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Extra comments on the sqlite3OsShmMap() call in walBeginUnlocked(). No

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changes to code.

FossilOrigin-Name: 033ee92bf4d5dc57f5cb8fd02d1154ae06f2d3261d214e7191a82c70c8ffebf7
This commit is contained in:
drh
2017-11-08 19:26:27 +00:00
parent 7316960cec
commit 9214c1efe8
3 changed files with 16 additions and 8 deletions
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10
src/wal.c
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@@ -2210,6 +2210,14 @@ static int walBeginUnlocked(Wal *pWal, int *pChanged){
/* Try to map the *-shm file again. If it succeeds this time, then
** a non-readonly_shm connection has already connected to the database.
** In this case, start over with opening the transaction.
**
** The *-shm file was opened read-only, so sqlite3OsShmMap() can never
** return SQLITE_OK here, as that would imply that it had established
** a read/write mapping. A return of SQLITE_READONLY means success - that
** a mapping has been established to a shared-memory segment that is actively
** maintained by a writer. SQLITE_READONLY_CANTINIT means that all
** all connections to the -shm file are read-only and hence the content
** of the -shm file might be out-of-date.
**
** The WAL_READ_LOCK(0) lock held by this client prevents a checkpoint
** from taking place. But it does not prevent the wal from being wrapped
@@ -2219,8 +2227,8 @@ static int walBeginUnlocked(Wal *pWal, int *pChanged){
** the unlocked transaction, as the other client may overwrite wal
** frames that this client is still using. */
rc = sqlite3OsShmMap(pWal->pDbFd, 0, WALINDEX_PGSZ, 0, &pDummy);
assert( rc!=SQLITE_OK ); /* SQLITE_OK not possible for read-only connection */
if( rc!=SQLITE_READONLY_CANTINIT ){
assert( rc!=SQLITE_OK );
rc = (rc==SQLITE_READONLY ? WAL_RETRY : rc);
goto begin_unlocked_out;
}