| Submitter | Durham Goode |
|---|---|
| Date | March 17, 2016, 6:31 a.m. |
| Message ID | <cdf4a94fdc46d27196f5.1458196310@dev8486.prn1.facebook.com> |
| Download | mbox | patch |
| Permalink | /patch/13918/ |
| State | Changes Requested |
| Headers | show |
Comments
On 3/16/2016 11:31 PM, Durham Goode wrote: > # HG changeset patch > # User Durham Goode <durham@fb.com> > # Date 1458196059 25200 > # Wed Mar 16 23:27:39 2016 -0700 > # Node ID cdf4a94fdc46d27196f5411fc7a4008690834fba > # Parent ed75909c4c670a7d9db4a2bef9817a0d5f0b4d9c > transaction: move changelog finalizer to be before bookmark finalizer > > Previously, transaction close would run the file generators before running the > finalizers (see the list below for what is in each). Since file generators > contain the bookmarks and the dirstate, this meant we made the dirstate and > bookmarks visible to external readers before we actually wrote the commits into > the changelog, which could result in missing bookmarks and missing working copy > parents (especially on servers with high commit throughput, since pulls might > fail to see certain bookmarks in this situation). > > By moving the changelog writing to be before the bookmark/dirstate writing, we > ensure the commits are present before they are referenced. > > For reference, file generators currently consist of: bookmarks, dirstate, and > phases. Finalizers currently consist of: changelog, revbranchcache, and fncache. > All of the former reference the latter, so therefore the latter should be > written first. > > Technically there's still plenty of race conditions (can the order of finalizers > affect how external readers see the repo?), but this is a step forward at least. > > diff --git a/mercurial/transaction.py b/mercurial/transaction.py > --- a/mercurial/transaction.py > +++ b/mercurial/transaction.py > @@ -407,10 +407,10 @@ class transaction(object): > '''commit the transaction''' > if self.count == 1: > self.validator(self) # will raise exception if needed > - self._generatefiles() > categories = sorted(self._finalizecallback) > for cat in categories: > self._finalizecallback[cat](self) > + self._generatefiles() > > self.count -= 1 > if self.count != 0: > diff --git a/tests/test-bookmarks.t b/tests/test-bookmarks.t > --- a/tests/test-bookmarks.t > +++ b/tests/test-bookmarks.t > @@ -846,3 +846,52 @@ updates the working directory and curren > 6:81dcce76aa0b > $ hg -R ../cloned-bookmarks-update bookmarks | grep ' Y ' > * Y 6:81dcce76aa0b > + > + $ cd .. > + > +ensure changelog is written before bookmarks > + $ hg init orderrepo > + $ cd orderrepo > + $ touch a > + $ hg commit -Aqm one > + $ hg book mybook > + $ echo a > a > + > + $ cat > $TESTTMP/pausefinalize.py <<EOF > + > from mercurial import extensions, localrepo > + > import os, time > + > def transaction(orig, self, desc, report=None): > + > tr = orig(self, desc, report) > + > def sleep(*args, **kwargs): > + > retry = 20 > + > while retry > 0 and not os.path.exists("$TESTTMP/unpause"): > + > retry -= 1 > + > time.sleep(0.5) > + > if os.path.exists("$TESTTMP/unpause"): > + > os.remove("$TESTTMP/unpause") > + > # It is important that this finalizer start with 'a', so it runs before > + > # the changelog finalizer appends to the changelog. > + > tr.addfinalize('a-sleep', sleep) > + > return tr > + > > + > def extsetup(ui): > + > # This extension inserts an artifical pause during the transaction > + > # finalizer, so we can run commands mid-transaction-close. > + > extensions.wrapfunction(localrepo.localrepository, 'transaction', > + > transaction) > + > EOF > + $ hg commit -qm two --config extensions.pausefinalize=$TESTTMP/pausefinalize.py & > + $ sleep 2 > + $ hg log -r . > + changeset: 0:867bc5792c8c > + bookmark: mybook > + tag: tip > + user: test > + date: Thu Jan 01 00:00:00 1970 +0000 > + summary: one > + > + $ hg bookmarks > + * mybook 0:867bc5792c8c > + $ touch $TESTTMP/unpause > + > + $ cd .. > This patch looks minimal and safe to me. The particular race condition that currently exists is really bad in high-commit-rate repositories -- bookmarks often "go missing"altogether, which is a terrible user experience.
