Add pg_get_snapshot_timestamp function

Author: knizhnik

src/backend/storage/file/fd.c

@@ -94,6 +94,7 @@
 #include "utils/guc.h"
 #include "utils/inval.h"
 #include "utils/resowner_private.h"
+#include "utils/timestamp.h"
 
 
 /* Define PG_FLUSH_DATA_WORKS if we have an implementation for pg_flush_data */
@@ -3965,6 +3966,40 @@ sfs_get_snapshot_size(SnapshotId snap_id)
 	return sfs_snapshot_size;
 }
 
+static time_t sfs_snapshot_time;
+
+static void
+sfs_snapshot_file_time(const char *fname, bool isdir, int elevel)
+{
+	if (!isdir)
+	{
+		char* suf = strstr(fname, ".snap.");
+		if (suf != NULL)
+		{
+			int snap_id;
+			if (sscanf(suf + 6, "%d", &snap_id) == 1
+				&& snap_id == sfs_current_snapshot)
+			{
+				struct stat fst;
+				if (!stat(fname, &fst))
+					elog(elevel, "[SFS] Failed to stat snapshot file %s: %m", fname);
+				if (sfs_snapshot_time > fst.st_ctime) {
+					sfs_snapshot_time = fst.st_ctime;
+				}
+			}
+		}
+	}
+}
+
+TimestampTz
+sfs_get_snapshot_timestamp(SnapshotId snap_id)
+{
+	sfs_snapshot_time = -1;
+	sfs_current_snapshot = snap_id;
+	walk_data_dir(sfs_snapshot_file_time, LOG);
+	return time_t_to_timestamptz(sfs_snapshot_time);
+}
+
 void
 sfs_remove_snapshot(SnapshotId snap_id)
 {
@@ -4008,6 +4043,3 @@ sfs_recover_to_snapshot(SnapshotId snap_id)
 
 	sfs_unlock_database();
 }
-
-int64
-sfs_get_shapshot_size(SnapshotId sid);

src/backend/storage/file/snapfs.c

@@ -289,3 +289,10 @@ Datum pg_get_backend_snapshot(PG_FUNCTION_ARGS)
 {
 	PG_RETURN_INT32(sfs_backend_snapshot);
 }
+
+Datum pg_get_snapshot_timestamp(PG_FUNCTION_ARGS)
+{
+	SnapshotId snap_id = PG_GETARG_INT32(0);
+	PG_RETURN_TIMESTAMPTZ(sfs_get_snapshot_timestamp(snap_id));
+}
+

src/include/catalog/pg_proc.dat

@@ -10251,4 +10251,10 @@
   prorettype => 'int4', proargtypes => '',
   prosrc => 'pg_get_backend_snapshot' },
 
+{ oid => '6023',
+  descr => 'get snapshot timestamp',
+  proname => 'pg_get_snapshot_timestamp', provolatile => 'v',
+  proisstrict => 't', prorettype => 'timestamptz', proargtypes => 'int4',
+  prosrc => 'pg_get_snapshot_timestamp' },
+
 ]

src/include/storage/snapfs.h

@@ -51,6 +51,11 @@ extern void sfs_set_backend_snapshot(SnapshotId sid);
  */
 extern int64 sfs_get_snapshot_size(SnapshotId sid);
 
+/*
+ * Get snapshot timestamp (time of first database modification in this snapshot)
+ */
+extern TimestampTz sfs_get_snapshot_timestamp(SnapshotId sid);
+
 /*
  * Iternal functions and variables
  */