Date: Sat, 24 Oct 2020 22:18:36 -0700
From: Noah Misch <noah@leadboat.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Andres Freund <andres@anarazel.de>,
	Phil Florent <philflorent@hotmail.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: pgstat_report_activity() and parallel CREATE INDEX (was:
 Parallel index creation & pg_stat_activity)
Message-ID: <20201025051836.GC2651494@rfd.leadboat.com>
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On Sat, Oct 17, 2020 at 08:39:35AM -0700, Peter Geoghegan wrote:
> On Tue, Oct 13, 2020 at 7:26 PM Noah Misch <noah@leadboat.com> wrote:
> > 1. Disable parallelism for the index build under ExecuteTruncateGuts().
> >    Nobody will mourn a performance loss from declining parallelism for an
> >    empty index, but I feel like this is fixing in the wrong place.
> > 2. Make _bt_begin_parallel() and begin_parallel_vacuum() recognize the
> >    debug_query_string==NULL case and reproduce it on the worker.
> > 3. Require bgworkers to set debug_query_string before entering code of vacuum,
> >    truncate, etc.  Logical replication might synthesize a DDL statement, or it
> >    might just use a constant string.
> >
> > I tend to prefer (2), but (3) isn't bad.  Opinions?
> 
> I also prefer 2.

Thanks.  Here is the pair of patches I plan to use.  The second is equivalent
to v0; I just added a log message.

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Content-Disposition: attachment; filename="null-debug_query_string-v1.patch"

Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Reproduce debug_query_string==NULL on parallel workers.
    
    Certain background workers initiate parallel queries while
    debug_query_string==NULL, at which point they attempted strlen(NULL) and
    died to SIGSEGV.  Older debug_query_string observers allow NULL, so do
    likewise in these newer ones.  Back-patch to v11, where commit
    7de4a1bcc56f494acbd0d6e70781df877dc8ecb5 introduced the first of these.
    
    Reviewed by FIXME.
    
    Discussion: https://postgr.es/m/20201014022636.GA1962668@rfd.leadboat.com

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index 4f2f381..34ae8aa 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -3238,7 +3238,6 @@ begin_parallel_vacuum(Oid relid, Relation *Irel, LVRelStats *vacrelstats,
 	WalUsage   *wal_usage;
 	bool	   *can_parallel_vacuum;
 	long		maxtuples;
-	char	   *sharedquery;
 	Size		est_shared;
 	Size		est_deadtuples;
 	int			nindexes_mwm = 0;
@@ -3335,9 +3334,14 @@ begin_parallel_vacuum(Oid relid, Relation *Irel, LVRelStats *vacrelstats,
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Finally, estimate PARALLEL_VACUUM_KEY_QUERY_TEXT space */
-	querylen = strlen(debug_query_string);
-	shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
-	shm_toc_estimate_keys(&pcxt->estimator, 1);
+	if (debug_query_string)
+	{
+		querylen = strlen(debug_query_string);
+		shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
+		shm_toc_estimate_keys(&pcxt->estimator, 1);
+	}
+	else
+		querylen = 0;			/* keep compiler quiet */
 
 	InitializeParallelDSM(pcxt);
 
@@ -3382,10 +3386,16 @@ begin_parallel_vacuum(Oid relid, Relation *Irel, LVRelStats *vacrelstats,
 	lps->wal_usage = wal_usage;
 
 	/* Store query string for workers */
-	sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
-	memcpy(sharedquery, debug_query_string, querylen + 1);
-	sharedquery[querylen] = '\0';
-	shm_toc_insert(pcxt->toc, PARALLEL_VACUUM_KEY_QUERY_TEXT, sharedquery);
+	if (debug_query_string)
+	{
+		char	   *sharedquery;
+
+		sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
+		memcpy(sharedquery, debug_query_string, querylen + 1);
+		sharedquery[querylen] = '\0';
+		shm_toc_insert(pcxt->toc,
+					   PARALLEL_VACUUM_KEY_QUERY_TEXT, sharedquery);
+	}
 
 	pfree(can_parallel_vacuum);
 	return lps;
@@ -3528,7 +3538,7 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
 		elog(DEBUG1, "starting parallel vacuum worker for bulk delete");
 
 	/* Set debug_query_string for individual workers */
-	sharedquery = shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_QUERY_TEXT, false);
+	sharedquery = shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_QUERY_TEXT, true);
 	debug_query_string = sharedquery;
 	pgstat_report_activity(STATE_RUNNING, debug_query_string);
 
diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index efee867..8730de2 100644
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -1466,7 +1466,6 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	WalUsage   *walusage;
 	BufferUsage *bufferusage;
 	bool		leaderparticipates = true;
-	char	   *sharedquery;
 	int			querylen;
 
