
* We are passed the list cell containing the next notification to write
* and return the first still-unwritten cell back. Eventually we will return
* NULL indicating all is done.
*
* We are holding AsyncQueueLock already from the caller and grab AsyncCtlLock
* locally in this function.
*/
static ListCell *
asyncQueueAddEntries(ListCell *nextNotify)
{
AsyncQueueEntry qe;
QueuePosition queue_head;
int pageno;
int offset;
int slotno;
/* We hold both AsyncQueueLock and AsyncCtlLock during this operation */
LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
/*
* We work with a local copy of QUEUE_HEAD, which we write back to shared
* memory upon exiting. The reason for this is that if we have to advance
* to a new page, SimpleLruZeroPage might fail (out of disk space, for
* instance), and we must not advance QUEUE_HEAD if it does. (Otherwise,
* subsequent insertions would try to put entries into a page that slru.c
* thinks doesn't exist yet.) So, use a local position variable. Note
* that if we do fail, any already-inserted queue entries are forgotten;
* this is okay, since they'd be useless anyway after our transaction
* rolls back.
*/
queue_head = QUEUE_HEAD;
/* Fetch the current page */
pageno = QUEUE_POS_PAGE(queue_head);
slotno = SimpleLruReadPage(AsyncCtl, pageno, true, InvalidTransactionId);
/* Note we mark the page dirty before writing in it */
AsyncCtl->shared->page_dirty[slotno] = true;
while (nextNotify != NULL)
{
Notification *n = (Notification *) lfirst(nextNotify);
/* Construct a valid queue entry in local variable qe */
asyncQueueNotificationToEntry(n, &qe);
offset = QUEUE_POS_OFFSET(queue_head);
/* Check whether the entry really fits on the current page */
if (offset + qe.length <= QUEUE_PAGESIZE)
{
/* OK, so advance nextNotify past this item */
nextNotify = lnext(nextNotify);
}
else
{
/*
* Write a dummy entry to fill up the page. Actually readers will
* only check dboid and since it won't match any reader's database
* OID, they will ignore this entry and move on.
*/
qe.length = QUEUE_PAGESIZE - offset;
qe.dboid = InvalidOid;
qe.data[0] = '\0'; /* empty channel */
qe.data[1] = '\0'; /* empty payload */
}
/* Now copy qe into the shared buffer page */
memcpy(AsyncCtl->shared->page_buffer[slotno] + offset,
&qe,
qe.length);
/* Advance queue_head appropriately, and detect if page is full */
if (asyncQueueAdvance(&(queue_head), qe.length))
{
/*
* Page is full, so we're done here, but first fill the next page
* with zeroes. The reason to do this is to ensure that slru.c's
* idea of the head page is always the same as ours, which avoids
* boundary problems in SimpleLruTruncate. The test in
* asyncQueueIsFull() ensured that there is room to create this
* page without overrunning the queue.
*/
slotno = SimpleLruZeroPage(AsyncCtl, QUEUE_POS_PAGE(queue_head));
/* And exit the loop */
break;
}
}
/* Success, so update the global QUEUE_HEAD */
QUEUE_HEAD = queue_head;
LWLockRelease(AsyncCtlLock);
return nextNotify;
}
/*
* SQL function to return the fraction of the notification queue currently
* occupied.
*/
Datum
pg_notification_queue_usage(PG_FUNCTION_ARGS)
{
double usage;
LWLockAcquire(AsyncQueueLock, LW_SHARED);
usage = asyncQueueUsage();
LWLockRelease(AsyncQueueLock);
PG_RETURN_FLOAT8(usage);
}
/*
* Return the fraction of the queue that is currently occupied.
*
* The caller must hold AsyncQueueLock in (at least) shared mode.
*/
static double
asyncQueueUsage(void)
{
int headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
int tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
int occupied;
occupied = headPage - tailPage;
if (occupied == 0)
return (double) 0; /* fast exit for common case */
if (occupied < 0)
{
/* head has wrapped around, tail not yet */
occupied += QUEUE_MAX_PAGE + 1;
}
return (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
}
/*
* Check whether the queue is at least half full, and emit a warning if so.
