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Re: [PATCH 8/8] reflog_expire(): lock symbolic refs themselves, not their referent

From
Junio C Hamano <gitster@pobox.com>
Date
Feb 13, 2015, 21:53 UTC
Message-ID
<xmqq3869mqbf.fsf@gitster.dls.corp.google.com>
In-Reply-To
<54DE5A72.6030106@alum.mit.edu>
Michael Haggerty <mhagger@alum.mit.edu> writes:
Show 7 quoted lines
> Now back to the real world. Currently, if R is changed *through* a
> symbolic reference S, then the reflogs for both R and S are updated, but
> not the reflogs for any other symbolic references that might point at R.
> If R is changed directly, then no symref's reflogs are affected, except
> for the special case that HEAD's reflog is changed if it points directly
> at R. This limitation is a hack to avoid having to walk symrefs
> backwards to find any symrefs that might be pointing at R.
Yup.
Show 5 quoted lines
> It might actually not be extremely expensive to follow symrefs
> backwards. Symbolic references cannot be packed, so we would only have
> to scan the loose references; we could ignore packed refs. But it would
> still be a lot more expensive than just updating one file. I don't know
> that it's worth it, given that symbolic references are used so sparingly.

I personally do not think it is worth it. I further think that it would be perfectly OK to do one of the following:

    - We only maintain reflogs for $GIT_DIR/HEAD; no other symrefs
      get their own reflog, and we only check $GIT_DIR/HEAD when
      updating refs/heads/* and no other refs for direct reference
      (i.e. HEAD -> refs/something/else -> refs/heads/master symref
      chain is ignored).
    - In addition to the above, we also maintain reflogs for
      $GIT_DIR/refs/remotes/*/HEAD but support only when they
      directly point into a remote tracking branch in the same
      hierarchy.  $GIT_DIR/refs/remotes/foo/HEAD that points at
      $GIT_DIR/refs/remotes/bar/master is ignored and will get an
      undefined behaviour.
Show 6 quoted lines
> I think that the rule about locks as expressed above can be carried over
> the the real world:
>
>     We should hold the locks on exactly those references (symbolic
>     or regular) whose reflogs we plan to change. We should acquire all
>     of the locks before making any changes.
Sure.
Previous: Michael HaggertyNext: Michael Haggerty
Message 42 of 46 in “Fix some problems with reflog expiration”
  1. 0/8 Fix some problems with reflog expirationMichael Haggerty, Feb 9, 2015
  2. 1/8 write_ref_sha1(): remove check for lock == NULLMichael Haggerty, Feb 9, 2015
  3. Stefan BellerFeb 10, 2015
  4. Jeff KingFeb 11, 2015
  5. 2/8 write_ref_sha1(): Move write elision test to callersMichael Haggerty, Feb 9, 2015
  6. Junio C HamanoFeb 12, 2015
  7. 3/8 lock_ref_sha1_basic(): do not set force_write for missing referencesMichael Haggerty, Feb 9, 2015
  8. Stefan BellerFeb 10, 2015
  9. Jeff KingFeb 11, 2015
  10. Stefan BellerFeb 11, 2015
  11. Michael HaggertyFeb 12, 2015
  12. 4/8 reflog: fix documentationMichael Haggerty, Feb 9, 2015
  13. Stefan BellerFeb 10, 2015
  14. 5/8 reflog: rearrange the manpageMichael Haggerty, Feb 9, 2015
  15. Stefan BellerFeb 10, 2015
  16. Michael HaggertyFeb 12, 2015
  17. Junio C HamanoFeb 12, 2015
  18. 6/8 reflog_expire(): ignore --updateref for symbolic referencesMichael Haggerty, Feb 9, 2015
  19. Stefan BellerFeb 11, 2015
  20. Michael HaggertyFeb 12, 2015
  21. Stefan BellerFeb 12, 2015
  22. Junio C HamanoFeb 12, 2015
  23. Jeff KingFeb 12, 2015
  24. Michael HaggertyFeb 13, 2015
  25. 7/8 reflog_expire(): never update a reference to null_sha1Michael Haggerty, Feb 9, 2015
  26. Eric SunshineFeb 9, 2015
  27. Michael HaggertyFeb 12, 2015
  28. 8/8 reflog_expire(): lock symbolic refs themselves, not their referentMichael Haggerty, Feb 9, 2015
  29. Stefan BellerFeb 11, 2015
  30. Junio C HamanoFeb 11, 2015
  31. Stefan BellerFeb 11, 2015
  32. Michael HaggertyFeb 12, 2015
  33. Stefan BellerFeb 12, 2015
  34. Michael HaggertyFeb 13, 2015
  35. Stefan BellerFeb 13, 2015
  36. Junio C HamanoFeb 13, 2015
  37. Stefan BellerFeb 13, 2015
  38. Junio C HamanoFeb 13, 2015
  39. Stefan BellerFeb 13, 2015
  40. Junio C HamanoFeb 13, 2015
  41. Michael HaggertyFeb 13, 2015
  42. Junio C HamanoFeb 13, 2015
  43. Michael HaggertyFeb 14, 2015
  44. Stefan BellerFeb 9, 2015
  45. refs.c: get rid of force_write flagStefan Beller, Feb 10, 2015
  46. Michael HaggertyFeb 12, 2015

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