Separate snapshot management code from tuple visibility code, create a

snapmgmt.c file for the former.  The header files have also been reorganized
in three parts: the most basic snapshot definitions are now in a new file
snapshot.h, and the also new snapmgmt.h keeps the definitions for snapmgmt.c.
tqual.h has been reduced to the bare minimum.

This patch is just a first step towards managing live snapshots within a
transaction; there is no functionality change.

Per my proposal to pgsql-patches on 20080318191940.GB27458@alvh.no-ip.org and
subsequent discussion.

src/include/utils/snapshot.h

@@ -0,0 +1,62 @@
+/*-------------------------------------------------------------------------
+ *
+ * snapshot.h
+ *	  POSTGRES snapshot definition
+ *
+ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/utils/snapshot.h,v 1.1 2008/03/26 16:20:48 alvherre Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SNAPSHOT_H
+#define SNAPSHOT_H
+
+#include "access/htup.h"
+#include "storage/buf.h"
+
+
+typedef struct SnapshotData *Snapshot;
+
+#define InvalidSnapshot		((Snapshot) NULL)
+
+/*
+ * We use SnapshotData structures to represent both "regular" (MVCC)
+ * snapshots and "special" snapshots that have non-MVCC semantics.
+ * The specific semantics of a snapshot are encoded by the "satisfies"
+ * function.
+ */
+typedef bool (*SnapshotSatisfiesFunc) (HeapTupleHeader tuple,
+									   Snapshot snapshot, Buffer buffer);
+
+typedef struct SnapshotData
+{
+	SnapshotSatisfiesFunc satisfies;	/* tuple test function */
+
+	/*
+	 * The remaining fields are used only for MVCC snapshots, and are normally
+	 * just zeroes in special snapshots.  (But xmin and xmax are used
+	 * specially by HeapTupleSatisfiesDirty.)
+	 *
+	 * An MVCC snapshot can never see the effects of XIDs >= xmax. It can see
+	 * the effects of all older XIDs except those listed in the snapshot. xmin
+	 * is stored as an optimization to avoid needing to search the XID arrays
+	 * for most tuples.
+	 */
+	TransactionId xmin;			/* all XID < xmin are visible to me */
+	TransactionId xmax;			/* all XID >= xmax are invisible to me */
+	uint32		xcnt;			/* # of xact ids in xip[] */
+	TransactionId *xip;			/* array of xact IDs in progress */
+	/* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
+	int32		subxcnt;		/* # of xact ids in subxip[], -1 if overflow */
+	TransactionId *subxip;		/* array of subxact IDs in progress */
+
+	/*
+	 * note: all ids in subxip[] are >= xmin, but we don't bother filtering
+	 * out any that are >= xmax
+	 */
+	CommandId	curcid;			/* in my xact, CID < curcid are visible */
+} SnapshotData;
+
+#endif /* SNAPSHOT_H */