*** 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 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.