pgstat: Use correct lock level in pgstat_drop_all_entries().

Previously we didn't, which lead to an assertion failure when resetting
partially loaded statistics. This was encountered on the buildfarm, for
as-of-yet unknown reasons.

Ttighten up a validity check when reading the stats file, verifying 'E'
signals the end of the file (rather than just stopping reading). That's then
used in a test appending to the stats file that crashed before the fix in
pgstat_drop_all_entries().

Reported by buildfarm animals mylodon and kestrel, via Tom Lane.

Discussion: https://postgr.es/m/1656446.1650043715@sss.pgh.pa.us

src/backend/utils/activity/pgstat.c

@@ -1583,6 +1583,10 @@ pgstat_read_statsfile(void)
 					break;
 				}
 			case 'E':
+				/* check that 'E' actually signals end of file */
+				if (fgetc(fpin) != EOF)
+					goto error;
+
 				goto done;
 
 			default:

src/backend/utils/activity/pgstat_shmem.c

@@ -878,7 +878,7 @@ pgstat_drop_all_entries(void)
 	PgStatShared_HashEntry *ps;
 	uint64		not_freed_count = 0;
 
-	dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
+	dshash_seq_init(&hstat, pgStatLocal.shared_hash, true);
 	while ((ps = dshash_seq_next(&hstat)) != NULL)
 	{
 		if (ps->dropped)

src/test/recovery/t/029_stats_restart.pl

@@ -113,7 +113,23 @@ overwrite_file($og_stats, "ZZZZZZZZZZZZZ");
 $node->start;
 
 # no stats present due to invalid stats file
-$sect = "invalid";
+$sect = "invalid_overwrite";
+is(have_stats('database', $dboid, 0), 'f', "$sect: db stats do not exist");
+is(have_stats('function', $dboid, $funcoid),
+	'f', "$sect: function stats do not exist");
+is(have_stats('relation', $dboid, $tableoid),
+	'f', "$sect: relation stats do not exist");
+
+
+## check invalid stats file starting with valid contents, but followed by
+## invalid content is handled.
+
+trigger_funcrel_stat();
+$node->stop;
+append_file($og_stats, "XYZ");
+$node->start;
+
+$sect = "invalid_append";
 is(have_stats('database', $dboid, 0), 'f', "$sect: db stats do not exist");
 is(have_stats('function', $dboid, $funcoid),
 	'f', "$sect: function stats do not exist");
@@ -285,7 +301,17 @@ sub overwrite_file
 {
 	my ($filename, $str) = @_;
 	open my $fh, ">", $filename
-	  or die "could not write \"$filename\": $!";
+	  or die "could not overwrite \"$filename\": $!";
+	print $fh $str;
+	close $fh;
+	return;
+}
+
+sub append_file
+{
+	my ($filename, $str) = @_;
+	open my $fh, ">>", $filename
+	  or die "could not append to \"$filename\": $!";
 	print $fh $str;
 	close $fh;
 	return;