Force immediate commit after CREATE DATABASE etc in extended protocol.

We have a few commands that "can't run in a transaction block",
meaning that if they complete their processing but then we fail
to COMMIT, we'll be left with inconsistent on-disk state.
However, the existing defenses for this are only watertight for
simple query protocol.  In extended protocol, we didn't commit
until receiving a Sync message.  Since the client is allowed to
issue another command instead of Sync, we're in trouble if that
command fails or is an explicit ROLLBACK.  In any case, sitting
in an inconsistent state while waiting for a client message
that might not come seems pretty risky.

This case wasn't reachable via libpq before we introduced pipeline
mode, but it's always been an intended aspect of extended query
protocol, and likely there are other clients that could reach it
before.

To fix, set a flag in PreventInTransactionBlock that tells
exec_execute_message to force an immediate commit.  This seems
to be the approach that does least damage to existing working
cases while still preventing the undesirable outcomes.

While here, add some documentation to protocol.sgml that explicitly
says how to use pipelining.  That's latent in the existing docs if
you know what to look for, but it's better to spell it out; and it
provides a place to document this new behavior.

Per bug #17434 from Yugo Nagata.  It's been wrong for ages,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/17434-d9f7a064ce2a88a3@postgresql.org

3 files changed, 46 insertions, 26 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index bd60b55574c..594d8da2cdc 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -3453,6 +3453,9 @@ AbortCurrentTransaction(void)
  *	could issue more commands and possibly cause a failure after the statement
  *	completes).  Subtransactions are verboten too.
  *
+ *	We must also set XACT_FLAGS_NEEDIMMEDIATECOMMIT in MyXactFlags, to ensure
+ *	that postgres.c follows through by committing after the statement is done.
+ *
  *	isTopLevel: passed down from ProcessUtility to determine whether we are
  *	inside a function.  (We will always fail if this is false, but it's
  *	convenient to centralize the check here instead of making callers do it.)
@@ -3494,7 +3497,9 @@ PreventInTransactionBlock(bool isTopLevel, const char *stmtType)
 	if (CurrentTransactionState->blockState != TBLOCK_DEFAULT &&
 		CurrentTransactionState->blockState != TBLOCK_STARTED)
 		elog(FATAL, "cannot prevent transaction chain");
-	/* all okay */
+
+	/* All okay.  Set the flag to make sure the right thing happens later. */
+	MyXactFlags |= XACT_FLAGS_NEEDIMMEDIATECOMMIT;
 }
 
 /*
@@ -3591,6 +3596,13 @@ IsInTransactionBlock(bool isTopLevel)
 		CurrentTransactionState->blockState != TBLOCK_STARTED)
 		return true;
 
+	/*
+	 * If we tell the caller we're not in a transaction block, then inform
+	 * postgres.c that it had better commit when the statement is done.
+	 * Otherwise our report could be a lie.
+	 */
+	MyXactFlags |= XACT_FLAGS_NEEDIMMEDIATECOMMIT;
+
 	return false;
 }
 
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index f84b2fa54e0..06dfe115e48 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -1285,6 +1285,13 @@ exec_simple_query(const char *query_string)
 		else
 		{
 			/*
+			 * We had better not see XACT_FLAGS_NEEDIMMEDIATECOMMIT set if
+			 * we're not calling finish_xact_command().  (The implicit
+			 * transaction block should have prevented it from getting set.)
+			 */
+			Assert(!(MyXactFlags & XACT_FLAGS_NEEDIMMEDIATECOMMIT));
+
+			/*
 			 * We need a CommandCounterIncrement after every query, except
 			 * those that start or end a transaction block.
 			 */
@@ -2099,32 +2106,16 @@ exec_execute_message(const char *portal_name, long max_rows)
 
 	/*
 	 * We must copy the sourceText and prepStmtName into MessageContext in
-	 * case the portal is destroyed during finish_xact_command. Can avoid the
-	 * copy if it's not an xact command, though.
+	 * case the portal is destroyed during finish_xact_command.  We do not
+	 * make a copy of the portalParams though, preferring to just not print
+	 * them in that case.
 	 */
-	if (is_xact_command)
-	{
-		sourceText = pstrdup(portal->sourceText);
-		if (portal->prepStmtName)
-			prepStmtName = pstrdup(portal->prepStmtName);
-		else
-			prepStmtName = "<unnamed>";
-
-		/*
-		 * An xact command shouldn't have any parameters, which is a good
-		 * thing because they wouldn't be around after finish_xact_command.
-		 */
-		portalParams = NULL;
-	}
+	sourceText = pstrdup(portal->sourceText);
+	if (portal->prepStmtName)
+		prepStmtName = pstrdup(portal->prepStmtName);
 	else
-	{
-		sourceText = portal->sourceText;
-		if (portal->prepStmtName)
-			prepStmtName = portal->prepStmtName;
-		else
-			prepStmtName = "<unnamed>";
-		portalParams = portal->portalParams;
-	}
+		prepStmtName = "<unnamed>";
+	portalParams = portal->portalParams;
 
 	/*
 	 * Report query to various monitoring facilities.
@@ -2223,13 +2214,24 @@ exec_execute_message(const char *portal_name, long max_rows)
 
 	if (completed)
 	{
-		if (is_xact_command)
+		if (is_xact_command || (MyXactFlags & XACT_FLAGS_NEEDIMMEDIATECOMMIT))
 		{
 			/*
 			 * If this was a transaction control statement, commit it.  We
 			 * will start a new xact command for the next command (if any).
+			 * Likewise if the statement required immediate commit.  Without
+			 * this provision, we wouldn't force commit until Sync is
+			 * received, which creates a hazard if the client tries to
+			 * pipeline immediate-commit statements.
 			 */
 			finish_xact_command();
+
+			/*
+			 * These commands typically don't have any parameters, and even if
+			 * one did we couldn't print them now because the storage went
+			 * away during finish_xact_command.  So pretend there were none.
+			 */
+			portalParams = NULL;
 		}
 		else
 		{
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index 4794941df31..65616ca2f79 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -108,6 +108,12 @@ extern PGDLLIMPORT int MyXactFlags;
 #define XACT_FLAGS_ACQUIREDACCESSEXCLUSIVELOCK	(1U << 1)
 
 /*
+ * XACT_FLAGS_NEEDIMMEDIATECOMMIT - records whether the top level statement
+ * is one that requires immediate commit, such as CREATE DATABASE.
+ */
+#define XACT_FLAGS_NEEDIMMEDIATECOMMIT			(1U << 2)
+
+/*
  *	start- and end-of-transaction callbacks for dynamically loaded modules
  */
 typedef enum