Fix low-probability loss of NOTIFY messages due to XID wraparound.

Up to now async.c has used TransactionIdIsInProgress() to detect whether
a notify message's source transaction is still running.  However, that
function has a quick-exit path that reports that XIDs before RecentXmin
are no longer running.  If a listening backend is doing nothing but
listening, and not running any queries, there is nothing that will advance
its value of RecentXmin.  Once 2 billion transactions elapse, the
RecentXmin check causes active transactions to be reported as not running.
If they aren't committed yet according to CLOG, async.c decides they
aborted and discards their messages.  The timing for that is a bit tight
but it can happen when multiple backends are sending notifies concurrently.
The net symptom therefore is that a sufficiently-long-surviving
listen-only backend starts to miss some fraction of NOTIFY traffic,
but only under heavy load.

The only function that updates RecentXmin is GetSnapshotData().
A brute-force fix would therefore be to take a snapshot before
processing incoming notify messages.  But that would add cycles,
as well as contention for the ProcArrayLock.  We can be smarter:
having taken the snapshot, let's use that to check for running
XIDs, and not call TransactionIdIsInProgress() at all.  In this
way we reduce the number of ProcArrayLock acquisitions from one
per message to one per notify interrupt; that's the same under
light load but should be a benefit under heavy load.  Light testing
says that this change is a wash performance-wise for normal loads.

I looked around for other callers of TransactionIdIsInProgress()
that might be at similar risk, and didn't find any; all of them
are inside transactions that presumably have already taken a
snapshot.

Problem report and diagnosis by Marko Tiikkaja, patch by me.
Back-patch to all supported branches, since it's been like this
since 9.0.

Discussion: https://postgr.es/m/20170926182935.14128.65278@wrigleys.postgresql.org

diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index bacc08eb84f0c7c108aec31a2f0d25814320052a..a93c81bca28798e64ef84be847eecf12c6c6c740 100644 (file)
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -137,7 +137,9 @@
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
+#include "utils/snapmgr.h"
 #include "utils/timestamp.h"
+#include "utils/tqual.h"
 
 
 /*
@@ -387,7 +389,8 @@ static bool SignalBackends(void);
 static void asyncQueueReadAllNotifications(void);
 static bool asyncQueueProcessPageEntries(volatile QueuePosition *current,
                             QueuePosition stop,
-                            char *page_buffer);
+                            char *page_buffer,
+                            Snapshot snapshot);
 static void asyncQueueAdvanceTail(void);
 static void ProcessIncomingNotify(void);
 static bool AsyncExistsPendingNotify(const char *channel, const char *payload);
@@ -798,7 +801,7 @@ PreCommit_Notify(void)
        }
    }
 
-   /* Queue any pending notifies */
+   /* Queue any pending notifies (must happen after the above) */
    if (pendingNotifies)
    {
        ListCell   *nextNotify;
@@ -987,7 +990,9 @@ Exec_ListenPreCommit(void)
     * 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.
+    * 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.
     */
@@ -1744,6 +1749,7 @@ asyncQueueReadAllNotifications(void)
    volatile QueuePosition pos;
    QueuePosition oldpos;
    QueuePosition head;
+   Snapshot    snapshot;
    bool        advanceTail;
 
    /* page_buffer must be adequately aligned, so use a union */
@@ -1767,6 +1773,9 @@ asyncQueueReadAllNotifications(void)
        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.
@@ -1854,7 +1863,8 @@ asyncQueueReadAllNotifications(void)
             * while sending the notifications to the frontend.
             */
            reachedStop = asyncQueueProcessPageEntries(&pos, head,
-                                                      page_buffer.buf);
+                                                      page_buffer.buf,
+                                                      snapshot);
        } while (!reachedStop);
    }
    PG_CATCH();
@@ -1882,6 +1892,9 @@ asyncQueueReadAllNotifications(void)
    /* If we were the laziest backend, try to advance the tail pointer */
    if (advanceTail)
        asyncQueueAdvanceTail();
+
+   /* Done with snapshot */
+   UnregisterSnapshot(snapshot);
 }
 
 /*
@@ -1903,7 +1916,8 @@ asyncQueueReadAllNotifications(void)
 static bool
 asyncQueueProcessPageEntries(volatile QueuePosition *current,
                             QueuePosition stop,
-                            char *page_buffer)
+                            char *page_buffer,
+                            Snapshot snapshot)
 {
    bool        reachedStop = false;
    bool        reachedEndOfPage;
@@ -1928,7 +1942,7 @@ asyncQueueProcessPageEntries(volatile QueuePosition *current,
        /* Ignore messages destined for other databases */
        if (qe->dboid == MyDatabaseId)
        {
-           if (TransactionIdIsInProgress(qe->xid))
+           if (XidInMVCCSnapshot(qe->xid, snapshot))
            {
                /*
                 * The source transaction is still in progress, so we can't
@@ -1939,10 +1953,15 @@ asyncQueueProcessPageEntries(volatile QueuePosition *current,
                 * this advance-then-back-up behavior when dealing with an
                 * uncommitted message.)
                 *
-                * Note that we must test TransactionIdIsInProgress 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 tqual.c.
+                * 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 tqual.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;

diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
index bbac4083c98e14930091875faa0d15887560a0bf..b7aab0dd190aa6c9e366cd39b154204950fe65d3 100644 (file)
--- a/src/backend/utils/time/tqual.c
+++ b/src/backend/utils/time/tqual.c
@@ -81,8 +81,6 @@
 SnapshotData SnapshotSelfData = {HeapTupleSatisfiesSelf};
 SnapshotData SnapshotAnyData = {HeapTupleSatisfiesAny};
 
-/* local functions */
-static bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
 
 /*
  * SetHintBits()
@@ -1479,10 +1477,10 @@ HeapTupleIsSurelyDead(HeapTuple htup, TransactionId OldestXmin)
  * Note: GetSnapshotData never stores either top xid or subxids of our own
  * backend into a snapshot, so these xids will not be reported as "running"
  * by this function.  This is OK for current uses, because we always check
- * TransactionIdIsCurrentTransactionId first, except for known-committed
- * XIDs which could not be ours anyway.
+ * TransactionIdIsCurrentTransactionId first, except when it's known the
+ * XID could not be ours anyway.
  */
-static bool
+bool
 XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
 {
    uint32      i;

diff --git a/src/include/utils/tqual.h b/src/include/utils/tqual.h
index 9a3b56e5f0324f45d42a8822e59c687682871b33..96eaf01ca0eeea133faec53046d026317151246c 100644 (file)
--- a/src/include/utils/tqual.h
+++ b/src/include/utils/tqual.h
@@ -78,6 +78,7 @@ extern HTSV_Result HeapTupleSatisfiesVacuum(HeapTuple htup,
                         TransactionId OldestXmin, Buffer buffer);
 extern bool HeapTupleIsSurelyDead(HeapTuple htup,
                      TransactionId OldestXmin);
+extern bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
 
 extern void HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
                     uint16 infomask, TransactionId xid);