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/include/utils/snapshot.h

@@ -30,6 +30,22 @@ typedef struct SnapshotData *Snapshot;
 typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
 									   Snapshot snapshot, Buffer buffer);
 
+/*
+ * Struct representing all kind of possible snapshots.
+ *
+ * There are several different kinds of snapshots:
+ * * Normal MVCC snapshots
+ * * MVCC snapshots taken during recovery (in Hot-Standby mode)
+ * * Historic MVCC snapshots used during logical decoding 
+ * * snapshots passed to HeapTupleSatisfiesDirty()
+ * * snapshots used for SatisfiesAny, Toast, Self where no members are
+ *   accessed.
+ *
+ * TODO: It's probably a good idea to split this struct using a NodeTag
+ * similar to how parser and executor nodes are handled, with one type for
+ * each different kind of snapshot to avoid overloading the meaning of
+ * individual fields.
+ */
 typedef struct SnapshotData
 {
 	SnapshotSatisfiesFunc satisfies;	/* tuple test function */
@@ -46,11 +62,23 @@ typedef struct SnapshotData
 	 */
 	TransactionId xmin;			/* all XID < xmin are visible to me */
 	TransactionId xmax;			/* all XID >= xmax are invisible to me */
-	TransactionId *xip;			/* array of xact IDs in progress */
+	/*
+	 * For normal MVCC snapshot this contains the all xact IDs that are in
+	 * progress, unless the snapshot was taken during recovery in which case
+	 * it's empty. For historic MVCC snapshots, the meaning is inverted,
+	 * i.e. it contains *committed* transactions between xmin and xmax.
+	 */
+	TransactionId *xip;
 	uint32		xcnt;			/* # of xact ids in xip[] */
 	/* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
 	int32		subxcnt;		/* # of xact ids in subxip[] */
-	TransactionId *subxip;		/* array of subxact IDs in progress */
+	/*
+	 * For non-historic MVCC snapshots, this contains subxact IDs that are in
+	 * progress (and other transactions that are in progress if taken during
+	 * recovery). For historic snapshot it contains *all* xids assigned to the
+	 * replayed transaction, including the toplevel xid.
+	 */
+	TransactionId *subxip;
 	bool		suboverflowed;	/* has the subxip array overflowed? */
 	bool		takenDuringRecovery;	/* recovery-shaped snapshot? */
 	bool		copied;			/* false if it's a static snapshot */