On 03/16/2016 11:31 PM, Durham Goode wrote: > # HG changeset patch > # User Durham Goode <durham@fb.com> > # Date 1458196059 25200 > # Wed Mar 16 23:27:39 2016 -0700 > # Node ID cdf4a94fdc46d27196f5411fc7a4008690834fba > # Parent ed75909c4c670a7d9db4a2bef9817a0d5f0b4d9c > transaction: move changelog finalizer to be before bookmark finalizer > > Previously, transaction close would run the file generators before running the > finalizers (see the list below for what is in each). Since file generators > contain the bookmarks and the dirstate, this meant we made the dirstate and > bookmarks visible to external readers before we actually wrote the commits into > the changelog, which could result in missing bookmarks and missing working copy > parents (especially on servers with high commit throughput, since pulls might > fail to see certain bookmarks in this situation). > > By moving the changelog writing to be before the bookmark/dirstate writing, we > ensure the commits are present before they are referenced. > > For reference, file generators currently consist of: bookmarks, dirstate, and > phases. Finalizers currently consist of: changelog, revbranchcache, and fncache. > All of the former reference the latter, so therefore the latter should be > written first. > > Technically there's still plenty of race conditions (can the order of finalizers > affect how external readers see the repo?), but this is a step forward at least. Can we get a more complete analyse on the race condition we have here and which ones we are trading for which others one? For example, this changes could result in phase root being visible after the changesets, making public being seen public while they are not. I would like to have a clear idea of where we are walking there and what tradeoff we are performing.
On 3/18/16 6:07 PM, Pierre-Yves David wrote: > > On 03/16/2016 11:31 PM, Durham Goode wrote: >> # HG changeset patch >> # User Durham Goode <durham@fb.com> >> # Date 1458196059 25200 >> # Wed Mar 16 23:27:39 2016 -0700 >> # Node ID cdf4a94fdc46d27196f5411fc7a4008690834fba >> # Parent ed75909c4c670a7d9db4a2bef9817a0d5f0b4d9c >> transaction: move changelog finalizer to be before bookmark finalizer >> >> Previously, transaction close would run the file generators before >> running the >> finalizers (see the list below for what is in each). Since file >> generators >> contain the bookmarks and the dirstate, this meant we made the >> dirstate and >> bookmarks visible to external readers before we actually wrote the >> commits into >> the changelog, which could result in missing bookmarks and missing >> working copy >> parents (especially on servers with high commit throughput, since >> pulls might >> fail to see certain bookmarks in this situation). >> >> By moving the changelog writing to be before the bookmark/dirstate >> writing, we >> ensure the commits are present before they are referenced. >> >> For reference, file generators currently consist of: bookmarks, >> dirstate, and >> phases. Finalizers currently consist of: changelog, revbranchcache, >> and fncache. >> All of the former reference the latter, so therefore the latter >> should be >> written first. >> >> Technically there's still plenty of race conditions (can the order of >> finalizers >> affect how external readers see the repo?), but this is a step >> forward at least. > > Can we get a more complete analyse on the race condition we have here > and which ones we are trading for which others one? > > For example, this changes could result in phase root being visible > after the changesets, making public being seen public while they are not. > > I would like to have a clear idea of where we are walking there and > what tradeoff we are performing K, given that file generators are "bookmarks, dirstate, and phases" and finalizers are "revlogs, revbranchcache, fncache", the old race conditions are: - bookmarks written before revlogs - causes bookmarks to appear missing during read - dirstate written before revlogs - causes missing working copy parent - phases written before revlogs - should be benign, since any phase on a commit that is missing only affects commits that are also missing - bookmarks/dirstate/phases written before revbranch cache - no issues - bookmarks/dirstate/phases written before fncache - no issues and the new race conditions are: - revlogs written before bookmarks - may see commits where the bookmark hasn't moved on top of them yet; odd but not an invalid state - revlogs written before dirstate - no issues - revlogs written before phases - some commits may show up with the wrong phase (potentially public) - revbranch cache written before bookmarks/dirstate/phases - no issues - fncache written before bookmarks/dirstate/phases - no issues Race conditions that existed before (within the two categories): - fncache before revlog - fncache could point to file that doesn't exist - revlog before fncache - fncache could be missing a file So in summary, my change removes the invalid bookmarks and invalid dirstate case, and replaces it with an out-of-date phase case. The out of date phase case could potentially be bad, since if we send public commits to the client, it may be hard to make them draft later. On the flip side, most central-server setups are publishing servers, so this wouldn't actually be an issue for them. This could be fixed by moving the phase writes to be before the revlogs (or by redoing the entire storage system to be more widely transactional).
