Move NEON_AUTH_TOKEN to a builtin GUC

This environment variable is used as the password to connect to another
postgres instance as the walreceiver. The purpose of moving to a GUC is
so that we can reload the storage auth token periodically.

Signed-off-by: Tristan Partin <tristan.partin@databricks.com>

Author: tristan957

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -131,7 +131,6 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 /* BEGIN_NEON */
 	const char *keys[7];
 	const char *vals[7];
-	char * neon_auth_token = NULL;
 /* END_NEON */
 	int			i = 0;
 
@@ -145,16 +144,14 @@ libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
 /* BEGIN_NEON */
 	if (pg_strcasecmp(appname, "walreceiver") == 0)
 	{
-		neon_auth_token = getenv("NEON_AUTH_TOKEN");
-		if (neon_auth_token != NULL)
+		if (neon_storage_token[0] != '\0')
 		{
-			elog(LOG, "Use NEON_AUTH_TOKEN to connect");
 			keys[++i] = "password";
-			vals[i] = neon_auth_token;
+			vals[i] = neon_storage_token;
 		}
 		else
 		{
-			elog(LOG, "NEON_AUTH_TOKEN is undefined in the environment");
+			elog(LOG, "no storage authentication token set");
 		}
 	}
 /* END_NEON */

src/backend/replication/walreceiver.c

@@ -90,6 +90,7 @@
 int			wal_receiver_status_interval;
 int			wal_receiver_timeout;
 bool		hot_standby_feedback;
+char	   *neon_storage_token;
 
 /* libpqwalreceiver connection */
 static WalReceiverConn *wrconn = NULL;

src/backend/utils/misc/guc.c

@@ -4801,6 +4801,17 @@ static struct config_string ConfigureNamesString[] =
 		check_restrict_nonsystem_relation_kind, assign_restrict_nonsystem_relation_kind, NULL
 	},
 
+	{
+		{"neon_storage_token", PGC_POSTMASTER, REPLICATION_STANDBY,
+			"Authentication token for Neon storage",
+			NULL,
+			GUC_SUPERUSER_ONLY
+		},
+		&neon_storage_token,
+		"",
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, NULL, NULL, NULL, NULL

src/include/replication/walreceiver.h

@@ -28,6 +28,7 @@
 extern PGDLLIMPORT int wal_receiver_status_interval;
 extern PGDLLIMPORT int wal_receiver_timeout;
 extern PGDLLIMPORT bool hot_standby_feedback;
+extern PGDLLIMPORT char *neon_storage_token;
 
 /*
  * MAXCONNINFO: maximum size of a connection string.