From 589c0e98ae913a61d7f2833091a89d0f4e1ce556 Mon Sep 17 00:00:00 2001
From: Andrey Borodin <amborodin@acm.org>
Date: Sat, 19 Dec 2020 21:53:00 +0500
Subject: [PATCH v3 1/2] Wait for prepared xacts in CREATE INDEX CONCURRENTLY
 locks infrastructure

---
 src/backend/storage/lmgr/lmgr.c |  5 ++---
 src/backend/storage/lmgr/lock.c | 37 ++++++++++++++++++++++-----------
 src/include/storage/lock.h      |  5 +++++
 3 files changed, 32 insertions(+), 15 deletions(-)

diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 7409de9405..c1e8e76f41 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -904,8 +904,7 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 
 	/*
 	 * Note: GetLockConflicts() never reports our own xid, hence we need not
-	 * check for that.  Also, prepared xacts are not reported, which is fine
-	 * since they certainly aren't going to do anything anymore.
+	 * check for that.  Also, prepared xacts are reported and awaited.
 	 */
 
 	/* Finally wait for each such transaction to complete */
@@ -913,7 +912,7 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 	{
 		VirtualTransactionId *lockholders = lfirst(lc);
 
-		while (VirtualTransactionIdIsValid(*lockholders))
+		while (VirtualTransactionIdIsValidOrPreparedXact(*lockholders))
 		{
 			/* If requested, publish who we're going to wait for. */
 			if (progress)
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index d86566f455..d870396504 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -2904,9 +2904,7 @@ FastPathGetRelationLockEntry(LOCALLOCK *locallock)
  * so use of this function has to be thought about carefully.
  *
  * Note we never include the current xact's vxid in the result array,
- * since an xact never blocks itself.  Also, prepared transactions are
- * ignored, which is a bit more debatable but is appropriate for current
- * uses of the result.
+ * since an xact never blocks itself.
  */
 VirtualTransactionId *
 GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
@@ -2931,15 +2929,17 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 
 	/*
 	 * Allocate memory to store results, and fill with InvalidVXID.  We only
-	 * need enough space for MaxBackends + a terminator, since prepared xacts
-	 * don't count. InHotStandby allocate once in TopMemoryContext.
+	 * need enough space for MaxBackends + max_prepared_xacts + a
+	 * terminator. InHotStandby allocate once in TopMemoryContext.
 	 */
 	if (InHotStandby)
 	{
 		if (vxids == NULL)
 			vxids = (VirtualTransactionId *)
 				MemoryContextAlloc(TopMemoryContext,
-								   sizeof(VirtualTransactionId) * (MaxBackends + 1));
+								   sizeof(VirtualTransactionId) * (MaxBackends 
+								   + max_prepared_xacts
+								   + 1));
 	}
 	else
 		vxids = (VirtualTransactionId *)
@@ -3085,10 +3085,11 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 
 				/*
 				 * If we see an invalid VXID, then either the xact has already
-				 * committed (or aborted), or it's a prepared xact.  In either
-				 * case we may ignore it.
+				 * committed (or aborted), or it's a prepared xact. We take
+				 * prepared xacts into account too.
 				 */
-				if (VirtualTransactionIdIsValid(vxid))
+				if (VirtualTransactionIdIsValid(vxid)
+					|| VirtualTransactionIdIsPreparedXact(vxid))
 				{
 					int			i;
 
@@ -3108,7 +3109,7 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 
 	LWLockRelease(partitionLock);
 
-	if (count > MaxBackends)	/* should never happen */
+	if (count > MaxBackends + max_prepared_xacts)	/* should never happen */
 		elog(PANIC, "too many conflicting locks found");
 
 	vxids[count].backendId = InvalidBackendId;
@@ -4461,9 +4462,21 @@ bool
 VirtualXactLock(VirtualTransactionId vxid, bool wait)
 {
 	LOCKTAG		tag;
-	PGPROC	   *proc;
+	PGPROC	   *proc = NULL;
 
-	Assert(VirtualTransactionIdIsValid(vxid));
+	Assert(VirtualTransactionIdIsValid(vxid) ||
+			VirtualTransactionIdIsPreparedXact(vxid));
+
+	if (VirtualTransactionIdIsPreparedXact(vxid))
+	{
+		LockAcquireResult lar;
+		/* If it's prepared xact - just wait for it */
+		SET_LOCKTAG_TRANSACTION(tag, vxid.localTransactionId);
+		lar = LockAcquire(&tag, ShareLock, false, !wait);
+		if (lar == LOCKACQUIRE_OK)
+			LockRelease(&tag, ShareLock, false);
+		return lar != LOCKACQUIRE_NOT_AVAIL;
+	}
 
 	SET_LOCKTAG_VIRTUALTRANSACTION(tag, vxid);
 
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index 1c3e9c1999..dbd2a32678 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -70,6 +70,11 @@ typedef struct
 #define VirtualTransactionIdIsValid(vxid) \
 	(((vxid).backendId != InvalidBackendId) && \
 	 LocalTransactionIdIsValid((vxid).localTransactionId))
+#define VirtualTransactionIdIsValidOrPreparedXact(vxid) \
+	(((vxid).backendId == InvalidBackendId) || \
+	 LocalTransactionIdIsValid((vxid).localTransactionId))
+#define VirtualTransactionIdIsPreparedXact(vxid) \
+	((vxid).backendId == InvalidBackendId)
 #define VirtualTransactionIdEquals(vxid1, vxid2) \
 	((vxid1).backendId == (vxid2).backendId && \
 	 (vxid1).localTransactionId == (vxid2).localTransactionId)
-- 
2.24.3 (Apple Git-128)