*
* This is unlikely given the size of the queue, but possible.
* The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
*
* Caller must hold exclusive AsyncQueueLock.
*/
static void
asyncQueueFillWarning(void)
{
double fillDegree;
TimestampTz t;
fillDegree = asyncQueueUsage();
if (fillDegree < 0.5)
return;
t = GetCurrentTimestamp();
if (TimestampDifferenceExceeds(asyncQueueControl->lastQueueFillWarn,
t, QUEUE_FULL_WARN_INTERVAL))
{
QueuePosition min = QUEUE_HEAD;
int32 minPid = InvalidPid;
int i;
for (i = 1; i <= MaxBackends; i++)
{
if (QUEUE_BACKEND_PID(i) != InvalidPid)
{
min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
if (QUEUE_POS_EQUAL(min, QUEUE_BACKEND_POS(i)))
minPid = QUEUE_BACKEND_PID(i);
}
}
ereport(WARNING,
(errmsg("NOTIFY queue is %.0f%% full", fillDegree * 100),
(minPid != InvalidPid ?
errdetail("The server process with PID %d is among those with the oldest transactions.", minPid)
: 0),
(minPid != InvalidPid ?
errhint("The NOTIFY queue cannot be emptied until that process ends its current transaction.")
: 0)));
asyncQueueControl->lastQueueFillWarn = t;
}
}
/*
* Send signals to all listening backends (except our own).
*
* Returns true if we sent at least one signal.
*
* Since we need EXCLUSIVE lock anyway we also check the position of the other
* backends and in case one is already up-to-date we don't signal it.
* This can happen if concurrent notifying transactions have sent a signal and
* the signaled backend has read the other notifications and ours in the same
* step.
*
* Since we know the BackendId and the Pid the signalling is quite cheap.
*/
static bool
SignalBackends(void)
{
bool signalled = false;
int32 *pids;
BackendId *ids;
int count;
int i;
int32 pid;
/*
* Identify all backends that are listening and not already up-to-date. We
* don't want to send signals while holding the AsyncQueueLock, so we just
* build a list of target PIDs.
*
* XXX in principle these pallocs could fail, which would be bad. Maybe
* preallocate the arrays? But in practice this is only run in trivial
* transactions, so there should surely be space available.
*/
pids = (int32 *) palloc(MaxBackends * sizeof(int32));
ids = (BackendId *) palloc(MaxBackends * sizeof(BackendId));
count = 0;
LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
for (i = 1; i <= MaxBackends; i++)
{
pid = QUEUE_BACKEND_PID(i);
if (pid != InvalidPid && pid != MyProcPid)
{
QueuePosition pos = QUEUE_BACKEND_POS(i);
if (!QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
{
pids[count] = pid;
ids[count] = i;
count++;
}
}
}
LWLockRelease(AsyncQueueLock);
/* Now send signals */
for (i = 0; i < count; i++)
{
pid = pids[i];
/*
* Note: assuming things aren't broken, a signal failure here could
* only occur if the target backend exited since we released
* AsyncQueueLock; which is unlikely but certainly possible. So we
* just log a low-level debug message if it happens.
*/
if (SendProcSignal(pid, PROCSIG_NOTIFY_INTERRUPT, ids[i]) < 0)
elog(DEBUG3, "could not signal backend with PID %d: %m", pid);
else
signalled = true;
}
pfree(pids);
pfree(ids);
return signalled;
}
/*
* AtAbort_Notify
*
* This is called at transaction abort.
*
* Gets rid of pending actions and outbound notifies that we would have
* executed if the transaction got committed.
*/
void
AtAbort_Notify(void)
{
/*
* If we LISTEN but then roll back the transaction after PreCommit_Notify,
* we have registered as a listener but have not made any entry in
* listenChannels. In that case, deregister again.
*/
if (amRegisteredListener && listenChannels == NIL)
asyncQueueUnregister();
/* And clean up */
ClearPendingActionsAndNotifies();
}
/*
* AtSubStart_Notify() --- Take care of subtransaction start.
*
* Push empty state for the new subtransaction.
