Fix some cases of indirectly casting away const.

Newest versions of gcc+glibc are able to detect cases where code
implicitly casts away const by assigning the result of strchr() or
a similar function applied to a "const char *" value to a target
variable that's just "char *".  This of course creates a hazard of
not getting a compiler warning about scribbling on a string one was
not supposed to, so fixing up such cases is good.

This patch fixes a dozen or so places where we were doing that.
Most are trivial additions of "const" to the target variable,
since no actually-hazardous change was occurring.

Thanks to Bertrand Drouvot for finding a couple more spots than
I had.

This commit back-patches relevant portions of 8f1791c61 and
9f7565c6c into supported branches.  However, there are two
places in ecpg (in v18 only) where a proper fix is more
complicated than seems appropriate for a back-patch.  I opted
to silence those two warnings by adding casts.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/1324889.1764886170@sss.pgh.pa.us
Discussion: https://postgr.es/m/3988414.1771950285@sss.pgh.pa.us
Backpatch-through: 14-18

src/backend/catalog/pg_type.c

@@ -927,7 +927,7 @@ char *
 makeMultirangeTypeName(const char *rangeTypeName, Oid typeNamespace)
 {
 	char	   *buf;
-	char	   *rangestr;
+	const char *rangestr;
 
 	/*
 	 * If the range type name contains "range" then change that to

src/backend/tsearch/spell.c

@@ -2308,7 +2308,7 @@ CheckCompoundAffixes(CMPDAffix **ptr, char *word, int len, bool CheckInPlace)
 	}
 	else
 	{
-		char	   *affbegin;
+		const char *affbegin;
 
 		while ((*ptr)->affix)
 		{

src/backend/utils/adt/formatting.c

@@ -1001,8 +1001,9 @@ typedef struct NUMProc
 	char	   *number,			/* string with number	*/
 			   *number_p,		/* pointer to current number position */
 			   *inout,			/* in / out buffer	*/
-			   *inout_p,		/* pointer to current inout position */
-			   *last_relevant,	/* last relevant number after decimal point */
+			   *inout_p;		/* pointer to current inout position */
+
+	const char *last_relevant,	/* last relevant number after decimal point */
 
 			   *L_negative_sign,	/* Locale */
 			   *L_positive_sign,

src/backend/utils/adt/pg_locale.c

@@ -1093,7 +1093,7 @@ get_iso_localename(const char *winlocname)
 	wchar_t		wc_locale_name[LOCALE_NAME_MAX_LENGTH];
 	wchar_t		buffer[LOCALE_NAME_MAX_LENGTH];
 	static char iso_lc_messages[LOCALE_NAME_MAX_LENGTH];
-	char	   *period;
+	const char *period;
 	int			len;
 	int			ret_val;
 

src/backend/utils/adt/xid8funcs.c

@@ -232,7 +232,7 @@ is_visible_fxid(FullTransactionId value, const pg_snapshot *snap)
 #ifdef USE_BSEARCH_IF_NXIP_GREATER
 	else if (snap->nxip > USE_BSEARCH_IF_NXIP_GREATER)
 	{
-		void	   *res;
+		const void *res;
 
 		res = bsearch(&value, snap->xip, snap->nxip, sizeof(FullTransactionId),
 					  cmp_fxid);

src/bin/pg_waldump/pg_waldump.c

@@ -160,7 +160,7 @@ create_fullpage_directory(char *path)
 static void
 split_path(const char *path, char **dir, char **fname)
 {
-	char	   *sep;
+	const char *sep;
 
 	/* split filepath into directory & filename */
 	sep = strrchr(path, '/');

src/bin/pgbench/pgbench.c

@@ -6149,7 +6149,7 @@ findBuiltin(const char *name)
 static int
 parseScriptWeight(const char *option, char **script)
 {
-	char	   *sep;
+	const char *sep;
 	int			weight;
 
 	if ((sep = strrchr(option, WSEP)))

src/common/compression.c

@@ -425,7 +425,7 @@ validate_compress_specification(pg_compress_specification *spec)
 void
 parse_compress_options(const char *option, char **algorithm, char **detail)
 {
-	char	   *sep;
+	const char *sep;
 	char	   *endp;
 	long		result;
 

src/interfaces/ecpg/pgtypeslib/datetime.c

@@ -335,8 +335,8 @@ PGTYPESdate_defmt_asc(date * d, const char *fmt, const char *str)
 	 */
 	int			token[3][2];
 	int			token_values[3] = {-1, -1, -1};
-	char	   *fmt_token_order;
-	char	   *fmt_ystart,
+	const char *fmt_token_order;
+	const char *fmt_ystart,
 			   *fmt_mstart,
 			   *fmt_dstart;
 	unsigned int i;

src/port/getopt.c

@@ -71,7 +71,7 @@ int
 getopt(int nargc, char *const *nargv, const char *ostr)
 {
 	static char *place = EMSG;	/* option letter processing */
-	char	   *oli;			/* option letter list index */
+	const char *oli;			/* option letter list index */
 
 	if (!*place)
 	{							/* update scanning pointer */

src/port/getopt_long.c

@@ -59,7 +59,7 @@ getopt_long(int argc, char *const argv[],
 			const struct option *longopts, int *longindex)
 {
 	static char *place = EMSG;	/* option letter processing */
-	char	   *oli;			/* option letter list index */
+	const char *oli;			/* option letter list index */
 
 	if (!*place)
 	{							/* update scanning pointer */

src/port/win32setlocale.c

@@ -119,9 +119,9 @@ map_locale(const struct locale_map *map, const char *locale)
 		const char *needle_start = map[i].locale_name_start;
 		const char *needle_end = map[i].locale_name_end;
 		const char *replacement = map[i].replacement;
-		char	   *match;
-		char	   *match_start = NULL;
-		char	   *match_end = NULL;
+		const char *match;
+		const char *match_start = NULL;
+		const char *match_end = NULL;
 
 		match = strstr(locale, needle_start);
 		if (match)
@@ -148,7 +148,7 @@ map_locale(const struct locale_map *map, const char *locale)
 			/* Found a match. Replace the matched string. */
 			int			matchpos = match_start - locale;
 			int			replacementlen = strlen(replacement);
-			char	   *rest = match_end;
+			const char *rest = match_end;
 			int			restlen = strlen(rest);
 
 			/* check that the result fits in the static buffer */

src/test/regress/pg_regress.c

@@ -672,7 +672,7 @@ static
 const char *
 get_expectfile(const char *testname, const char *file)
 {
-	char	   *file_type;
+	const char *file_type;
 	_resultmap *rm;
 
 	/*

src/timezone/zic.c

@@ -2631,7 +2631,7 @@ doabbr(char *abbr, struct zone const *zp, char const *letters,
 	   bool isdst, zic_t save, bool doquotes)
 {
 	char	   *cp;
-	char	   *slashp;
+	char const *slashp;
 	size_t		len;
 	char const *format = zp->z_format;