Julian Foad wrote on Wed, Feb 16, 2011 at 19:27:19 +0000: > On Wed, 2011-02-16, Daniel Shahaf wrote: > > Julian Foad wrote on Tue, Feb 15, 2011 at 15:06:43 +0000: > > > * > > > * 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. > > > * > > > > Nit: > > This doesn't say how this spec ensures the disk is consistent with the DB. > > Do you mean this intro should be more specific and say sthg like > > "This spec defines how the store operates so as to ensure > * consistency between disk and DB; > * atomicity of, and arbitration between, add and delete and read > operations." >
Obviously not the most critical change to make, but the proposed text sounds good. > > > * ==== 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). > > > > What happens if you crash (kill -9) between these two steps? > > > > (Reading ahead: I assume that in that case you'll leave a stray wc lock > > or wq entry behind?) > > Not at this level of the spec. Here, all that happens is that the SDB > txn prepares to commit a change to the table, but if we crash that txn > would be abandoned and that change would never happen. > > On the other hand, if we crash after those two steps but before > committing (finalizing) the SDB txn, then the file would exist on disk > as an "orphan", which is harmless. > > Moving up a layer to the "Reference Counting" layer of the spec, then > yes a WC lock is taken out before running this txn, so yes that WC lock > would be left behind if we crash anywhere in this sequence (between the > steps or before or after them). > Clear, thanks. > > > * * 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? > > > * > > > > Does that mean "We need an SDB lock that blocks both readers and writers" > > in non-sqlite-specific terms? > > Yes. > OK. (Having it in non-sqlite-specific terms help follow the spec, IMO.) > > > * ==== 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? > > > * > > > > Yes please. (It bypasses some of the DB-keyed logic so I think it'll be > > good to spell it out) > > Okay, will do. > Thanks. > > > * 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. > > > * > > > > To clarify: I believe you mean "Needs" as it's currently designed, > > rather than "Needs" due to the inherent semantics of a 'commit' > > operation. (Extreme example: 'commit' of rN could leave the wc at > > rN-1.) > > Yes. > > > > * 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. > > > * > > > > I don't understand this paragraph. What does it propose to do in order > > to avoid purging pristines that have refcount=0 while a commit is in > > progress? Does it propose that 'commit' take a lock for the duration > > from adding the pristine to incrementing its refcount? > > Yes: in the case of the way "commit" is currently implemented, where > "add a pristine" comes first and then "reference it" comes later, the > commit operation should hold a WC lock during that whole time. > Understood, thanks. > > > * 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. > > > * > > > > I think (c) has some good points. There's a variant, (c'), where > > instead of overloading the refcount field you use a separate > > refcounts_by_commitsinprogress flag; in that scheme, 'find & purge' > > doesn't need a lock, it just needs to respect that flag. > > Food for thought, but I find that working carefully through these > scenarios is quite time consuming. A large part of my aim here is to > try to find a good scheme without having to first define all the > possible schemes in detail. Your comments are helping me to do that. > Glad to help. > > > * 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. > > > * > > > > Surely you mean "any *other* process" :-) > > Yes - something like that, though that's still not quite right. > *nod* due to multi-threaded processes. Perhaps s/process/WC context object/, or some other appropriate API entity. > > > * 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. > > > * > > > > Will we have wc-checks.sql triggers enforce these? > > The first one is enforced by constraints defined in wc-metadata.sql. > > The second one is currently assisted by triggers defined in > wc-metadata.sql but not strictly enforced. I would like to enforce it > but haven't looked at doing so yet. > Sounds good. > > > * 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. > > > * > > > > IOW, the refcount may be zero if the work queue is non empty. > > No, I meant the number of refs does not necessarily match the refcount > column if the work queue is non empty. When WQ is empty, refcount and > number of refs always match each other and may (both together) be zero > or more than zero. > I haven't re-read the >>>-quoted bullet poitns, but the >-quoted paragraph (i.e., what you just said) sounds good. (Hmm, there's another invariant --- if refcount>0 then the file exists --- but I'm not sure offhand what section of the spec it belongs in.) > > But a few pages above you said that this specification doesn't use the > > work queue to manage the pristine files. Why is the work queue involved > > here then? Does this spec use the work queue for the disk-tree files > > but not for the .svn/pristine/ files? > > I think I've messed up this part. I'll take another look. I'm not sure > whether the Work Queue needs to be involved at all. > OK. > > > * ==== 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. > > > * > > > > So the first bullet will take an SDB lock, release it, and then the > > reference will be added while the WC lock is held (and within a separate > > SDB txn/lock)? > > Yes. That's to support cases like the current "commit" implementation. > In certain other cases, these could be collapsed into a single txn. > OK. > > > * * 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. > > > > Why is 'purge an unreferenced pristine text' part of the operating > > procedures for incrementing/decrementing refcounts? It doesn't belong > > here, does it? > > > > (i.e., there is a 'manage the pristine store' layer --- first part of > > your email --- and a 'manage the references' layer, here) > > Well, it's because in the lower "manage the pristine store" layer, > purging depends only on refcount==0 (and no simultaneous add or remove). > At this higher "manage the references" layer, purging must be avoided > when the WC is in the middle of a WC-lock operation. In other words, > this upper layer's "purge" is a wrapper that adds an extra level of > locking around the lower layer's "purge". It's not safe to call the > lower-layer API directly. > In other words, the lower-layer API is oblivious of WC locks. I see. That said, I just noticed that your bullet points have both a "wc lock" and an "exclusive wc lock" --- I had overlooked that distinction earlier. I assume that an 'exclusive' lock is a recursive lock from the wc root and below? Thanks, Daniel (I'm behind on following the commits@ list --- if there are relevant commits/milestones of the doc, feel free to direct my attention to them explicitly)
