Correct constness of a few variables.

This allows the compiler / linker to mark affected pages as read-only.

There's other cases, but they're a bit more invasive, and should go
through some review. These are easy.

They were found with
objdump -j .data -t src/backend/postgres|awk '{print $4, $5, $6}'|sort -r|less

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

src/backend/commands/event_trigger.c

@@ -85,7 +85,7 @@ typedef enum
 } event_trigger_command_tag_check_result;
 
 /* XXX merge this with ObjectTypeMap? */
-static event_trigger_support_data event_trigger_support[] = {
+static const event_trigger_support_data event_trigger_support[] = {
 	{"ACCESS METHOD", true},
 	{"AGGREGATE", true},
 	{"CAST", true},
@@ -282,7 +282,7 @@ static event_trigger_command_tag_check_result
 check_ddl_tag(const char *tag)
 {
 	const char *obtypename;
-	event_trigger_support_data *etsd;
+	const event_trigger_support_data *etsd;
 
 	/*
 	 * Handle some idiosyncratic special cases.

src/backend/foreign/foreign.c

@@ -560,7 +560,7 @@ struct ConnectionOption
  *
  * The list is small - don't bother with bsearch if it stays so.
  */
-static struct ConnectionOption libpq_conninfo_options[] = {
+static const struct ConnectionOption libpq_conninfo_options[] = {
 	{"authtype", ForeignServerRelationId},
 	{"service", ForeignServerRelationId},
 	{"user", UserMappingRelationId},
@@ -587,7 +587,7 @@ static struct ConnectionOption libpq_conninfo_options[] = {
 static bool
 is_conninfo_option(const char *option, Oid context)
 {
-	struct ConnectionOption *opt;
+	const struct ConnectionOption *opt;
 
 	for (opt = libpq_conninfo_options; opt->optname; opt++)
 		if (context == opt->optcontext && strcmp(opt->optname, option) == 0)
@@ -622,7 +622,7 @@ postgresql_fdw_validator(PG_FUNCTION_ARGS)
 
 		if (!is_conninfo_option(def->defname, catalog))
 		{
-			struct ConnectionOption *opt;
+			const struct ConnectionOption *opt;
 			StringInfoData buf;
 
 			/*

src/backend/libpq/pqcomm.c

@@ -170,7 +170,7 @@ static int	Lock_AF_UNIX(char *unixSocketDir, char *unixSocketPath);
 static int	Setup_AF_UNIX(char *sock_path);
 #endif							/* HAVE_UNIX_SOCKETS */
 
-static PQcommMethods PqCommSocketMethods = {
+static const PQcommMethods PqCommSocketMethods = {
 	socket_comm_reset,
 	socket_flush,
 	socket_flush_if_writable,
@@ -181,7 +181,7 @@ static PQcommMethods PqCommSocketMethods = {
 	socket_endcopyout
 };
 
-PQcommMethods *PqCommMethods = &PqCommSocketMethods;
+const PQcommMethods *PqCommMethods = &PqCommSocketMethods;
 
 WaitEventSet *FeBeWaitSet;
 

src/backend/libpq/pqmq.c

@@ -36,7 +36,7 @@ static void mq_putmessage_noblock(char msgtype, const char *s, size_t len);
 static void mq_startcopyout(void);
 static void mq_endcopyout(bool errorAbort);
 
-static PQcommMethods PqCommMqMethods = {
+static const PQcommMethods PqCommMqMethods = {
 	mq_comm_reset,
 	mq_flush,
 	mq_flush_if_writable,

src/backend/replication/basebackup.c

@@ -190,7 +190,7 @@ static const char *excludeFiles[] =
 /*
  * List of files excluded from checksum validation.
  */
-static const char *noChecksumFiles[] = {
+static const char *const noChecksumFiles[] = {
 	"pg_control",
 	"pg_filenode.map",
 	"pg_internal.init",
@@ -1321,7 +1321,7 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 static bool
 is_checksummed_file(const char *fullpath, const char *filename)
 {
-	const char **f;
+	const char *const *f;
 
 	/* Check that the file is in a tablespace */
 	if (strncmp(fullpath, "./global/", 9) == 0 ||

src/backend/storage/lmgr/generate-lwlocknames.pl

@@ -23,7 +23,7 @@ print $h $autogen;
 print $h "/* there is deliberately not an #ifndef LWLOCKNAMES_H here */\n\n";
 print $c $autogen, "\n";
 
-print $c "char *MainLWLockNames[] = {";
+print $c "const char *const MainLWLockNames[] = {";
 
 while (<$lwlocknames>)
 {

src/backend/utils/adt/cash.c

@@ -39,13 +39,13 @@ static const char *
 num_word(Cash value)
 {
 	static char buf[128];
-	static const char *small[] = {
+	static const char *const small[] = {
 		"zero", "one", "two", "three", "four", "five", "six", "seven",
 		"eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen",
 		"fifteen", "sixteen", "seventeen", "eighteen", "nineteen", "twenty",
 		"thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"
 	};
-	const char **big = small + 18;
+	const char *const *big = small + 18;
 	int			tu = value % 100;
 
 	/* deal with the simple cases first */

src/include/libpq/libpq.h

@@ -33,7 +33,7 @@ typedef struct
 	void		(*endcopyout) (bool errorAbort);
 } PQcommMethods;
 
-extern PGDLLIMPORT PQcommMethods *PqCommMethods;
+extern const PGDLLIMPORT PQcommMethods *PqCommMethods;
 
 #define pq_comm_reset() (PqCommMethods->comm_reset())
 #define pq_flush() (PqCommMethods->flush())

src/include/storage/lwlock.h

@@ -88,7 +88,7 @@ typedef union LWLockMinimallyPadded
 } LWLockMinimallyPadded;
 
 extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
-extern char *MainLWLockNames[];
+extern const char *const MainLWLockNames[];
 
 /* struct for storing named tranche information */
 typedef struct NamedLWLockTranche