Would anyone be able to review this spec please? I'm trying to get straight what locking / access control rules need to be.
/* * THE PRISTINE STORE * ================== * * === Introduction === * * The Pristine Store is the part of the Working Copy metadata that holds * a local copy of the full text of the base version of each WC file. * * Texts in the Pristine Store are addressed only by their SHA-1 checksum. * The Pristine Store does not track which text relates to which repository * and revision and path. The Pristine Store does not hold pristine copies * of directories, nor of properties. * * The Pristine Store data is held in * * the 'PRISTINE' table in the SQLite Data Base (SDB), and * * the files in the 'pristine' directory. * * This specification uses SDB transactions to ensure the consistency of * writes and reads. * * ==== Invariants ==== * * The operating procedures below maintain the following invariants. * These invariants apply at all times except within the SDB txns defined * below. * * * Each row in the PRISTINE table has an associated pristine text file * that is not open for writing and is available for reading and whose * content matches the columns 'size', 'checksum', 'md5_checksum'. * * ==== Operating Procedures ==== * * The steps should be carried out in the order specified. (See rationale.) * * * To add a pristine, do the following inside an SDB txn: * * Add the table row, and set the refcount as desired. If a row * already exists, add the desired refcount to its refcount, and * preferably verify the old row matches the new metadata. * * Create the file. Creation should be fs-atomic, e.g. by moving a * new file into place, so as never to orphan a partial file. If a * file already exists, preferably leave it rather than replace it, * and optionally verify it matches the new metadata (e.g. length). * * * To remove a pristine, do the following inside an SDB txn: * * First, check refcount == 0, and abort if not. * * Delete the table row. * * Delete the file or move it away. (If not present, log a * consistency error but, in a release build, return success.) * * * To query a pristine's existence or SDB metadata, the reader must: * * Ensure no pristine-remove txn is in progress while querying it. * * * To read a pristine text, the reader must: * * Ensure no pristine-remove txn is in progress while querying and * opening it. * * Ensure the pristine text remains in the store continuously from * opening it for the duration of the read. (Perhaps by ensuring * refcount remains >= 1 and/or by cooperating with the clean-up * code.) * * ==== Rationale ==== * * * Adding a pristine: * * We can't add the file *before* the SDB txn takes out a lock, * because that would leave a gap in which another process could * see this file as an orphan and delete it. * * Within the txn, the table row could be added after creating the * file; it makes no difference as it will not become externally * visible until commit. But then we would have to take out a lock * explicitly before adding the file. Adding the row takes out a * lock implicitly, so doing it first avoids an extra step. * * Leaving an existing file in place is less likely to interfere with * processes that are currently reading from the file. Replacing it * might also be acceptable, but that would need further * investigation. * * * Removing a pristine: * * We can't remove the file *after* the SDB txn that updates the * table, because that would leave a gap in which another process * might re-add this same pristine file and then we would delete it. * * Within the txn, the table row could be removed after creating the * file, but see the rationale for adding a pristine. * * In a typical use case for removing a pristine text, the caller * would check the refcount before starting this txn, but * nevertheless it may have changed and so must be checked again * inside the txn. * * * In the add and remove txns, we need to acquire an SDB 'RESERVED' * lock before adding or removing the file. This can be done by starting * the txn with 'BEGIN IMMEDIATE' and/or by performing an SDB write (such * as the table row update). ### Would a 'SHARED' lock be sufficient, * and if so would it be noticably better? * * ==== Notes ==== * * * This procedure can leave orphaned pristine files (files without a * corresponding SDB row) if Subvsersion crashes. The Pristine Store * will still operate correctly. It should be easy to teach "svn cleanup" * to safely delete these. ### Do we need to define the clean-up * procedure here? * * * This specification is conceptually simple, but requires completing disk * operations within SDB transactions, which may make it too inefficient * in practice. An alternative specification could use the Work Queue to * enable more efficient processing of multiple transactions. * * * REFERENCE COUNTING * ================== * * The Pristine Store spec above defines how texts are added and removed * from the store. This spec defines how the addition and removal of * pristine text references within the WC DB are co-ordinated with the * addition and removal of the pristine texts themselves. * * One requirement is to allow a pristine text to be stored some * time before the reference to it is written into the NODES table. The * 'commit' code path, for example, needs to store a file's new pristine * text somewhere (and the pristine store is an obvious option) and then, * when the commit succeeds, update the WC to reference it. * * Store-then-reference could be achieved by: * * (a) Store text outside Pristine Store. When commit succeeds, add it * to the Pristine Store and reference it in the WC; if commit * fails, remove the temporary text. * (b) Store text in Pristine Store with initial ref count = 0. When * commit succeeds, add the reference and update the ref count; if * commit fails, optionally try to purge this pristine text. * (c) Store text in Pristine Store with initial ref count = 1. When * commit succeeds, add the reference; if commit fails, decrement * the ref count and optionally try to purge it. * * Method (a) would require, in effect, implementing an ad-hoc temporary * Pristine Store, which seems needless duplication of effort. It would * also require changing the way the commit code path passes information * around, which might be no bad thing in the long term, but the result * would not appear to have any advantage over method (b). * * Method (b) plays well with automatically maintaining the ref counts * equal to the number of in-SDB references, at the granularity of SDB * txns. It requires an interlock between adding/deleting references and * purging unreferenced pristines - e.g. guard each of these operations by * a WC lock. * * Add a pristine & reference it => any WC lock * (To prevent purging it while adding.) * * Unreference a pristine => no lock needed. * * Unreference a pristine & purge-if-0 => Same as doing these separately. * * Purge any/all refcount==0 pristines => an exclusive WC lock. * (To prevent adding a ref while purging.) * * If a WC lock remains after a crash, then purge refcount==0 pristines. * * Method (c): * * ### Not sure about this one - haven't thought it through in detail... * * Add a pristine & reference in separate steps => any WC lock (?) * * Remove a reference requires ... (nothing more?) * * Find & purge unreferenced pristines requires an exclusive WC lock. * * Ref counts are sometimes too high while a WC lock is held, so * uncertain after a crash if WC locks remain, so need to be re-counted * during clean-up. * * We choose method (b). * * * === Invariants in a Valid WC DB State === * * * No pristine text, even if refcount == 0, will be deleted from the store * as long as any process holds any WC lock in this WC. * * The following conditions are always true outside of a SQL txn: * * * The 'checksum' column in each NODES table row is either NULL or * references a primary key in the 'pristine' table. * * * The 'refcount' column in each PRISTINE table row is equal to the * number of NODES table rows whose 'checksum' column references this * pristine row. * * The following conditions are always true * outside of a SQL txn, * when the Work Queue is empty: * (### ?) when no WC locks are held by any process: * * * The 'refcount' column in a PRISTINE table row equals the number of * NODES table rows whose 'checksum' column references that pristine row. * It may be zero. * * ==== Operating Procedures ==== * * The steps should be carried out in the order specified. * * * To add a pristine text reference to the WC, obtain the text and its * checksum, and then do this while holding a WC lock: * * Add the pristine text to the Pristine Store, setting the desired * refcount >= 1. * * Add the reference(s) in the NODES table. * * * To remove a pristine text reference from the WC, do this while holding * a WC lock: * * Remove the reference(s) in the NODES table. * * Decrement the pristine text's 'refcount' column. * * * To purge an unreferenced pristine text, do this with an *exclusive* * WC lock: * * Check refcount == 0; skip if not. * * Remove it from the pristine store. */ - Julian
