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Overview
Comment:Extra comments on the sqlite3OsShmMap() call in walBeginUnlocked(). No changes to code.
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SHA3-256:033ee92bf4d5dc57f5cb8fd02d1154ae06f2d3261d214e7191a82c70c8ffebf7
User & Date: drh 2017-11-08 19:26:27
Context
2017-11-09
16:30
Initial work on porting the changes on this branch to Win32. check-in: 3738bfd0 user: mistachkin tags: readonly-wal-recovery
2017-11-08
19:26
Extra comments on the sqlite3OsShmMap() call in walBeginUnlocked(). No changes to code. check-in: 033ee92b user: drh tags: readonly-wal-recovery
17:51
Turns out that SQLITE_READONLY_CANTLOCK is an historical name that must be preserved. So make a new SQLITE_READLOCK_CANTINIT name instead. check-in: 04974a8b user: drh tags: readonly-wal-recovery
Changes
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Changes to src/wal.c.

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    goto begin_unlocked_out;
  }
  pWal->readLock = 0;

  /* 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 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
  ** if a checkpoint has already taken place. This means that if another
  ** client is connected at this point, it may have already checkpointed 
  ** the entire wal. In that case it would not be safe to continue with
  ** 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);

  if( rc!=SQLITE_READONLY_CANTINIT ){
    assert( rc!=SQLITE_OK );
    rc = (rc==SQLITE_READONLY ? WAL_RETRY : rc);
    goto begin_unlocked_out;
  }

  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));
  rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
  if( rc!=SQLITE_OK ){







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    goto begin_unlocked_out;
  }
  pWal->readLock = 0;

  /* 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
  ** if a checkpoint has already taken place. This means that if another
  ** client is connected at this point, it may have already checkpointed 
  ** the entire wal. In that case it would not be safe to continue with
  ** 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 ){

    rc = (rc==SQLITE_READONLY ? WAL_RETRY : rc);
    goto begin_unlocked_out;
  }

  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));
  rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
  if( rc!=SQLITE_OK ){