In the comments at the head of pager.c, there is this:
** The page cache comes up in PAGER_UNLOCK. The first time a ** sqlite3pager_get() occurs, the state transitions to PAGER_SHARED. ** After all pages have been released using sqlite_page_unref(), ** the state transitions back to PAGER_UNLOCK. The first time ** that sqlite3pager_write() is called, the state transitions to ** PAGER_RESERVED. (Note that sqlite_page_write() can only be ** called on an outstanding page which means that the pager must ** be in PAGER_SHARED before it transitions to PAGER_RESERVED.) ** The transition to PAGER_EXCLUSIVE occurs when before any changes ** are made to the database file. After an sqlite3pager_rollback() ** or sqlite_pager_commit(), the state goes back to PAGER_SHARED.
There is a minor typo in the second to last sentence -- "occurs when before" doesn't make sense.
b.bum