On 03/19/2016 10:04 AM, Durham Goode wrote: > > On 3/18/16 6:07 PM, Pierre-Yves David wrote: >> I would like to have a clear idea of where we are walking there and >> what tradeoff we are performing > K, given that file generators are "bookmarks, dirstate, and phases" and > finalizers are "revlogs, revbranchcache, fncache", the old race > conditions are: > > - bookmarks written before revlogs - causes bookmarks to appear missing > during read > - dirstate written before revlogs - causes missing working copy parent I'm not too worried about this one (well, it is annoying and I'm suspecting my thg is getting hit by that from time to time). However, server usually does not have a working copy and clients have no way to access it anyway. > - phases written before revlogs - should be benign, since any phase on a > commit that is missing only affects commits that are also missing We could be seeing changeset as draft while they should be public. This can have some annoying consequence but the next pull should fix it silently, so it is not too bad. (This impact people who have hooks to enforce constraint on there repository state, the race could breaks this guarantee) > - bookmarks/dirstate/phases written before revbranch cache - no issues > - bookmarks/dirstate/phases written before fncache - no issues > > and the new race conditions are: > > - revlogs written before bookmarks - may see commits where the bookmark > hasn't moved on top of them yet; odd but not an invalid state Less bad than the other one but still annoying: (This impact people who have hooks to enforce constraint on there repository state, the race could breaks this guarantee) > - revlogs written before dirstate - no issues > - revlogs written before phases - some commits may show up with the > wrong phase (potentially public) Yep, this pretty bad, having changeset being published "by chance" is pretty bad as this is not really recoverable. Especially there is people using non-publishing repository in production with strict permission for publishing (including, to some extend the mercurial project). We cannot regress this. > - revbranch cache written before bookmarks/dirstate/phases - no issues > - fncache written before bookmarks/dirstate/phases - no issues > > Race conditions that existed before (within the two categories): > - fncache before revlog - fncache could point to file that doesn't exist > - revlog before fncache - fncache could be missing a file > > > So in summary, my change removes the invalid bookmarks and invalid > dirstate case, and replaces it with an out-of-date phase case. The out > of date phase case could potentially be bad, since if we send public > commits to the client, it may be hard to make them draft later. On the > flip side, most central-server setups are publishing servers, so this > wouldn't actually be an issue for them. This could be fixed by moving > the phase writes to be before the revlogs Can you poke at what it would look like to have phases written before and bookmark written after as mitigation? > (or by redoing the entire > storage system to be more widely transactional).
Patch
diff --git a/mercurial/transaction.py b/mercurial/transaction.py --- a/mercurial/transaction.py +++ b/mercurial/transaction.py @@ -407,10 +407,10 @@ class transaction(object): '''commit the transaction''' if self.count == 1: self.validator(self) # will raise exception if needed - self._generatefiles() categories = sorted(self._finalizecallback) for cat in categories: self._finalizecallback[cat](self) + self._generatefiles() self.count -= 1 if self.count != 0: diff --git a/tests/test-bookmarks.t b/tests/test-bookmarks.t --- a/tests/test-bookmarks.t +++ b/tests/test-bookmarks.t @@ -846,3 +846,52 @@ updates the working directory and curren 6:81dcce76aa0b $ hg -R ../cloned-bookmarks-update bookmarks | grep ' Y ' * Y 6:81dcce76aa0b + + $ cd .. + +ensure changelog is written before bookmarks + $ hg init orderrepo + $ cd orderrepo + $ touch a + $ hg commit -Aqm one + $ hg book mybook + $ echo a > a + + $ cat > $TESTTMP/pausefinalize.py <<EOF + > from mercurial import extensions, localrepo + > import os, time + > def transaction(orig, self, desc, report=None): + > tr = orig(self, desc, report) + > def sleep(*args, **kwargs): + > retry = 20 + > while retry > 0 and not os.path.exists("$TESTTMP/unpause"): + > retry -= 1 + > time.sleep(0.5) + > if os.path.exists("$TESTTMP/unpause"): + > os.remove("$TESTTMP/unpause") + > # It is important that this finalizer start with 'a', so it runs before + > # the changelog finalizer appends to the changelog. + > tr.addfinalize('a-sleep', sleep) + > return tr + > + > def extsetup(ui): + > # This extension inserts an artifical pause during the transaction + > # finalizer, so we can run commands mid-transaction-close. + > extensions.wrapfunction(localrepo.localrepository, 'transaction', + > transaction) + > EOF + $ hg commit -qm two --config extensions.pausefinalize=$TESTTMP/pausefinalize.py & + $ sleep 2 + $ hg log -r . + changeset: 0:867bc5792c8c + bookmark: mybook + tag: tip + user: test + date: Thu Jan 01 00:00:00 1970 +0000 + summary: one + + $ hg bookmarks + * mybook 0:867bc5792c8c + $ touch $TESTTMP/unpause + + $ cd ..