No comments on the content, but reading the ensuing email thread, it
may be useful to put the document in notes/wc-ng/pristines, and add
questions / comments / corrections there. It would allow folks down
the road to see all the critiques in one location, rather than reading
On Tue, Feb 15, 2011 at 3:06 PM, Julian Foad <julian.foad_at_wandisco.com> wrote:
> 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
Received on 2011-02-16 14:51:17 CET