[Bug 3564] New: obsolete store can be read from partial data

[bugzilla-daemon at bz.selenic.com]

http://bz.selenic.com/show_bug.cgi?id=3564

          Priority: normal
            Bug ID: 3564
                CC: mercurial-devel at selenic.com
          Assignee: bugzilla at selenic.com
           Summary: obsolete store can be read from partial data
          Severity: bug
    Classification: Unclassified
                OS: Mac OS
          Reporter: patrick at mezard.eu
          Hardware: PC
            Status: UNCONFIRMED
           Version: 2.2.3
         Component: Mercurial
           Product: Mercurial

Writing to localrepo.obsstore is done by locking the store, starting a
transaction and appending markers data.

Reading does not lock anything, relying on the wrong assumption the transaction
and log structured format would prevent every issue. But transactions do not
provide isolation and obsolete._readmarkers() can perfectly read partial data.

An easy fix is to write the obstore with atomictemp=True, as we do with other
metadata files like dirstate or bookmarks. I was a little concerned doing this
with obsstore which can potentially gets much bigger than bookmarks or
branchheads. But we already do it for dirstate...

Or we can have a sibling file storing the obsstore valid data size, which is
updated atomically. It is tempting to try to read the transaction file if any
to get this number, but I do not think there is a reliable way to do that.

(As a side note, obsolete markers being pushed after the changegroup, clients
could pull revisions before the related markers are made available).

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