*/
void
AtSubStart_Notify(void)
{
MemoryContext old_cxt;
/* Keep the list-of-lists in TopTransactionContext for simplicity */
old_cxt = MemoryContextSwitchTo(TopTransactionContext);
upperPendingActions = lcons(pendingActions, upperPendingActions);
Assert(list_length(upperPendingActions) ==
GetCurrentTransactionNestLevel() - 1);
pendingActions = NIL;
upperPendingNotifies = lcons(pendingNotifies, upperPendingNotifies);
Assert(list_length(upperPendingNotifies) ==
GetCurrentTransactionNestLevel() - 1);
pendingNotifies = NIL;
MemoryContextSwitchTo(old_cxt);
}
/*
* AtSubCommit_Notify() --- Take care of subtransaction commit.
*
* Reassign all items in the pending lists to the parent transaction.
*/
void
AtSubCommit_Notify(void)
{
List *parentPendingActions;
List *parentPendingNotifies;
parentPendingActions = linitial_node(List, upperPendingActions);
upperPendingActions = list_delete_first(upperPendingActions);
Assert(list_length(upperPendingActions) ==
GetCurrentTransactionNestLevel() - 2);
/*
* Mustn't try to eliminate duplicates here --- see queue_listen()
*/
pendingActions = list_concat(parentPendingActions, pendingActions);
parentPendingNotifies = linitial_node(List, upperPendingNotifies);
upperPendingNotifies = list_delete_first(upperPendingNotifies);
Assert(list_length(upperPendingNotifies) ==
GetCurrentTransactionNestLevel() - 2);
/*
* We could try to eliminate duplicates here, but it seems not worthwhile.
*/
pendingNotifies = list_concat(parentPendingNotifies, pendingNotifies);
}
/*
* AtSubAbort_Notify() --- Take care of subtransaction abort.
*/
void
AtSubAbort_Notify(void)
{
int my_level = GetCurrentTransactionNestLevel();
/*
* All we have to do is pop the stack --- the actions/notifies made in
* this subxact are no longer interesting, and the space will be freed
* when CurTransactionContext is recycled.
*
* This routine could be called more than once at a given nesting level if
* there is trouble during subxact abort. Avoid dumping core by using
* GetCurrentTransactionNestLevel as the indicator of how far we need to
* prune the list.
*/
while (list_length(upperPendingActions) > my_level - 2)
{
pendingActions = linitial_node(List, upperPendingActions);
upperPendingActions = list_delete_first(upperPendingActions);
}
while (list_length(upperPendingNotifies) > my_level - 2)
{
pendingNotifies = linitial_node(List, upperPendingNotifies);
upperPendingNotifies = list_delete_first(upperPendingNotifies);
}
}
/*
* HandleNotifyInterrupt
*
* Signal handler portion of interrupt handling. Let the backend know
* that there's a pending notify interrupt. If we're currently reading
* from the client, this will interrupt the read and
* ProcessClientReadInterrupt() will call ProcessNotifyInterrupt().
*/
void
HandleNotifyInterrupt(void)
{
/*
* Note: this is called by a SIGNAL HANDLER. You must be very wary what
* you do here.
*/
/* signal that work needs to be done */
notifyInterruptPending = true;
/* make sure the event is processed in due course */
SetLatch(MyLatch);
}
/*
* ProcessNotifyInterrupt
*
* This is called just after waiting for a frontend command. If a
* interrupt arrives (via HandleNotifyInterrupt()) while reading, the
* read will be interrupted via the process's latch, and this routine
* will get called. If we are truly idle (ie, *not* inside a transaction
* block), process the incoming notifies.
*/
void
ProcessNotifyInterrupt(void)
{
if (IsTransactionOrTransactionBlock())
return; /* not really idle */
while (notifyInterruptPending)
ProcessIncomingNotify();
}
/*
* Read all pending notifications from the queue, and deliver appropriate
* ones to my frontend. Stop when we reach queue head or an uncommitted
* notification.
