 *** Throughout this document, a page is deemed to have been synced
     automatically as soon as it is written when PRAGMA synchronous=OFF.
     Otherwise, the page is not synced until the xSync method of the VFS
     is called successfully on the file containing the page.

 *** Definition:  A page of the database file is said to be "overwriteable" if
     one or more of the following are true about the page:

     (a)  The original content of the page as it was at the beginning of
          the transaction has been written into the rollback journal and
          synced.

     (b)  The page was a freelist leaf page at the start of the transaction.

     (c)  The page number is greater than the largest page that existed in
          the database file at the start of the transaction.

 (1) A page of the database file is never overwritten unless one of the
     following are true:

     (a) The page and all other pages on the same sector are overwriteable.

     (b) The atomic page write optimization is enabled, and the entire
         transaction other than the update of the transaction sequence
         number consists of a single page change.

 (2) The content of a page written into the rollback journal exactly matches
     both the content in the database when the rollback journal was written
     and the content in the database at the beginning of the current
     transaction.

 (3) Writes to the database file are an integer multiple of the page size
     in length and are aligned to a page boundary.

 (4) Reads from the database file are either aligned on a page boundary and
     an integer multiple of the page size in length or are taken from the
     first 100 bytes of the database file.

 (5) All writes to the database file are synced prior to the rollback journal
     being deleted, truncated, or zeroed.

 (6) If a master journal file is used, then all writes to the database file
     are synced prior to the master journal being deleted.

 *** Definition: Two databases (or the same database at two points it time)
     are said to be "logically equivalent" if they give the same answer to
     all queries.  Note in particular the content of freelist leaf
     pages can be changed arbitarily without effecting the logical equivalence
     of the database.

 (7) At any time, if any subset, including the empty set and the total set,
     of the unsynced changes to a rollback journal are removed and the
     journal is rolled back, the resulting database file will be logical
     equivalent to the database file at the beginning of the transaction.

 (8) When a transaction is rolled back, the xTruncate method of the VFS
     is called to restore the database file to the same size it was at
     the beginning of the transaction.  (In some VFSes, the xTruncate
     method is a no-op, but that does not change the fact the SQLite will
     invoke it.)

 (9) Whenever the database file is modified, at least one bit in the range
     of bytes from 24 through 39 inclusive will be changed prior to releasing
     the EXCLUSIVE lock.

(10) The pattern of bits in bytes 24 through 39 shall not repeat in less
     than one billion transactions.

(11) A database file is well-formed at the beginning and at the conclusion
     of every transaction.

(12) An EXCLUSIVE lock must be held on the database file before making
     any changes to the database file.

(13) A SHARED lock must be held on the database file before reading any
     content out of the database file.
