Make various variables const (read-only).

These changes should generally improve correctness/maintainability.
A nice side benefit is that several kilobytes move from initialized
data to text segment, allowing them to be shared across processes and
probably reducing copy-on-write overhead while forking a new backend.
Unfortunately this doesn't seem to help libpq in the same way (at least
not when it's compiled with -fpic on x86_64), but we can hope the linker
at least collects all nominally-const data together even if it's not
actually part of the text segment.

Also, make pg_encname_tbl[] static in encnames.c, since there seems
no very good reason for any other code to use it; per a suggestion
from Wim Lewis, who independently submitted a patch that was mostly
a subset of this one.

Oskari Saarenmaa, with some editorialization by me

src/backend/utils/mb/encnames.c

@@ -29,7 +29,13 @@
  * Karel Zak, Aug 2001
  * ----------
  */
-pg_encname	pg_encname_tbl[] =
+typedef struct pg_encname
+{
+	const char *name;
+	pg_enc		encoding;
+} pg_encname;
+
+static const pg_encname pg_encname_tbl[] =
 {
 	{
 		"abc", PG_WIN1258
@@ -285,15 +291,9 @@ pg_encname	pg_encname_tbl[] =
 	},							/* alias for UHC */
 	{
 		"windows950", PG_BIG5
-	},							/* alias for BIG5 */
-	{
-		NULL, 0
-	}							/* last */
+	}							/* alias for BIG5 */
 };
 
-unsigned int pg_encname_tbl_sz = \
-sizeof(pg_encname_tbl) / sizeof(pg_encname_tbl[0]) - 1;
-
 /* ----------
  * These are "official" encoding names.
  * XXX must be sorted by the same order as enum pg_enc (in mb/pg_wchar.h)
@@ -304,7 +304,7 @@ sizeof(pg_encname_tbl) / sizeof(pg_encname_tbl[0]) - 1;
 #else
 #define DEF_ENC2NAME(name, codepage) { #name, PG_##name, codepage }
 #endif
-pg_enc2name pg_enc2name_tbl[] =
+const pg_enc2name pg_enc2name_tbl[] =
 {
 	DEF_ENC2NAME(SQL_ASCII, 0),
 	DEF_ENC2NAME(EUC_JP, 20932),
@@ -356,7 +356,7 @@ pg_enc2name pg_enc2name_tbl[] =
  * This covers all encodings except MULE_INTERNAL, which is alien to gettext.
  * ----------
  */
-pg_enc2gettext pg_enc2gettext_tbl[] =
+const pg_enc2gettext pg_enc2gettext_tbl[] =
 {
 	{PG_SQL_ASCII, "US-ASCII"},
 	{PG_UTF8, "UTF-8"},
@@ -467,13 +467,15 @@ clean_encoding_name(const char *key, char *newkey)
 
 /* ----------
  * Search encoding by encoding name
+ *
+ * Returns encoding ID, or -1 for error
  * ----------
  */
-pg_encname *
-pg_char_to_encname_struct(const char *name)
+int
+pg_char_to_encoding(const char *name)
 {
-	unsigned int nel = pg_encname_tbl_sz;
-	pg_encname *base = pg_encname_tbl,
+	unsigned int nel = lengthof(pg_encname_tbl);
+	const pg_encname *base = pg_encname_tbl,
 			   *last = base + nel - 1,
 			   *position;
 	int			result;
@@ -481,13 +483,13 @@ pg_char_to_encname_struct(const char *name)
 			   *key;
 
 	if (name == NULL || *name == '\0')
-		return NULL;
+		return -1;
 
 	if (strlen(name) >= NAMEDATALEN)
 	{
 #ifdef FRONTEND
 		fprintf(stderr, "encoding name too long\n");
-		return NULL;
+		return -1;
 #else
 		ereport(ERROR,
 				(errcode(ERRCODE_NAME_TOO_LONG),
@@ -505,29 +507,14 @@ pg_char_to_encname_struct(const char *name)
 		{
 			result = strcmp(key, position->name);
 			if (result == 0)
-				return position;
+				return position->encoding;
 		}
 		if (result < 0)
 			last = position - 1;
 		else
 			base = position + 1;
 	}
-	return NULL;
-}
-
-/*
- * Returns encoding or -1 for error
- */
-int
-pg_char_to_encoding(const char *name)
-{
-	pg_encname *p;
-
-	if (!name)
-		return -1;
-
-	p = pg_char_to_encname_struct(name);
-	return p ? p->encoding : -1;
+	return -1;
 }
 
 #ifndef FRONTEND
@@ -545,7 +532,7 @@ pg_encoding_to_char(int encoding)
 {
 	if (PG_VALID_ENCODING(encoding))
 	{
-		pg_enc2name *p = &pg_enc2name_tbl[encoding];
+		const pg_enc2name *p = &pg_enc2name_tbl[encoding];
 
 		Assert(encoding == p->encoding);
 		return p->name;

src/backend/utils/mb/mbutils.c

@@ -55,9 +55,9 @@ static FmgrInfo *ToClientConvProc = NULL;
 /*
  * These variables track the currently-selected encodings.
  */
-static pg_enc2name *ClientEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
-static pg_enc2name *DatabaseEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
-static pg_enc2name *MessageEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
+static const pg_enc2name *ClientEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
+static const pg_enc2name *DatabaseEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
+static const pg_enc2name *MessageEncoding = &pg_enc2name_tbl[PG_SQL_ASCII];
 
 /*
  * During backend startup we can't set client encoding because we (a)

src/backend/utils/mb/wchar.c

@@ -1720,7 +1720,7 @@ pg_eucjp_increment(unsigned char *charptr, int length)
  * XXX must be sorted by the same order as enum pg_enc (in mb/pg_wchar.h)
  *-------------------------------------------------------------------
  */
-pg_wchar_tbl pg_wchar_table[] = {
+const pg_wchar_tbl pg_wchar_table[] = {
 	{pg_ascii2wchar_with_len, pg_wchar2single_with_len, pg_ascii_mblen, pg_ascii_dsplen, pg_ascii_verifier, 1}, /* PG_SQL_ASCII */
 	{pg_eucjp2wchar_with_len, pg_wchar2euc_with_len, pg_eucjp_mblen, pg_eucjp_dsplen, pg_eucjp_verifier, 3},	/* PG_EUC_JP */
 	{pg_euccn2wchar_with_len, pg_wchar2euc_with_len, pg_euccn_mblen, pg_euccn_dsplen, pg_euccn_verifier, 2},	/* PG_EUC_CN */