| *** 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. | |