*/
static void
asyncQueueReadAllNotifications(void)
{
volatile QueuePosition pos;
QueuePosition oldpos;
QueuePosition head;
Snapshot snapshot;
bool advanceTail;
/* page_buffer must be adequately aligned, so use a union */
union
{
char buf[QUEUE_PAGESIZE];
AsyncQueueEntry align;
} page_buffer;
/* Fetch current state */
LWLockAcquire(AsyncQueueLock, LW_SHARED);
/* Assert checks that we have a valid state entry */
Assert(MyProcPid == QUEUE_BACKEND_PID(MyBackendId));
pos = oldpos = QUEUE_BACKEND_POS(MyBackendId);
head = QUEUE_HEAD;
LWLockRelease(AsyncQueueLock);
if (QUEUE_POS_EQUAL(pos, head))
{
/* Nothing to do, we have read all notifications already. */
return;
}
/* Get snapshot we'll use to decide which xacts are still in progress */
snapshot = RegisterSnapshot(GetLatestSnapshot());
/*----------
* Note that we deliver everything that we see in the queue and that
* matches our _current_ listening state.
* Especially we do not take into account different commit times.
* Consider the following example:
*
* Backend 1: Backend 2:
*
* transaction starts
* NOTIFY foo;
* commit starts
* transaction starts
* LISTEN foo;
* commit starts
* commit to clog
* commit to clog
*
* It could happen that backend 2 sees the notification from backend 1 in
* the queue. Even though the notifying transaction committed before
* the listening transaction, we still deliver the notification.
*
* The idea is that an additional notification does not do any harm, we
* just need to make sure that we do not miss a notification.
*
* It is possible that we fail while trying to send a message to our
* frontend (for example, because of encoding conversion failure).
* If that happens it is critical that we not try to send the same
* message over and over again. Therefore, we place a PG_TRY block
* here that will forcibly advance our backend position before we lose
* control to an error. (We could alternatively retake AsyncQueueLock
* and move the position before handling each individual message, but
* that seems like too much lock traffic.)
*----------
*/
PG_TRY();
{
bool reachedStop;
do
{
int curpage = QUEUE_POS_PAGE(pos);
int curoffset = QUEUE_POS_OFFSET(pos);
int slotno;
int copysize;
/*
* We copy the data from SLRU into a local buffer, so as to avoid
* holding the AsyncCtlLock while we are examining the entries and
* possibly transmitting them to our frontend. Copy only the part
* of the page we will actually inspect.
*/
slotno = SimpleLruReadPage_ReadOnly(AsyncCtl, curpage, InvalidTransactionId);
if (curpage == QUEUE_POS_PAGE(head))
{
/* we only want to read as far as head */
copysize = QUEUE_POS_OFFSET(head) - curoffset;
if (copysize < 0)
copysize = 0; /* just for safety */
}
else
{
/* fetch all the rest of the page */
copysize = QUEUE_PAGESIZE - curoffset;
}
memcpy(page_buffer.buf + curoffset,
AsyncCtl->shared->page_buffer[slotno] + curoffset,
copysize);
/* Release lock that we got from SimpleLruReadPage_ReadOnly() */
LWLockRelease(AsyncCtlLock);
/*
* Process messages up to the stop position, end of page, or an
* uncommitted message.
*
* Our stop position is what we found to be the head's position
* when we entered this function. It might have changed already.
* But if it has, we will receive (or have already received and
* queued) another signal and come here again.
*
* We are not holding AsyncQueueLock here! The queue can only
* extend beyond the head pointer (see above) and we leave our
* backend's pointer where it is so nobody will truncate or
* rewrite pages under us. Especially we don't want to hold a lock
* while sending the notifications to the frontend.
*/
reachedStop = asyncQueueProcessPageEntries(&pos, head,
page_buffer.buf,
snapshot);
} while (!reachedStop);
}
PG_CATCH();
{
/* Update shared state */
LWLockAcquire(AsyncQueueLock, LW_SHARED);
QUEUE_BACKEND_POS(MyBackendId) = pos;
advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
LWLockRelease(AsyncQueueLock);
/* If we were the laziest backend, try to advance the tail pointer */
if (advanceTail)
asyncQueueAdvanceTail();
PG_RE_THROW();
}
PG_END_TRY();
/* Update shared state */
LWLockAcquire(AsyncQueueLock, LW_SHARED);
QUEUE_BACKEND_POS(MyBackendId) = pos;
advanceTail = QUEUE_POS_EQUAL(oldpos, QUEUE_TAIL);
LWLockRelease(AsyncQueueLock);
/* If we were the laziest backend, try to advance the tail pointer */
if (advanceTail)
asyncQueueAdvanceTail();
/* Done with snapshot */
UnregisterSnapshot(snapshot);
}
/*
* Fetch notifications from the shared queue, beginning at position current,
* and deliver relevant ones to my frontend.
