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Re: [PATCH 07/23] expire_reflog(): use a lock_file for rewriting the reflog file

From
Michael Haggerty <mhagger@alum.mit.edu>
Date
Dec 8, 2014, 10:40 UTC
Message-ID
<54858038.9090004@alum.mit.edu>
In-Reply-To
<CAN05THTH2XDaUmchehF0gY0-GSgD_O9rJH7F5Gc3dHLCPz3GsA@mail.gmail.com>
On 12/05/2014 03:59 AM, ronnie sahlberg wrote:
Show 15 quoted lines
> On Thu, Dec 4, 2014 at 3:08 PM, Michael Haggerty <mhagger@alum.mit.edu> wrote:
>> We don't actually need the locking functionality, because we already
>> hold the lock on the reference itself,
> 
> No. You do need the lock.
> The ref is locked only during transaction_commit()
> 
> If you don't want to lock the reflog file and instead rely on the lock
> on the ref itself you will need to
> rework your patches so that the lock on the ref is taken already
> during, for example, transaction_update_ref() instead.
> 
> But without doing those changes and moving the ref locking from
> _commit() to _update_ref() you will risk reflog corruption/surprises
> if two operations collide and both rewrite the reflog without any lock held.
Ronnie, I don't understand your comments.

It is a statement of fact (to the best of my knowledge) that reflogs are supposed to be modified only under a lock on the corresponding reference, namely "$GIT_DIR/refs/$refname.lock". We do not require reflog writers to hold "$GIT_DIR/logs/$refname.lock".

In this function, "$GIT_DIR/logs/$refname.lock" happens to be the name of the temporary file being used to stage the new contents of the reflog. But that is more or less a coincidence; we could call the temporary file whatever we want because it has no locking implications. However, what we want to do with the file in this code path (write a new version then rename the new version on top of the old version, deleting the temporary file if the program is interrupted) is the same as what we do with lockfiles, so we use the lockfile code because it is convenient.

This patch series has nothing to do with ref_transaction_commit() or any of the transaction machinery. It has to do with expire_reflog(), which is invoked outside of any transaction.

Michael
-- 
Michael Haggerty
mhagger@alum.mit.edu
Previous: ronnie sahlbergNext: Michael Haggerty
Message 27 of 63 in “Add reflog_expire() to the references API”
  1. 00/23 Add reflog_expire() to the references APIMichael Haggerty, Dec 4, 2014
  2. 01/23 refs.c: make ref_transaction_create a wrapper for ref_transaction_updateMichael Haggerty, Dec 4, 2014
  3. 02/23 refs.c: make ref_transaction_delete a wrapper for ref_transaction_updateMichael Haggerty, Dec 4, 2014
  4. 03/23 refs.c: add a function to append a reflog entry to a fdMichael Haggerty, Dec 4, 2014
  5. 04/23 expire_reflog(): remove unused parameterMichael Haggerty, Dec 4, 2014
  6. Jonathan NiederDec 4, 2014
  7. Jonathan NiederDec 4, 2014
  8. Michael HaggertyDec 5, 2014
  9. 05/23 expire_reflog(): rename "ref" parameter to "refname"Michael Haggerty, Dec 4, 2014
  10. Jonathan NiederDec 4, 2014
  11. 06/23 expire_reflog(): exit early if the reference has no reflogMichael Haggerty, Dec 4, 2014
  12. Jonathan NiederDec 4, 2014
  13. Jonathan NiederDec 4, 2014
  14. Michael HaggertyDec 5, 2014
  15. 07/23 expire_reflog(): use a lock_file for rewriting the reflog fileMichael Haggerty, Dec 4, 2014
  16. Jonathan NiederDec 5, 2014
  17. Stefan BellerDec 5, 2014
  18. Michael HaggertyDec 8, 2014
  19. Junio C HamanoDec 9, 2014
  20. Jeff KingDec 9, 2014
  21. Stefan BellerDec 5, 2014
  22. Junio C HamanoDec 5, 2014
  23. Stefan BellerDec 5, 2014
  24. Junio C HamanoDec 5, 2014
  25. Michael HaggertyDec 8, 2014
  26. ronnie sahlbergDec 5, 2014
  27. Michael HaggertyDec 8, 2014
  28. 08/23 Extract function should_expire_reflog_ent()Michael Haggerty, Dec 4, 2014
  29. Stefan BellerDec 8, 2014
  30. 09/23 expire_reflog(): extract two policy-related functionsMichael Haggerty, Dec 4, 2014
  31. Stefan BellerDec 5, 2014
  32. 10/23 expire_reflog(): add a "flags" argumentMichael Haggerty, Dec 4, 2014
  33. Stefan BellerDec 8, 2014
  34. 11/23 expire_reflog(): move dry_run to flags argumentMichael Haggerty, Dec 4, 2014
  35. Stefan BellerDec 8, 2014
  36. 12/23 expire_reflog(): move updateref to flags argumentMichael Haggerty, Dec 4, 2014
  37. Stefan BellerDec 8, 2014
  38. 13/23 Rename expire_reflog_cb to expire_reflog_policy_cbMichael Haggerty, Dec 4, 2014
  39. Stefan BellerDec 8, 2014
  40. 14/23 struct expire_reflog_cb: a new callback data typeMichael Haggerty, Dec 4, 2014
  41. Stefan BellerDec 8, 2014
  42. 15/23 expire_reflog(): pass flags through to expire_reflog_ent()Michael Haggerty, Dec 4, 2014
  43. Stefan BellerDec 8, 2014
  44. 16/23 expire_reflog(): move verbose to flags argumentMichael Haggerty, Dec 4, 2014
  45. Stefan BellerDec 8, 2014
  46. 17/23 expire_reflog(): move rewrite to flags argumentMichael Haggerty, Dec 4, 2014
  47. Stefan BellerDec 8, 2014
  48. 18/23 Move newlog and last_kept_sha1 to "struct expire_reflog_cb"Michael Haggerty, Dec 4, 2014
  49. Stefan BellerDec 8, 2014
  50. 19/23 expire_reflog(): treat the policy callback data as opaqueMichael Haggerty, Dec 4, 2014
  51. Stefan BellerDec 8, 2014
  52. 20/23 reflog_expire(): new function in the reference APIMichael Haggerty, Dec 4, 2014
  53. Stefan BellerDec 8, 2014
  54. Michael HaggertyDec 12, 2014
  55. Jeff KingDec 12, 2014
  56. Junio C HamanoDec 12, 2014
  57. 21/23 refs.c: remove unlock_ref/close_ref/commit_ref from the refs apiMichael Haggerty, Dec 4, 2014
  58. 22/23 lock_any_ref_for_update(): inline functionMichael Haggerty, Dec 4, 2014
  59. Stefan BellerDec 8, 2014
  60. Michael HaggertyDec 11, 2014
  61. 23/23 refs.c: don't expose the internal struct ref_lock in the header fileMichael Haggerty, Dec 4, 2014
  62. Junio C HamanoDec 4, 2014
  63. Stefan BellerDec 5, 2014

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