Handle better implicit transaction state of pipeline mode

michaelpq · michaelpq · commit 5c3067282ec1 · 2024-11-27T09:31:43.000+09:00
When using a pipeline, a transaction starts from the first command and is committed with a Sync message or when the pipeline ends. Functions like IsInTransactionBlock() or PreventInTransactionBlock() were already able to understand a pipeline as being in a transaction block, but it was not the case of CheckTransactionBlock(). This function is called for example to generate a WARNING for SET LOCAL, complaining that it is used outside of a transaction block. The current state of the code caused multiple problems, like: - SET LOCAL executed at any stage of a pipeline issued a WARNING, even if the command was at least second in line where the pipeline is in a transaction state. - LOCK TABLE failed when invoked at any step of a pipeline, even if it should be able to work within a transaction block. The pipeline protocol assumes that the first command of a pipeline is not part of a transaction block, and that any follow-up commands is considered as within a transaction block. This commit changes the backend so as an implicit transaction block is started each time the first Execute message of a pipeline has finished processing, with this implicit transaction block ended once a sync is processed. The checks based on XACT_FLAGS_PIPELINING in the routines checking if we are in a transaction block are not necessary: it is enough to rely on the existing ones. Some tests are added to pgbench, that can be backpatched down to v17 when \syncpipeline is involved and down to v14 where \startpipeline and \endpipeline are available. This is unfortunately limited regarding the error patterns that can be checked, but it provides coverage for various pipeline combinations to check if these succeed or fail. These tests are able to capture the case of SET LOCAL's WARNING. The author has proposed a different feature to improve the coverage by adding similar meta-commands to psql where error messages could be checked, something more useful for the cases where commands cannot be used in transaction blocks, like REINDEX CONCURRENTLY or VACUUM. This is considered as future work for v18~. Author: Anthonin Bonnefoy Reviewed-by: Jelte Fennema-Nio, Michael Paquier Discussion: https://postgr.es/m/CAO6_XqrWO8uNBQrSu5r6jh+vTGi5Oiyk4y8yXDORdE2jbzw8xw@mail.gmail.com Backpatch-through: 13
diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
@@ -1088,16 +1088,17 @@ SELCT 1/0;<!-- this typo is intentional -->
 
    <para>
     If the client has not issued an explicit <command>BEGIN</command>,
-    then each Sync ordinarily causes an implicit <command>COMMIT</command>
-    if the preceding step(s) succeeded, or an
-    implicit <command>ROLLBACK</command> if they failed.  However, there
-    are a few DDL commands (such as <command>CREATE DATABASE</command>)
-    that cannot be executed inside a transaction block.  If one of
-    these is executed in a pipeline, it will fail unless it is the first
-    command in the pipeline.  Furthermore, upon success it will force an
-    immediate commit to preserve database consistency.  Thus a Sync
-    immediately following one of these commands has no effect except to
-    respond with ReadyForQuery.
+    then an implicit transaction block is started and each Sync ordinarily
+    causes an implicit <command>COMMIT</command> if the preceding step(s)
+    succeeded, or an implicit <command>ROLLBACK</command> if they failed.
+    This implicit transaction block will only be detected by the server
+    when the first command ends without a sync.  There are a few DDL
+    commands (such as <command>CREATE DATABASE</command>) that cannot be
+    executed inside a transaction block. If one of these is executed in a
+    pipeline, it will fail unless it is the first command after a Sync.
+    Furthermore, upon success it will force an immediate commit to preserve
+    database consistency. Thus a Sync immediately following one of these
+    commands has no effect except to respond with ReadyForQuery.
    </para>
 
    <para>
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
@@ -3405,16 +3405,6 @@ PreventInTransactionBlock(bool isTopLevel, const char *stmtType)
 				 errmsg("%s cannot run inside a subtransaction",
 						stmtType)));
 
-	/*
-	 * inside a pipeline that has started an implicit transaction?
-	 */
-	if (MyXactFlags & XACT_FLAGS_PIPELINING)
-		ereport(ERROR,
-				(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
-		/* translator: %s represents an SQL statement name */
-				 errmsg("%s cannot be executed within a pipeline",
-						stmtType)));
-
 	/*
 	 * inside a function call?
 	 */
@@ -3526,9 +3516,6 @@ IsInTransactionBlock(bool isTopLevel)
 	if (IsSubTransaction())
 		return true;
 
-	if (MyXactFlags & XACT_FLAGS_PIPELINING)
-		return true;
-
 	if (!isTopLevel)
 		return true;
 
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -2656,6 +2656,17 @@ start_xact_command(void)
 
 		xact_started = true;
 	}
+	else if (MyXactFlags & XACT_FLAGS_PIPELINING)
+	{
+		/*
+		 * When the first Execute message is completed, following commands
+		 * will be done in an implicit transaction block created via
+		 * pipelining. The transaction state needs to be updated to an
+		 * implicit block if we're not already in a transaction block (like
+		 * one started by an explicit BEGIN).
+		 */
+		BeginImplicitTransactionBlock();
+	}
 
 	/*
 	 * Start statement timeout if necessary.  Note that this'll intentionally
@@ -4605,6 +4616,13 @@ PostgresMain(int argc, char *argv[],
 
 			case 'S':			/* sync */
 				pq_getmsgend(&input_message);
+
+				/*
+				 * If pipelining was used, we may be in an implicit
+				 * transaction block. Close it before calling
+				 * finish_xact_command.
+				 */
+				EndImplicitTransactionBlock();
 				finish_xact_command();
 				send_ready_for_query = true;
 				break;
