Fix bug in early startup of Hot Standby with subtransactions.

When HS startup is deferred because of overflowed subtransactions, ensure
that we re-initialize KnownAssignedXids for when both existing and incoming
snapshots have non-zero qualifying xids.

Fixes bug #6661 reported by Valentine Gogichashvili.

Analysis and fix by Andres Freund

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 62e2f650c04fa4639c36fd80fb93227961ae8bed..1dd238f55bfec8f2a8beac85cf7e3294bea770bd 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -158,6 +158,7 @@ static int KnownAssignedXidsGetAndSetXmin(TransactionId *xarray,
                               TransactionId xmax);
 static TransactionId KnownAssignedXidsGetOldestXmin(void);
 static void KnownAssignedXidsDisplay(int trace_level);
+static void KnownAssignedXidsReset(void);
 
 /*
  * Report shared-memory space needed by CreateSharedProcArray.
@@ -493,6 +494,11 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
         */
        if (!running->subxid_overflow || running->xcnt == 0)
        {
+           /*
+            * If we have already collected known assigned xids, we need to
+            * throw them away before we apply the recovery snapshot.
+            */
+           KnownAssignedXidsReset();
            standbyState = STANDBY_INITIALIZED;
        }
        else
@@ -536,7 +542,6 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
     * xids to subtrans. If RunningXacts is overflowed then we don't have
     * enough information to correctly update subtrans anyway.
     */
-   Assert(procArray->numKnownAssignedXids == 0);
 
    /*
     * Allocate a temporary array to avoid modifying the array passed as
@@ -566,6 +571,12 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 
    if (nxids > 0)
    {
+       if (procArray->numKnownAssignedXids != 0)
+       {
+           LWLockRelease(ProcArrayLock);
+           elog(ERROR, "KnownAssignedXids is not empty");
+       }
+
        /*
         * Sort the array so that we can add them safely into
         * KnownAssignedXids.
@@ -3133,3 +3144,22 @@ KnownAssignedXidsDisplay(int trace_level)
 
    pfree(buf.data);
 }
+
+/*
+ * KnownAssignedXidsReset
+ *     Resets KnownAssignedXids to be empty
+ */
+static void
+KnownAssignedXidsReset(void)
+{
+   /* use volatile pointer to prevent code rearrangement */
+   volatile ProcArrayStruct *pArray = procArray;
+
+   LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+
+   pArray->numKnownAssignedXids = 0;
+   pArray->tailKnownAssignedXids = 0;
+   pArray->headKnownAssignedXids = 0;
+
+   LWLockRelease(ProcArrayLock);
+}