*
* The current page must have been fetched into page_buffer from shared
* memory. (We could access the page right in shared memory, but that
* would imply holding the AsyncCtlLock throughout this routine.)
*
* We stop if we reach the "stop" position, or reach a notification from an
* uncommitted transaction, or reach the end of the page.
*
* The function returns true once we have reached the stop position or an
* uncommitted notification, and false if we have finished with the page.
* In other words: once it returns true there is no need to look further.
* The QueuePosition *current is advanced past all processed messages.
*/
static bool
asyncQueueProcessPageEntries(volatile QueuePosition *current,
QueuePosition stop,
char *page_buffer,
Snapshot snapshot)
{
bool reachedStop = false;
bool reachedEndOfPage;
AsyncQueueEntry *qe;
do
{
QueuePosition thisentry = *current;
if (QUEUE_POS_EQUAL(thisentry, stop))
break;
qe = (AsyncQueueEntry *) (page_buffer + QUEUE_POS_OFFSET(thisentry));
/*
* Advance *current over this message, possibly to the next page. As
* noted in the comments for asyncQueueReadAllNotifications, we must
* do this before possibly failing while processing the message.
*/
reachedEndOfPage = asyncQueueAdvance(current, qe->length);
/* Ignore messages destined for other databases */
if (qe->dboid == MyDatabaseId)
{
if (XidInMVCCSnapshot_Local(qe->xid, snapshot))
{
/*
* The source transaction is still in progress, so we can't
* process this message yet. Break out of the loop, but first
* back up *current so we will reprocess the message next
* time. (Note: it is unlikely but not impossible for
* TransactionIdDidCommit to fail, so we can't really avoid
* this advance-then-back-up behavior when dealing with an
* uncommitted message.)
*
* Note that we must test XidInMVCCSnapshot before we test
* TransactionIdDidCommit, else we might return a message from
* a transaction that is not yet visible to snapshots; compare
* the comments at the head of heapam_visibility.c.
*
* Also, while our own xact won't be listed in the snapshot,
* we need not check for TransactionIdIsCurrentTransactionId
* because our transaction cannot (yet) have queued any
* messages.
*/
*current = thisentry;
reachedStop = true;
break;
}
else if (TransactionIdDidCommit(qe->xid))
{
/* qe->data is the null-terminated channel name */
char *channel = qe->data;
if (IsListeningOn(channel))
{
/* payload follows channel name */
char *payload = qe->data + strlen(channel) + 1;
NotifyMyFrontEnd(channel, payload, qe->srcPid);
}
}
else
{
/*
* The source transaction aborted or crashed, so we just
* ignore its notifications.
*/
}
}
/* Loop back if we're not at end of page */
} while (!reachedEndOfPage);
if (QUEUE_POS_EQUAL(*current, stop))
reachedStop = true;
return reachedStop;
}
/*
* Advance the shared queue tail variable to the minimum of all the
* per-backend tail pointers. Truncate pg_notify space if possible.
*/
static void
asyncQueueAdvanceTail(void)
{
QueuePosition min;
int i;
int oldtailpage;
int newtailpage;
int boundary;
LWLockAcquire(AsyncQueueLock, LW_EXCLUSIVE);
min = QUEUE_HEAD;
for (i = 1; i <= MaxBackends; i++)
{
if (QUEUE_BACKEND_PID(i) != InvalidPid)
min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
}
oldtailpage = QUEUE_POS_PAGE(QUEUE_TAIL);
QUEUE_TAIL = min;
LWLockRelease(AsyncQueueLock);
/*
* We can truncate something if the global tail advanced across an SLRU
* segment boundary.
*
* XXX it might be better to truncate only once every several segments, to
* reduce the number of directory scans.
