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Improve comments and add new assert() statements in WAL to help document

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how everything works.

FossilOrigin-Name: 23b08fe9db24a953cc231b093cf74d140c9965d22964d0401ff8ab3d4ecba443
This commit is contained in:
drh
2021-07-29 16:48:21 +00:00
parent 02a9996e40
commit fd4c7862ee
3 changed files with 41 additions and 10 deletions
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37
src/wal.c
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@@ -161,7 +161,10 @@
** HASHTABLE_NPAGE_ONE frames. The values of HASHTABLE_NPAGE_ONE and
** HASHTABLE_NPAGE are selected so that together the wal-index header and
** first index block are the same size as all other index blocks in the
** wal-index.
** wal-index. The values are:
**
** HASHTABLE_NPAGE 4096
** HASHTABLE_NPAGE_ONE 4062
**
** Each index block contains two sections, a page-mapping that contains the
** database page number associated with each wal frame, and a hash-table
@@ -1410,14 +1413,42 @@ int sqlite3WalOpen(
assert( zWalName && zWalName[0] );
assert( pDbFd );
/* Verify the values of various constants. Any changes to the values
** of these constants would result in an incompatible on-disk format
** for the -shm file. Any change that causes one of these asserts to
** fail is a backward compatibility problem, even if the change otherwise
** works.
**
** This table also serves as a helpful cross-reference when trying to
** interpret hex dumps of the -shm file.
*/
assert( 48 == sizeof(WalIndexHdr) );
assert( 40 == sizeof(WalCkptInfo) );
assert( 120 == WALINDEX_LOCK_OFFSET );
assert( 136 == WALINDEX_HDR_SIZE );
assert( 4096 == HASHTABLE_NPAGE );
assert( 4062 == HASHTABLE_NPAGE_ONE );
assert( 8192 == HASHTABLE_NSLOT );
assert( 383 == HASHTABLE_HASH_1 );
assert( 32768 == WALINDEX_PGSZ );
assert( 8 == SQLITE_SHM_NLOCK );
assert( 5 == WAL_NREADER );
assert( 24 == WAL_HDRSIZE );
assert( 120 == WALINDEX_LOCK_OFFSET + WAL_WRITE_LOCK );
assert( 121 == WALINDEX_LOCK_OFFSET + WAL_CKPT_LOCK );
assert( 122 == WALINDEX_LOCK_OFFSET + WAL_RECOVER_LOCK );
assert( 123 == WALINDEX_LOCK_OFFSET + WAL_READ_LOCK(0) );
assert( 124 == WALINDEX_LOCK_OFFSET + WAL_READ_LOCK(1) );
assert( 125 == WALINDEX_LOCK_OFFSET + WAL_READ_LOCK(2) );
assert( 126 == WALINDEX_LOCK_OFFSET + WAL_READ_LOCK(3) );
assert( 127 == WALINDEX_LOCK_OFFSET + WAL_READ_LOCK(4) );
/* In the amalgamation, the os_unix.c and os_win.c source files come before
** this source file. Verify that the #defines of the locking byte offsets
** in os_unix.c and os_win.c agree with the WALINDEX_LOCK_OFFSET value.
** For that matter, if the lock offset ever changes from its initial design
** value of 120, we need to know that so there is an assert() to check it.
*/
assert( 120==WALINDEX_LOCK_OFFSET );
assert( 136==WALINDEX_HDR_SIZE );
#ifdef WIN_SHM_BASE
assert( WIN_SHM_BASE==WALINDEX_LOCK_OFFSET );
#endif