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Overview
Comment:Instead of extra locks, use F_GETLK to ensure that readonly_shm clients cannot connect to a wal-mode database if there are no writers.
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SHA3-256:5492f457dc7cc5c416de4b4e61e84bd2f10b4e6ce54011b7a60feb47f629c923
User & Date: dan 2017-10-26 17:05:22
Context
2017-10-26
17:34
Fix an error in the previous commit on this branch. check-in: f71dfee0 user: dan tags: readonly-wal-recovery
17:05
Instead of extra locks, use F_GETLK to ensure that readonly_shm clients cannot connect to a wal-mode database if there are no writers. check-in: 5492f457 user: dan tags: readonly-wal-recovery
2017-10-25
23:28
Use extra locks to prevent a readonly_shm=1 process from connecting to a WAL-mode database if there are no writers. check-in: 35d97908 user: drh tags: readonly-wal-recovery
Changes
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Changes to src/os_unix.c.

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};

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE   ((22+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
#define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */
#define UNIX_SHM_N_DMS  1000000000                        /* Size of the DMS */

/*
** Apply posix advisory locks for all bytes from ofst through ofst+n-1.
**
** Locks block if the mask is exactly UNIX_SHM_C and are non-blocking
** otherwise.
*/
................................................................................
  struct flock f;        /* The posix advisory locking structure */
  int rc = SQLITE_OK;    /* Result code form fcntl() */

  /* Access to the unixShmNode object is serialized by the caller */
  pShmNode = pFile->pInode->pShmNode;
  assert( sqlite3_mutex_held(pShmNode->mutex) || pShmNode->nRef==0 );







  if( pShmNode->h>=0 ){
    /* Initialize the locking parameters */
    memset(&f, 0, sizeof(f));
    f.l_type = lockType;
    f.l_whence = SEEK_SET;
    f.l_start = ofst;
    f.l_len = n;
................................................................................

    rc = osFcntl(pShmNode->h, F_SETLK, &f);
    rc = (rc!=(-1)) ? SQLITE_OK : SQLITE_BUSY;
  }

  /* Update the global lock state and do debug tracing */
#ifdef SQLITE_DEBUG
  if( ofst<UNIX_SHM_DMS ){
    u16 mask;

    /* Shared locks never span more than one byte */
    assert( n==1 || lockType!=F_RDLCK );

    /* Locks are within range */
    assert( n>=1 && n<=SQLITE_SHM_NLOCK );

    OSTRACE(("SHM-LOCK "));
    mask = ofst>31 ? 0xffff : (1<<(ofst+n)) - (1<<ofst);
    if( rc==SQLITE_OK ){
      if( lockType==F_UNLCK ){
        OSTRACE(("unlock %d ok", ofst));
        pShmNode->exclMask &= ~mask;
        pShmNode->sharedMask &= ~mask;
      }else if( lockType==F_RDLCK ){
        OSTRACE(("read-lock %d ok", ofst));
        pShmNode->exclMask &= ~mask;
        pShmNode->sharedMask |= mask;
      }else{
        assert( lockType==F_WRLCK );
        OSTRACE(("write-lock %d ok", ofst));
        pShmNode->exclMask |= mask;
        pShmNode->sharedMask &= ~mask;
      }
    }else{
      if( lockType==F_UNLCK ){
        OSTRACE(("unlock %d failed", ofst));
      }else if( lockType==F_RDLCK ){
        OSTRACE(("read-lock failed"));
      }else{
        assert( lockType==F_WRLCK );
        OSTRACE(("write-lock %d failed", ofst));
      }
    }
    OSTRACE((" - afterwards %03x,%03x\n",
             pShmNode->sharedMask, pShmNode->exclMask));
  }
#endif

  return rc;        
}

/*
................................................................................

      /* If this process is running as root, make sure that the SHM file
      ** is owned by the same user that owns the original database.  Otherwise,
      ** the original owner will not be able to connect.
      */
      robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
  
      /* Do not allow a read-only process to connect if there are no
      ** writers, because a read-only process is unable to recover the
      ** shm file following a system crash.
      */

      rc = SQLITE_OK;
      if( pShmNode->isReadonly ){
        if( !unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, UNIX_SHM_N_DMS) ){








          rc = SQLITE_CANTOPEN_DIRTYWAL;
        }
      }

      /* If we are able to grab the dead-man switch, that means this is the
      ** first (write-enable) process to connect to the database.  In that
      ** case, truncate the shm file because the contents found on disk might
      ** be invalid leftovers from a system crash.  The shm will be rebuilt
      */
      if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
        if( robust_ftruncate(pShmNode->h, 0) ){
          rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
        }
      }

