Introduce logical decoding.

This feature, building on previous commits, allows the write-ahead log
stream to be decoded into a series of logical changes; that is,
inserts, updates, and deletes and the transactions which contain them.
It is capable of handling decoding even across changes to the schema
of the effected tables.  The output format is controlled by a
so-called "output plugin"; an example is included.  To make use of
this in a real replication system, the output plugin will need to be
modified to produce output in the format appropriate to that system,
and to perform filtering.

Currently, information can be extracted from the logical decoding
system only via SQL; future commits will add the ability to stream
changes via walsender.

Andres Freund, with review and other contributions from many other
people, including Álvaro Herrera, Abhijit Menon-Sen, Peter Gheogegan,
Kevin Grittner, Robert Haas, Heikki Linnakangas, Fujii Masao, Abhijit
Menon-Sen, Michael Paquier, Simon Riggs, Craig Ringer, and Steve
Singer.

src/backend/utils/time/snapmgr.c

@@ -19,6 +19,10 @@
  * have regd_count = 1 and are counted in RegisteredSnapshots, but are not
  * tracked by any resource owner.
  *
+ * The same is true for historic snapshots used during logical decoding,
+ * their lifetime is managed separately (as they life longer as one xact.c
+ * transaction).
+ *
  * These arrangements let us reset MyPgXact->xmin when there are no snapshots
  * referenced by this transaction.	(One possible improvement would be to be
  * able to advance Xmin when the snapshot with the earliest Xmin is no longer
@@ -69,12 +73,13 @@
  */
 static SnapshotData CurrentSnapshotData = {HeapTupleSatisfiesMVCC};
 static SnapshotData SecondarySnapshotData = {HeapTupleSatisfiesMVCC};
-static SnapshotData CatalogSnapshotData = {HeapTupleSatisfiesMVCC};
+SnapshotData CatalogSnapshotData = {HeapTupleSatisfiesMVCC};
 
 /* Pointers to valid snapshots */
 static Snapshot CurrentSnapshot = NULL;
 static Snapshot SecondarySnapshot = NULL;
 static Snapshot CatalogSnapshot = NULL;
+static Snapshot HistoricSnapshot = NULL;
 
 /*
  * Staleness detection for CatalogSnapshot.
@@ -86,13 +91,18 @@ static bool CatalogSnapshotStale = true;
  * for the convenience of TransactionIdIsInProgress: even in bootstrap
  * mode, we don't want it to say that BootstrapTransactionId is in progress.
  *
- * RecentGlobalXmin is initialized to InvalidTransactionId, to ensure that no
- * one tries to use a stale value.	Readers should ensure that it has been set
- * to something else before using it.
+ * RecentGlobalXmin and RecentGlobalDataXmin are initialized to
+ * InvalidTransactionId, to ensure that no one tries to use a stale
+ * value. Readers should ensure that it has been set to something else
+ * before using it.
  */
 TransactionId TransactionXmin = FirstNormalTransactionId;
 TransactionId RecentXmin = FirstNormalTransactionId;
 TransactionId RecentGlobalXmin = InvalidTransactionId;
+TransactionId RecentGlobalDataXmin = InvalidTransactionId;
+
+/* (table, ctid) => (cmin, cmax) mapping during timetravel */
+static HTAB *tuplecid_data = NULL;
 
 /*
  * Elements of the active snapshot stack.
@@ -158,6 +168,18 @@ static void SnapshotResetXmin(void);
 Snapshot
 GetTransactionSnapshot(void)
 {
+	/*
+	 * Return historic snapshot if doing logical decoding. We'll never
+	 * need a non-historic transaction snapshot in this (sub-)transaction, so
+	 * there's no need to be careful to set one up for later calls to
+	 * GetTransactionSnapshot().
+	 */
+	if (HistoricSnapshotActive())
+	{
+		Assert(!FirstSnapshotSet);
+		return HistoricSnapshot;
+	}
+
 	/* First call in transaction? */
 	if (!FirstSnapshotSet)
 	{
@@ -214,6 +236,13 @@ GetTransactionSnapshot(void)
 Snapshot
 GetLatestSnapshot(void)
 {
+	/*
+	 * So far there are no cases requiring support for GetLatestSnapshot()
+	 * during logical decoding, but it wouldn't be hard to add if
+	 * required.
+	 */
+	Assert(!HistoricSnapshotActive());
+
 	/* If first call in transaction, go ahead and set the xact snapshot */
 	if (!FirstSnapshotSet)
 		return GetTransactionSnapshot();
@@ -230,6 +259,26 @@ GetLatestSnapshot(void)
  */
 Snapshot
 GetCatalogSnapshot(Oid relid)
+{
+	/*
+	 * Return historic snapshot if we're doing logical decoding, but
+	 * return a non-historic, snapshot if we temporarily are doing up2date
+	 * lookups.
+	 */
+	if (HistoricSnapshotActive())
+		return HistoricSnapshot;
+
+	return GetNonHistoricCatalogSnapshot(relid);
+}
+
+/*
+ * GetNonHistoricCatalogSnapshot
+ *		Get a snapshot that is sufficiently up-to-date for scan of the system
+ *		catalog with the specified OID, even while historic snapshots are set
+ *		up.
+ */
+Snapshot
+GetNonHistoricCatalogSnapshot(Oid relid)
 {
 	/*
 	 * If the caller is trying to scan a relation that has no syscache,
@@ -303,6 +352,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, TransactionId sourcexid)
 
 	Assert(RegisteredSnapshots == 0);
 	Assert(FirstXactSnapshot == NULL);
+	Assert(HistoricSnapshotActive());
 
 	/*
 	 * Even though we are not going to use the snapshot it computes, we must
@@ -796,7 +846,7 @@ AtEOXact_Snapshot(bool isCommit)
  *		Returns the token (the file name) that can be used to import this
  *		snapshot.
  */
-static char *
+char *
 ExportSnapshot(Snapshot snapshot)
 {
 	TransactionId topXid;
@@ -1258,3 +1308,45 @@ ThereAreNoPriorRegisteredSnapshots(void)
 
 	return false;
 }
+
+/*
+ * Setup a snapshot that replaces normal catalog snapshots that allows catalog
+ * access to behave just like it did at a certain point in the past.
+ *
+ * Needed for logical decoding.
+ */
+void
+SetupHistoricSnapshot(Snapshot historic_snapshot, HTAB *tuplecids)
+{
+	Assert(historic_snapshot != NULL);
+
+	/* setup the timetravel snapshot */
+	HistoricSnapshot = historic_snapshot;
+
+	/* setup (cmin, cmax) lookup hash */
+	tuplecid_data = tuplecids;
+}
+
+
+/*
+ * Make catalog snapshots behave normally again.
+ */
+void
+TeardownHistoricSnapshot(bool is_error)
+{
+	HistoricSnapshot = NULL;
+	tuplecid_data = NULL;
+}
+
+bool
+HistoricSnapshotActive(void)
+{
+	return HistoricSnapshot != NULL;
+}
+
+HTAB *
+HistoricSnapshotGetTupleCids(void)
+{
+	Assert(HistoricSnapshotActive());
+	return tuplecid_data;
+}