*/
newtailpage = QUEUE_POS_PAGE(min);
boundary = newtailpage - (newtailpage % SLRU_PAGES_PER_SEGMENT);
if (asyncQueuePagePrecedes(oldtailpage, boundary))
{
/*
* SimpleLruTruncate() will ask for AsyncCtlLock but will also release
* the lock again.
*/
SimpleLruTruncate(AsyncCtl, newtailpage);
}
}
/*
* ProcessIncomingNotify
*
* Deal with arriving NOTIFYs from other backends as soon as it's safe to
* do so. This used to be called from the PROCSIG_NOTIFY_INTERRUPT
* signal handler, but isn't anymore.
*
* Scan the queue for arriving notifications and report them to my front
* end.
*
* NOTE: since we are outside any transaction, we must create our own.
*/
static void
ProcessIncomingNotify(void)
{
/* We *must* reset the flag */
notifyInterruptPending = false;
/* Do nothing else if we aren't actively listening */
if (listenChannels == NIL)
return;
if (Trace_notify)
elog(DEBUG1, "ProcessIncomingNotify");
set_ps_display("notify interrupt", false);
/*
* We must run asyncQueueReadAllNotifications inside a transaction, else
* bad things happen if it gets an error.
*/
StartTransactionCommand();
asyncQueueReadAllNotifications();
CommitTransactionCommand();
/*
* Must flush the notify messages to ensure frontend gets them promptly.
*/
pq_flush();
set_ps_display("idle", false);
if (Trace_notify)
elog(DEBUG1, "ProcessIncomingNotify: done");
}
/*
* Send NOTIFY message to my front end.
*
* GPDB: We have exposed this function globally for our dispatch-notify
* mechanism. We overload the srcPid field to pass in the gp_session_id
* from GPDB specific callsites.
*/
void
NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
{
if (whereToSendOutput == DestRemote)
{
StringInfoData buf;
pq_beginmessage(&buf, 'A');
pq_sendint32(&buf, srcPid);
pq_sendstring(&buf, channel);
if (PG_PROTOCOL_MAJOR(FrontendProtocol) >= 3)
pq_sendstring(&buf, payload);
pq_endmessage(&buf);
/*
* NOTE: we do not do pq_flush() here. For a self-notify, it will
* happen at the end of the transaction, and for incoming notifies
* ProcessIncomingNotify will do it after finding all the notifies.
*/
}
else
elog(INFO, "NOTIFY for \"%s\" payload \"%s\"", channel, payload);
}
/* Does pendingNotifies include the given channel/payload? */
static bool
AsyncExistsPendingNotify(const char *channel, const char *payload)
{
ListCell *p;
Notification *n;
if (pendingNotifies == NIL)
return false;
if (payload == NULL)
payload = "";
/*----------
* We need to append new elements to the end of the list in order to keep
* the order. However, on the other hand we'd like to check the list
* backwards in order to make duplicate-elimination a tad faster when the
* same condition is signaled many times in a row. So as a compromise we
* check the tail element first which we can access directly. If this
* doesn't match, we check the whole list.
*
* As we are not checking our parents' lists, we can still get duplicates
* in combination with subtransactions, like in:
*
* begin;
* notify foo '1';
* savepoint foo;
* notify foo '1';
* commit;
*----------
*/
n = (Notification *) llast(pendingNotifies);
if (strcmp(n->channel, channel) == 0 &&
strcmp(n->payload, payload) == 0)
return true;
foreach(p, pendingNotifies)
{
n = (Notification *) lfirst(p);
if (strcmp(n->channel, channel) == 0 &&
strcmp(n->payload, payload) == 0)
return true;
}
return false;
}
/* Clear the pendingActions and pendingNotifies lists. */
static void
ClearPendingActionsAndNotifies(void)
{
/*
* We used to have to explicitly deallocate the list members and nodes,
* because they were malloc'd. Now, since we know they are palloc'd in
* CurTransactionContext, we need not do that --- they'll go away
* automatically at transaction exit. We need only reset the list head
* pointers.
*/
pendingActions = NIL;
pendingNotifies = NIL;
}
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