pg_controldata: Prevent division-by-zero errors

If the control file is corrupted and specifies the WAL segment size
to be 0 bytes, calculating the latest checkpoint's REDO WAL file
will fail with a division-by-zero error.  Show it as "???" instead.

Also reword the warning message a bit and send it to stdout, like the
other pre-existing warning messages.

Add some tests for dealing with a corrupted pg_control file.

Author: Nathan Bossart <bossartn@amazon.com>, tests by me

src/bin/pg_controldata/pg_controldata.c

@@ -95,7 +95,6 @@ main(int argc, char *argv[])
 	char		mock_auth_nonce_str[MOCK_AUTH_NONCE_LEN * 2 + 1];
 	const char *strftime_fmt = "%c";
 	const char *progname;
-	XLogSegNo	segno;
 	char		xlogfilename[MAXFNAMELEN];
 	int			c;
 	int			i;
@@ -169,10 +168,11 @@ main(int argc, char *argv[])
 	WalSegSz = ControlFile->xlog_seg_size;
 
 	if (!IsValidWalSegSize(WalSegSz))
-		fprintf(stderr,
-				_("WARNING: WAL segment size specified, %d bytes, is not a power of two between 1MB and 1GB.\n"
-				  "The file is corrupt and the results below are untrustworthy.\n"),
-				WalSegSz);
+		printf(_("WARNING: invalid WAL segment size\n"
+				 "The WAL segment size stored in the file, %d bytes, is not a power of two\n"
+				 "between 1 MB and 1 GB.  The file is corrupt and the results below are\n"
+				 "untrustworthy.\n\n"),
+			   WalSegSz);
 
 	/*
 	 * This slightly-chintzy coding will work as long as the control file
@@ -193,10 +193,20 @@ main(int argc, char *argv[])
 	/*
 	 * Calculate name of the WAL file containing the latest checkpoint's REDO
 	 * start point.
+	 *
+	 * A corrupted control file could report a WAL segment size of 0, and to
+	 * guard against division by zero, we need to treat that specially.
 	 */
-	XLByteToSeg(ControlFile->checkPointCopy.redo, segno, WalSegSz);
-	XLogFileName(xlogfilename, ControlFile->checkPointCopy.ThisTimeLineID,
-				 segno, WalSegSz);
+	if (WalSegSz != 0)
+	{
+		XLogSegNo	segno;
+
+		XLByteToSeg(ControlFile->checkPointCopy.redo, segno, WalSegSz);
+		XLogFileName(xlogfilename, ControlFile->checkPointCopy.ThisTimeLineID,
+					 segno, WalSegSz);
+	}
+	else
+		strcpy(xlogfilename, _("???"));
 
 	/*
 	 * Format system_identifier and mock_authentication_nonce separately to

src/bin/pg_controldata/t/001_pg_controldata.pl

@@ -2,7 +2,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 13;
+use Test::More tests => 17;
 
 program_help_ok('pg_controldata');
 program_version_ok('pg_controldata');
@@ -16,3 +16,22 @@ $node->init;
 
 command_like([ 'pg_controldata', $node->data_dir ],
 	qr/checkpoint/, 'pg_controldata produces output');
+
+
+# check with a corrupted pg_control
+
+my $pg_control = $node->data_dir . '/global/pg_control';
+my $size = (stat($pg_control))[7];
+
+open my $fh, '>', $pg_control or BAIL_OUT($!);
+binmode $fh;
+# fill file with zeros
+print $fh pack("x[$size]");
+close $fh;
+
+command_checks_all([ 'pg_controldata', $node->data_dir ],
+				   0,
+				   [ qr/WARNING: Calculated CRC checksum does not match value stored in file/,
+					 qr/WARNING: invalid WAL segment size/ ],
+				   [ qr/^$/ ],
+				   'pg_controldata with corrupted pg_control');