Add warning to pg_controldata on PG_CONTROL_VERSION mismatchHEADmaster

If you run pg_controldata on a cluster that has been initialized with
different PG_CONTROL_VERSION than what the pg_controldata program has
been compiled with, pg_controldata will still try to interpret the
control file, but the result is likely to be somewhat nonsensical. How
nonsensical it is depends on the differences between the versions. If
sizeof(ControlFileData) differs between the versions, the CRC will not
match and you get a warning of that, but otherwise you get no
warning.

Looking back at recent PG_CONTROL_VERSION updates, all changes that
would mess up the printed values have also changed
sizeof(ControlFileData), but there's no guarantee of that in future
versions.

Add an explicit check and warning for version number mismatch before
the CRC check. That way, you get a more clear warning if you use the
pg_controldata binary from wrong version, and if we change the control
file in the future in a way that doesn't change
sizeof(ControlFileData), this ensures that you get a warning in that
case too.

Discussion: https://www.postgresql.org/message-id/[email protected]

2 files changed, 18 insertions, 5 deletions

diff --git a/src/bin/pg_controldata/pg_controldata.c b/src/bin/pg_controldata/pg_controldata.c
index 5c77f40313b..30ad46912e1 100644
--- a/src/bin/pg_controldata/pg_controldata.c
+++ b/src/bin/pg_controldata/pg_controldata.c
@@ -167,7 +167,14 @@ main(int argc, char *argv[])
 
 	/* get a copy of the control file */
 	ControlFile = get_controlfile(DataDir, &crc_ok);
-	if (!crc_ok)
+	if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
+	{
+		pg_log_warning("control file version (%u) does not match the version understood by this program (%u)",
+					   ControlFile->pg_control_version, PG_CONTROL_VERSION);
+		pg_log_warning_detail("Either the control file has been created with a different version of PostgreSQL, "
+							  "or it is corrupt.  The results below are untrustworthy.");
+	}
+	else if (!crc_ok)
 	{
 		pg_log_warning("calculated CRC checksum does not match value stored in control file");
 		pg_log_warning_detail("Either the control file is corrupt, or it has a different layout than this program "
diff --git a/src/bin/pg_controldata/t/001_pg_controldata.pl b/src/bin/pg_controldata/t/001_pg_controldata.pl
index 4aea00d6d5a..6ace61f9314 100644
--- a/src/bin/pg_controldata/t/001_pg_controldata.pl
+++ b/src/bin/pg_controldata/t/001_pg_controldata.pl
@@ -21,16 +21,22 @@ command_like([ 'pg_controldata', $node->data_dir ],
 	qr/checkpoint/, 'pg_controldata produces output');
 
 
-# check with a corrupted pg_control
+# Check with a corrupted pg_control
+#
+# To corrupt it, overwrite most of it with zeros. We leave the
+# beginning portion that contains the pg_control version number (first
+# 16 bytes) unmodified because otherwise you get an error about the
+# version number, instead of checksum mismatch.
 
 my $pg_control = $node->data_dir . '/global/pg_control';
 my $size = -s $pg_control;
 
-open my $fh, '>', $pg_control or BAIL_OUT($!);
+open my $fh, '+<', $pg_control or BAIL_OUT($!);
 binmode $fh;
 
-# fill file with zeros
-print $fh pack("x[$size]");
+my ($overwrite_off, $overwrite_len) = (16, $size - 16);
+seek $fh, $overwrite_off, 0 or BAIL_OUT($!);
+print $fh pack("x[$overwrite_len]");
 close $fh;
 
 command_checks_all(