      /* Acquires locks to tell other processes that a this process is
      ** running and therefore the shm is valid they do not need to run
      ** recovery.
      */
      if( rc==SQLITE_OK ){
        unsigned r;
        rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
        sqlite3_randomness(sizeof(r), &r);
        r = 1 + (r%(UNIX_SHM_N_DMS-1));
        (void)unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS+r, 1);
      }
      if( rc ) goto shm_open_err;
    }
  }

  /* Make the new connection a child of the unixShmNode */
  p->pShmNode = pShmNode;







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};

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE   ((22+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
#define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */


/*
** Apply posix advisory locks for all bytes from ofst through ofst+n-1.
**
** Locks block if the mask is exactly UNIX_SHM_C and are non-blocking
** otherwise.
*/
................................................................................
  struct flock f;        /* The posix advisory locking structure */
  int rc = SQLITE_OK;    /* Result code form fcntl() */

  /* Access to the unixShmNode object is serialized by the caller */
  pShmNode = pFile->pInode->pShmNode;
  assert( sqlite3_mutex_held(pShmNode->mutex) || pShmNode->nRef==0 );

  /* Shared locks never span more than one byte */
  assert( n==1 || lockType!=F_RDLCK );

  /* Locks are within range */
  assert( n>=1 && n<=SQLITE_SHM_NLOCK );

  if( pShmNode->h>=0 ){
    /* Initialize the locking parameters */
    memset(&f, 0, sizeof(f));
    f.l_type = lockType;
    f.l_whence = SEEK_SET;
    f.l_start = ofst;
    f.l_len = n;
................................................................................

    rc = osFcntl(pShmNode->h, F_SETLK, &f);
    rc = (rc!=(-1)) ? SQLITE_OK : SQLITE_BUSY;
  }

  /* Update the global lock state and do debug tracing */
#ifdef SQLITE_DEBUG

  { u16 mask;







  OSTRACE(("SHM-LOCK "));
  mask = ofst>31 ? 0xffff : (1<<(ofst+n)) - (1<<ofst);
  if( rc==SQLITE_OK ){
    if( lockType==F_UNLCK ){
      OSTRACE(("unlock %d ok", ofst));
      pShmNode->exclMask &= ~mask;
      pShmNode->sharedMask &= ~mask;
    }else if( lockType==F_RDLCK ){
      OSTRACE(("read-lock %d ok", ofst));
      pShmNode->exclMask &= ~mask;
      pShmNode->sharedMask |= mask;
    }else{
      assert( lockType==F_WRLCK );
      OSTRACE(("write-lock %d ok", ofst));
      pShmNode->exclMask |= mask;
      pShmNode->sharedMask &= ~mask;
    }
  }else{
    if( lockType==F_UNLCK ){
      OSTRACE(("unlock %d failed", ofst));
    }else if( lockType==F_RDLCK ){
      OSTRACE(("read-lock failed"));
    }else{
      assert( lockType==F_WRLCK );
      OSTRACE(("write-lock %d failed", ofst));
    }
  }
  OSTRACE((" - afterwards %03x,%03x\n",
           pShmNode->sharedMask, pShmNode->exclMask));
  }
#endif

  return rc;        
}

/*
................................................................................

      /* If this process is running as root, make sure that the SHM file
      ** is owned by the same user that owns the original database.  Otherwise,
      ** the original owner will not be able to connect.
      */
      robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
  
      /* Check to see if another process is holding the dead-man switch.
      ** For a readonly_shm client, if no other process holds the DMS lock,
      ** the file cannot be opened and SQLITE_CANTOPEN_DIRTYWAL is returned.
      ** Or, for a read-write connection, if no other process holds a
      ** DMS lock the file is truncated to zero bytes in size.  */
      rc = SQLITE_OK;
      if( pShmNode->isReadonly ){

        struct flock lock;
        lock.l_whence = SEEK_SET;
        lock.l_start = UNIX_SHM_DMS;
        lock.l_len = 1;
        lock.l_type = F_WRLCK;
        if( osFcntl(pShmNode->h, F_GETLK, &lockInfo)!=0 ) {
          rc = SQLITE_IOERR_LOCK;
        }else if( lock.l_type==F_UNLCK ){
          rc = SQLITE_CANTOPEN_DIRTYWAL;
        }







      }else if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
        if( robust_ftruncate(pShmNode->h, 0) ){
          rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
        }
      }





      if( rc==SQLITE_OK ){

        rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);



      }
      if( rc ) goto shm_open_err;
    }
  }

  /* Make the new connection a child of the unixShmNode */
  p->pShmNode = pShmNode;