Add GUC enabling global snapshots.

Author: arssher

src/backend/access/transam/global_snapshot.c

@@ -84,6 +84,9 @@ typedef struct
 #define DTM_TRACE(x)
 /* #define DTM_TRACE(x) fprintf x */
 
+/* GUC enabling global snapshots */
+bool track_global_snapshots;
+
 // static shmem_startup_hook_type prev_shmem_startup_hook;
 static HTAB *xid2status;
 static HTAB *gtid2xid;
@@ -1019,8 +1022,12 @@ Datum
 pg_global_snaphot_create(PG_FUNCTION_ARGS)
 {
 	GlobalTransactionId gtid = text_to_cstring(PG_GETARG_TEXT_PP(0));
-	cid_t		cid = DtmLocalExtend(&dtm_tx, gtid);
+	cid_t		cid;
+
+	if (!track_global_snapshots)
+		elog(ERROR, "Global snapshots are disabled");
 
+	cid = DtmLocalExtend(&dtm_tx, gtid);
 	DTM_TRACE((stderr, "Backend %d extends transaction %u(%s) to global with cid=%lu\n", getpid(), dtm_tx.xid, gtid, cid));
 	PG_RETURN_INT64(cid);
 }
@@ -1031,6 +1038,9 @@ pg_global_snaphot_join(PG_FUNCTION_ARGS)
 	cid_t		cid = PG_GETARG_INT64(0);
 	GlobalTransactionId gtid = text_to_cstring(PG_GETARG_TEXT_PP(1));
 
+	if (!track_global_snapshots)
+		elog(ERROR, "Global snapshots are disabled");
+
 	DTM_TRACE((stderr, "Backend %d joins transaction %u(%s) with cid=%lu\n", getpid(), dtm_tx.xid, gtid, cid));
 	cid = DtmLocalAccess(&dtm_tx, gtid, cid);
 	PG_RETURN_INT64(cid);
@@ -1041,6 +1051,9 @@ pg_global_snaphot_begin_prepare(PG_FUNCTION_ARGS)
 {
 	GlobalTransactionId gtid = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	if (!track_global_snapshots)
+		elog(ERROR, "Global snapshots are disabled");
+
 	DtmLocalBeginPrepare(gtid);
 	DTM_TRACE((stderr, "Backend %d begins prepare of transaction %s\n", getpid(), gtid));
 	PG_RETURN_VOID();
@@ -1052,6 +1065,10 @@ pg_global_snaphot_prepare(PG_FUNCTION_ARGS)
 	GlobalTransactionId gtid = text_to_cstring(PG_GETARG_TEXT_PP(0));
 	cid_t		cid = PG_GETARG_INT64(1);
 
+	if (!track_global_snapshots)
+		elog(ERROR, "Global snapshots are disabled");
+
+
 	cid = DtmLocalPrepare(gtid, cid);
 	DTM_TRACE((stderr, "Backend %d prepares transaction %s with cid=%lu\n", getpid(), gtid, cid));
 	PG_RETURN_INT64(cid);
@@ -1063,6 +1080,9 @@ pg_global_snaphot_end_prepare(PG_FUNCTION_ARGS)
 	GlobalTransactionId gtid = text_to_cstring(PG_GETARG_TEXT_PP(0));
 	cid_t		cid = PG_GETARG_INT64(1);
 
+	if (!track_global_snapshots)
+		elog(ERROR, "Global snapshots are disabled");
+
 	DTM_TRACE((stderr, "Backend %d ends prepare of transactions %s with cid=%lu\n", getpid(), gtid, cid));
 	DtmLocalEndPrepare(gtid, cid);
 	PG_RETURN_VOID();

src/backend/storage/ipc/ipci.c

@@ -17,6 +17,7 @@
 #include "access/clog.h"
 #include "access/commit_ts.h"
 #include "access/heapam.h"
+#include "access/global_snapshot.h"
 #include "access/multixact.h"
 #include "access/nbtree.h"
 #include "access/subtrans.h"
@@ -272,7 +273,9 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
 	SyncScanShmemInit();
 	AsyncShmemInit();
 	BackendRandomShmemInit();
-	DtmInitialize();
+
+	if (track_global_snapshots)
+		DtmInitialize();
 
 #ifdef EXEC_BACKEND
 

src/backend/utils/misc/guc.c

@@ -28,6 +28,7 @@
 
 #include "access/commit_ts.h"
 #include "access/gin.h"
+#include "access/global_snapshot.h"
 #include "access/rmgr.h"
 #include "access/transam.h"
 #include "access/twophase.h"
@@ -1666,6 +1667,16 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"track_global_snapshots", PGC_POSTMASTER, CUSTOM_OPTIONS,
+		 gettext_noop("Track global snapshots, allowing to use global_snapshot API"),
+		 NULL
+		},
+		&track_global_snapshots,
+		false,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL

src/include/access/global_snapshot.h

@@ -6,6 +6,8 @@
 typedef int nodeid_t;
 typedef uint64 cid_t;
 
+extern bool track_global_snapshots;
+
 typedef struct
 {
 	TransactionId xid;