Fix initdb's -c option to treat the GUC name case-insensitively.

The backend treats GUC names case-insensitively, so this code should
too.  This avoids ending up with a confusing set of redundant entries
in the generated postgresql.conf file.

Per report from Kyotaro Horiguchi.  Back-patch to v16 where this
feature was added (in commit 3e51b27).

Discussion: https://postgr.es/m/20230928.164904.2153358973162534034.horikyota.ntt@gmail.com

Author: tglsfdc

src/bin/initdb/initdb.c

@@ -484,6 +484,7 @@ replace_guc_value(char **lines, const char *guc_name, const char *guc_value,
 	for (i = 0; lines[i]; i++)
 	{
 		const char *where;
+		const char *namestart;
 
 		/*
 		 * Look for a line assigning to guc_name.  Typically it will be
@@ -494,15 +495,19 @@ replace_guc_value(char **lines, const char *guc_name, const char *guc_value,
 		where = lines[i];
 		while (*where == '#' || isspace((unsigned char) *where))
 			where++;
-		if (strncmp(where, guc_name, namelen) != 0)
+		if (pg_strncasecmp(where, guc_name, namelen) != 0)
 			continue;
+		namestart = where;
 		where += namelen;
 		while (isspace((unsigned char) *where))
 			where++;
 		if (*where != '=')
 			continue;
 
-		/* found it -- append the original comment if any */
+		/* found it -- let's use the canonical casing shown in the file */
+		memcpy(&newline->data[mark_as_comment ? 1 : 0], namestart, namelen);
+
+		/* now append the original comment if any */
 		where = strrchr(where, '#');
 		if (where)
 		{

src/bin/initdb/t/001_initdb.pl

@@ -199,4 +199,18 @@
 command_fails([ 'initdb', '--no-sync', '--set', 'foo=bar', "$tempdir/dataX" ],
 	'fails for invalid --set option');
 
+# Make sure multiple invocations of -c parameters are added case insensitive
+command_ok(
+	[
+		'initdb', '-cwork_mem=128',
+		'-cWork_Mem=256', '-cWORK_MEM=512',
+		"$tempdir/dataY"
+	],
+	'multiple -c options with different case');
+
+my $conf = slurp_file("$tempdir/dataY/postgresql.conf");
+ok($conf !~ qr/^WORK_MEM = /m, "WORK_MEM should not be configured");
+ok($conf !~ qr/^Work_Mem = /m, "Work_Mem should not be configured");
+ok($conf =~ qr/^work_mem = 512/m, "work_mem should be in config");
+
 done_testing();