From 5028f22f6eb0579890689655285a4778b4ffc460 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas
Date: Tue, 4 Nov 2014 11:35:15 +0200
Subject: Switch to CRC-32C in WAL and other places.

The old algorithm was found to not be the usual CRC-32 algorithm, used by
Ethernet et al. We were using a non-reflected lookup table with code meant
for a reflected lookup table. That's a strange combination that AFAICS does
not correspond to any bit-wise CRC calculation, which makes it difficult to
reason about its properties. Although it has worked well in practice, seems
safer to use a well-known algorithm.

Since we're changing the algorithm anyway, we might as well choose a
different polynomial. The Castagnoli polynomial has better error-correcting
properties than the traditional CRC-32 polynomial, even if we had
implemented it correctly. Another reason for picking that is that some new
CPUs have hardware support for calculating CRC-32C, but not CRC-32, let
alone our strange variant of it. This patch doesn't add any support for such
hardware, but a future patch could now do that.

The old algorithm is kept around for tsquery and pg_trgm, which use the
values in indexes that need to remain compatible so that pg_upgrade works.
While we're at it, share the old lookup table for CRC-32 calculation
between hstore, ltree and core. They all use the same table, so might as
well.
---
 src/backend/utils/cache/relmapper.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

(limited to 'src/backend/utils/cache')

diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c
index 95a2689fd42..b6b13308773 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -84,7 +84,7 @@ typedef struct RelMapFile
 	int32		magic;			/* always RELMAPPER_FILEMAGIC */
 	int32		num_mappings;	/* number of valid RelMapping entries */
 	RelMapping	mappings[MAX_MAPPINGS];
-	int32		crc;			/* CRC of all above */
+	pg_crc32	crc;			/* CRC of all above */
 	int32		pad;			/* to make the struct size be 512 exactly */
 } RelMapFile;
 
@@ -673,11 +673,11 @@ load_relmap_file(bool shared)
 						mapfilename)));
 
 	/* verify the CRC */
-	INIT_CRC32(crc);
-	COMP_CRC32(crc, (char *) map, offsetof(RelMapFile, crc));
-	FIN_CRC32(crc);
+	INIT_CRC32C(crc);
+	COMP_CRC32C(crc, (char *) map, offsetof(RelMapFile, crc));
+	FIN_CRC32C(crc);
 
-	if (!EQ_CRC32(crc, map->crc))
+	if (!EQ_CRC32C(crc, map->crc))
 		ereport(FATAL,
 		  (errmsg("relation mapping file \"%s\" contains incorrect checksum",
 				  mapfilename)));
@@ -719,9 +719,9 @@ write_relmap_file(bool shared, RelMapFile *newmap,
 	if (newmap->num_mappings < 0 || newmap->num_mappings > MAX_MAPPINGS)
 		elog(ERROR, "attempt to write bogus relation mapping");
 
-	INIT_CRC32(newmap->crc);
-	COMP_CRC32(newmap->crc, (char *) newmap, offsetof(RelMapFile, crc));
-	FIN_CRC32(newmap->crc);
+	INIT_CRC32C(newmap->crc);
+	COMP_CRC32C(newmap->crc, (char *) newmap, offsetof(RelMapFile, crc));
+	FIN_CRC32C(newmap->crc);
 
 	/*
 	 * Open the target file.  We prefer to do this before entering the
-- 
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