Refactor error messages for unsupported providers in pg_locale.c

These code paths should not be reached normally, but if they are an
error with "(null)" as information for the collation provider would show
up if no locale is set, while we can assume that we are referring to
libc.

This refactors the code so as the provider is always reported even if no
locale is set.  The name of the function where the error happens is
added, while on it, as it can be helpful for debugging.

Issue introduced by d87d548cd0, so backpatch down to 16.

Author: Michael Paquier, Ranier Vilela
Reviewed-by: Jeff Davis, Kyotaro Horiguchi
Discussion: https://postgr.es/m/7073610042fcf97e1bea2ce08b7e0214b5e11094.camel@j-davis.com
Backpatch-through: 16

src/backend/utils/adt/pg_locale.c

@@ -81,6 +81,10 @@
 #include <shlwapi.h>
 #endif
 
+/* Error triggered for locale-sensitive subroutines */
+#define		PGLOCALE_SUPPORT_ERROR(provider) \
+	elog(ERROR, "unsupported collprovider for %s: %c", __func__, provider)
+
 /*
  * This should be large enough that most strings will fit, but small enough
  * that we feel comfortable putting it on the stack
@@ -2031,7 +2035,7 @@ pg_strcoll(const char *arg1, const char *arg2, pg_locale_t locale)
 #endif
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }
@@ -2067,7 +2071,7 @@ pg_strncoll(const char *arg1, size_t len1, const char *arg2, size_t len2,
 #endif
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }
@@ -2086,7 +2090,7 @@ pg_strxfrm_libc(char *dest, const char *src, size_t destsize,
 		return strxfrm(dest, src, destsize);
 #else
 	/* shouldn't happen */
-	elog(ERROR, "unsupported collprovider: %c", locale->provider);
+	PGLOCALE_SUPPORT_ERROR(locale->provider);
 	return 0;					/* keep compiler quiet */
 #endif
 }
@@ -2282,7 +2286,7 @@ pg_strxfrm_enabled(pg_locale_t locale)
 		return true;
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return false;				/* keep compiler quiet */
 }
@@ -2314,7 +2318,7 @@ pg_strxfrm(char *dest, const char *src, size_t destsize, pg_locale_t locale)
 #endif
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }
@@ -2351,7 +2355,7 @@ pg_strnxfrm(char *dest, size_t destsize, const char *src, size_t srclen,
 #endif
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }
@@ -2369,7 +2373,7 @@ pg_strxfrm_prefix_enabled(pg_locale_t locale)
 		return true;
 	else
 		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return false;				/* keep compiler quiet */
 }
@@ -2393,16 +2397,14 @@ pg_strxfrm_prefix(char *dest, const char *src, size_t destsize,
 {
 	size_t		result = 0;		/* keep compiler quiet */
 
-	if (!locale || locale->provider == COLLPROVIDER_LIBC)
-		elog(ERROR, "collprovider '%c' does not support pg_strxfrm_prefix()",
-			 locale->provider);
+	if (!locale)
+		PGLOCALE_SUPPORT_ERROR(COLLPROVIDER_LIBC);
 #ifdef USE_ICU
 	else if (locale->provider == COLLPROVIDER_ICU)
 		result = pg_strnxfrm_prefix_icu(dest, src, -1, destsize, locale);
 #endif
 	else
-		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }
@@ -2430,16 +2432,14 @@ pg_strnxfrm_prefix(char *dest, size_t destsize, const char *src,
 {
 	size_t		result = 0;		/* keep compiler quiet */
 
-	if (!locale || locale->provider == COLLPROVIDER_LIBC)
-		elog(ERROR, "collprovider '%c' does not support pg_strnxfrm_prefix()",
-			 locale->provider);
+	if (!locale)
+		PGLOCALE_SUPPORT_ERROR(COLLPROVIDER_LIBC);
 #ifdef USE_ICU
 	else if (locale->provider == COLLPROVIDER_ICU)
 		result = pg_strnxfrm_prefix_icu(dest, src, -1, destsize, locale);
 #endif
 	else
-		/* shouldn't happen */
-		elog(ERROR, "unsupported collprovider: %c", locale->provider);
+		PGLOCALE_SUPPORT_ERROR(locale->provider);
 
 	return result;
 }