Avoid assuming there will be only 3 states for synchronous_commit.

Also avoid hardcoding the current default state by giving it the name
"on" and replace with a meaningful name that reflects its behaviour.
Coding only, no change in behaviour.

Author: simonat2ndQuadrant

src/backend/access/transam/xact.c

@@ -1056,7 +1056,7 @@ RecordTransactionCommit(void)
 	 * if all to-be-deleted tables are temporary though, since they are lost
 	 * anyway if we crash.)
 	 */
-	if ((wrote_xlog && synchronous_commit >= SYNCHRONOUS_COMMIT_LOCAL) ||
+	if ((wrote_xlog && synchronous_commit > SYNCHRONOUS_COMMIT_OFF) ||
 		forceSyncCommit || nrels > 0)
 	{
 		/*

src/backend/postmaster/autovacuum.c

@@ -1531,7 +1531,7 @@ AutoVacWorkerMain(int argc, char *argv[])
 	 * if we are waiting for standbys to connect. This is important to
 	 * ensure we aren't blocked from performing anti-wraparound tasks.
 	 */
-	if (synchronous_commit == SYNCHRONOUS_COMMIT_ON)
+	if (synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)
 		SetConfigOption("synchronous_commit", "local", PGC_SUSET, PGC_S_OVERRIDE);
 
 	/*

src/backend/utils/misc/guc.c

@@ -355,7 +355,7 @@ static const struct config_enum_entry constraint_exclusion_options[] = {
  * accept all the likely variants of "on" and "off".
  */
 static const struct config_enum_entry synchronous_commit_options[] = {
-	{"local", SYNCHRONOUS_COMMIT_LOCAL, false},
+	{"local", SYNCHRONOUS_COMMIT_LOCAL_FLUSH, false},
 	{"on", SYNCHRONOUS_COMMIT_ON, false},
 	{"off", SYNCHRONOUS_COMMIT_OFF, false},
 	{"true", SYNCHRONOUS_COMMIT_ON, true},

src/include/access/xact.h

@@ -54,11 +54,14 @@ extern bool XactDeferrable;
 
 typedef enum
 {
-	SYNCHRONOUS_COMMIT_OFF,		/* asynchronous commit */
-	SYNCHRONOUS_COMMIT_LOCAL,	/* wait for only local flush */
-	SYNCHRONOUS_COMMIT_ON		/* wait for local flush and sync rep */
+	SYNCHRONOUS_COMMIT_OFF,			/* asynchronous commit */
+	SYNCHRONOUS_COMMIT_LOCAL_FLUSH,	/* wait for local flush only */
+	SYNCHRONOUS_COMMIT_REMOTE_FLUSH	/* wait for local and remote flush */
 } SyncCommitLevel;
 
+/* Define the default setting for synchonous_commit */
+#define SYNCHRONOUS_COMMIT_ON 	SYNCHRONOUS_COMMIT_REMOTE_FLUSH
+
 /* Synchronous commit level */
 extern int	synchronous_commit;
 

src/include/replication/syncrep.h

@@ -20,7 +20,7 @@
 #include "utils/guc.h"
 
 #define SyncRepRequested() \
-	(max_wal_senders > 0 && synchronous_commit == SYNCHRONOUS_COMMIT_ON)
+	(max_wal_senders > 0 && synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)
 
 /* syncRepState */
 #define SYNC_REP_NOT_WAITING		0