XLOG (and related) changes:

* Store two past checkpoint locations, not just one, in pg_control.
  On startup, we fall back to the older checkpoint if the newer one
  is unreadable.  Also, a physical copy of the newest checkpoint record
  is kept in pg_control for possible use in disaster recovery (ie,
  complete loss of pg_xlog).  Also add a version number for pg_control
  itself.  Remove archdir from pg_control; it ought to be a GUC
  parameter, not a special case (not that it's implemented yet anyway).

* Suppress successive checkpoint records when nothing has been entered
  in the WAL log since the last one.  This is not so much to avoid I/O
  as to make it actually useful to keep track of the last two
  checkpoints.  If the things are right next to each other then there's
  not a lot of redundancy gained...

* Change CRC scheme to a true 64-bit CRC, not a pair of 32-bit CRCs
  on alternate bytes.  Polynomial borrowed from ECMA DLT1 standard.

* Fix XLOG record length handling so that it will work at BLCKSZ = 32k.

* Change XID allocation to work more like OID allocation.  (This is of
  dubious necessity, but I think it's a good idea anyway.)

* Fix a number of minor bugs, such as off-by-one logic for XLOG file
  wraparound at the 4 gig mark.

* Add documentation and clean up some coding infelicities; move file
  format declarations out to include files where planned contrib
  utilities can get at them.

* Checkpoint will now occur every CHECKPOINT_SEGMENTS log segments or
  every CHECKPOINT_TIMEOUT seconds, whichever comes first.  It is also
  possible to force a checkpoint by sending SIGUSR1 to the postmaster
  (undocumented feature...)

* Defend against kill -9 postmaster by storing shmem block's key and ID
  in postmaster.pid lockfile, and checking at startup to ensure that no
  processes are still connected to old shmem block (if it still exists).

* Switch backends to accept SIGQUIT rather than SIGUSR1 for emergency
  stop, for symmetry with postmaster and xlog utilities.  Clean up signal
  handling in bootstrap.c so that xlog utilities launched by postmaster
  will react to signals better.

* Standalone bootstrap now grabs lockfile in target directory, as added
  insurance against running it in parallel with live postmaster.

src/include/utils/pg_crc.h

@@ -0,0 +1,115 @@
+/*
+ * pg_crc.h
+ *
+ * PostgreSQL 64-bit CRC support
+ *
+ * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $Id: pg_crc.h,v 1.1 2001/03/13 01:17:06 tgl Exp $
+ */
+#ifndef PG_CRC_H
+#define PG_CRC_H
+
+/*
+ * If we have a 64-bit integer type, then a 64-bit CRC looks just like the
+ * usual sort of implementation.  (See Ross Williams' excellent introduction
+ * A PAINLESS GUIDE TO CRC ERROR DETECTION ALGORITHMS, available from
+ * ftp://ftp.rocksoft.com/papers/crc_v3.txt or several other net sites.)
+ * If we have no working 64-bit type, then fake it with two 32-bit registers.
+ *
+ * The present implementation is a normal (not "reflected", in Williams'
+ * terms) 64-bit CRC, using initial all-ones register contents and a final
+ * bit inversion.  The chosen polynomial is borrowed from the DLT1 spec
+ * (ECMA-182, available from http://www.ecma.ch/ecma1/STAND/ECMA-182.HTM):
+ *
+ * x^64 + x^62 + x^57 + x^55 + x^54 + x^53 + x^52 + x^47 + x^46 + x^45 +
+ * x^40 + x^39 + x^38 + x^37 + x^35 + x^33 + x^32 + x^31 + x^29 + x^27 +
+ * x^24 + x^23 + x^22 + x^21 + x^19 + x^17 + x^13 + x^12 + x^10 + x^9 +
+ * x^7 + x^4 + x + 1
+ */
+
+#ifdef INT64_IS_BUSTED
+
+/*
+ * crc0 represents the LSBs of the 64-bit value, crc1 the MSBs.  Note that
+ * with crc0 placed first, the output of 32-bit and 64-bit implementations
+ * will be bit-compatible only on little-endian architectures.  If it were
+ * important to make the two possible implementations bit-compatible on
+ * all machines, we could do a configure test to decide how to order the
+ * two fields, but it seems not worth the trouble.
+ */
+typedef struct crc64
+{
+	uint32		crc0;
+	uint32		crc1;
+} crc64;
+
+/* Initialize a CRC accumulator */
+#define INIT_CRC64(crc)	((crc).crc0 = 0xffffffff, (crc).crc1 = 0xffffffff)
+
+/* Finish a CRC calculation */
+#define FIN_CRC64(crc)	((crc).crc0 ^= 0xffffffff, (crc).crc1 ^= 0xffffffff)
+
+/* Accumulate some (more) bytes into a CRC */
+#define COMP_CRC64(crc, data, len)	\
+do { \
+	uint32		__crc0 = (crc).crc0; \
+	uint32		__crc1 = (crc).crc1; \
+	unsigned char *__data = (unsigned char *) (data); \
+	uint32		__len = (len); \
+\
+	while (__len-- > 0) \
+	{ \
+		int		__tab_index = ((int) (__crc1 >> 24) ^ *__data++) & 0xFF; \
+		__crc1 = crc_table1[__tab_index] ^ ((__crc1 << 8) | (__crc0 >> 24)); \
+		__crc0 = crc_table0[__tab_index] ^ (__crc0 << 8); \
+	} \
+	(crc).crc0 = __crc0; \
+	(crc).crc1 = __crc1; \
+} while (0)
+
+/* Check for equality of two CRCs */
+#define EQ_CRC64(c1,c2)  ((c1).crc0 == (c2).crc0 && (c1).crc1 == (c2).crc1)
+
+/* Constant table for CRC calculation */
+extern const uint32 crc_table0[];
+extern const uint32 crc_table1[];
+
+#else /* int64 works */
+
+typedef struct crc64
+{
+	uint64		crc0;
+} crc64;
+
+/* Initialize a CRC accumulator */
+#define INIT_CRC64(crc)	((crc).crc0 = (uint64) 0xffffffffffffffff)
+
+/* Finish a CRC calculation */
+#define FIN_CRC64(crc)	((crc).crc0 ^= (uint64) 0xffffffffffffffff)
+
+/* Accumulate some (more) bytes into a CRC */
+#define COMP_CRC64(crc, data, len)	\
+do { \
+	uint64		__crc0 = (crc).crc0; \
+	unsigned char *__data = (unsigned char *) (data); \
+	uint32		__len = (len); \
+\
+	while (__len-- > 0) \
+	{ \
+		int		__tab_index = ((int) (__crc0 >> 56) ^ *__data++) & 0xFF; \
+		__crc0 = crc_table[__tab_index] ^ (__crc0 << 8); \
+	} \
+	(crc).crc0 = __crc0; \
+} while (0)
+
+/* Check for equality of two CRCs */
+#define EQ_CRC64(c1,c2)  ((c1).crc0 == (c2).crc0)
+
+/* Constant table for CRC calculation */
+extern const uint64 crc_table[];
+
+#endif /* INT64_IS_BUSTED */
+
+#endif	 /* PG_CRC_H */