 #ifdef DISABLE_LEADER_PARTICIPATION
@@ -1533,9 +1532,14 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Finally, estimate PARALLEL_KEY_QUERY_TEXT space */
-	querylen = strlen(debug_query_string);
-	shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
-	shm_toc_estimate_keys(&pcxt->estimator, 1);
+	if (debug_query_string)
+	{
+		querylen = strlen(debug_query_string);
+		shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
+		shm_toc_estimate_keys(&pcxt->estimator, 1);
+	}
+	else
+		querylen = 0;			/* keep compiler quiet */
 
 	/* Everyone's had a chance to ask for space, so now create the DSM */
 	InitializeParallelDSM(pcxt);
@@ -1599,9 +1603,14 @@ _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
 	}
 
 	/* Store query string for workers */
-	sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
-	memcpy(sharedquery, debug_query_string, querylen + 1);
-	shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, sharedquery);
+	if (debug_query_string)
+	{
+		char	   *sharedquery;
+
+		sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
+		memcpy(sharedquery, debug_query_string, querylen + 1);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, sharedquery);
+	}
 
 	/*
 	 * Allocate space for each worker's WalUsage and BufferUsage; no need to
@@ -1806,7 +1815,7 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 #endif							/* BTREE_BUILD_STATS */
 
 	/* Set debug_query_string for individual workers first */
-	sharedquery = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, false);
+	sharedquery = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, true);
 	debug_query_string = sharedquery;
 
 	/* Report the query string from leader */

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 filename="worker_spi-debug_query_string-v1.patch"

Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Set debug_query_string in worker_spi.
    
    This makes elog.c emit the string, which is good practice for a
    background worker that executes SQL strings.
    
    Reviewed by FIXME.
    
    Discussion: https://postgr.es/m/20201014022636.GA1962668@rfd.leadboat.com

diff --git a/src/test/modules/worker_spi/worker_spi.c b/src/test/modules/worker_spi/worker_spi.c
index 1c7b17c..daf6c1d 100644
--- a/src/test/modules/worker_spi/worker_spi.c
+++ b/src/test/modules/worker_spi/worker_spi.c
@@ -113,16 +113,17 @@ initialize_worker_spi(worktable *table)
 	SPI_connect();
 	PushActiveSnapshot(GetTransactionSnapshot());
 	pgstat_report_activity(STATE_RUNNING, "initializing worker_spi schema");
 
 	/* XXX could we use CREATE SCHEMA IF NOT EXISTS? */
 	initStringInfo(&buf);
 	appendStringInfo(&buf, "select count(*) from pg_namespace where nspname = '%s'",
 					 table->schema);
+	debug_query_string = buf.data;
 
 	ret = SPI_execute(buf.data, true, 0);
 	if (ret != SPI_OK_SELECT)
 		elog(FATAL, "SPI_execute failed: error code %d", ret);
 
 	if (SPI_processed != 1)
 		elog(FATAL, "not a singleton result");
 
@@ -138,30 +139,32 @@ initialize_worker_spi(worktable *table)
 		appendStringInfo(&buf,
 						 "CREATE SCHEMA \"%s\" "
 						 "CREATE TABLE \"%s\" ("
 						 "		type text CHECK (type IN ('total', 'delta')), "
 						 "		value	integer)"
 						 "CREATE UNIQUE INDEX \"%s_unique_total\" ON \"%s\" (type) "
 						 "WHERE type = 'total'",
 						 table->schema, table->name, table->name, table->name);
+		debug_query_string = buf.data;
 
 		/* set statement start time */
 		SetCurrentStatementStartTimestamp();
 
 		ret = SPI_execute(buf.data, false, 0);
 
 		if (ret != SPI_OK_UTILITY)
 			elog(FATAL, "failed to create my schema");
 	}
 
 	SPI_finish();
 	PopActiveSnapshot();
 	CommitTransactionCommand();
 	pgstat_report_activity(STATE_IDLE, NULL);
+	debug_query_string = NULL;
 }
 
 void
 worker_spi_main(Datum main_arg)
 {
 	int			index = DatumGetInt32(main_arg);
 	worktable  *table;
 	StringInfoData buf;
@@ -206,16 +209,17 @@ worker_spi_main(Datum main_arg)
 					 "UPDATE %s.%s "
 					 "SET value = %s.value + total.sum "
 					 "FROM total WHERE type = 'total' "
 					 "RETURNING %s.value",
 					 table->schema, table->name,
 					 table->schema, table->name,
 					 table->name,
 					 table->name);
+	debug_query_string = buf.data;
 
 	/*
 	 * Main loop: do this until the SIGTERM handler tells us to terminate
 	 */
 	while (!got_sigterm)
 	{
 		int			ret;
 

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