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Re: [PATCH 28/31] dir: make untracked cache extension hash size independent

From
brian m. carlson <sandals@crustytoothpaste.net>
Date
Feb 13, 2019, 00:30 UTC
Message-ID
<20190213003041.GI684736@genre.crustytoothpaste.net>
In-Reply-To
<87y36ljmcu.fsf@evledraar.gmail.com>
On Tue, Feb 12, 2019 at 12:08:01PM +0100, Ævar Arnfjörð Bjarmason wrote:
Show 9 quoted lines
> Both this & the follow-up 29/31 look scary since this is an on-disk
> structure and this patch & the next one rather than implementing some
> transition, just changes the structs & code we use to read & write to
> use the current hash size.
> 
> So what are we going to do when the "current" size is SHA-256 and our
> on-disk cache is SHA-1? It seems like with this we'd at best (I haven't
> tested) throw away the SHA-1 UC data, and at worst introduce some silent
> persistent read failure.

That's a good question. The current design has everything in the .git directory other than the loose object table and the pack indices in one algorithm. That is, the untracked cache extension, like the rest of the index, will always be SHA-256 when the objects are stored in SHA-256.

Having mixed algorithms in the .git directory would add a lot of complexity.

Show 5 quoted lines
> In any case that seems like something we should have tests for with an
> on-disk format, i.e. write in one hash, see what happens when we read in
> another, and perhaps instead of not understanding SHA-1 hashes in
> SHA-256 mode we'd read the SHA-1 and consult the SHA-1<->SHA-256 lookup
> table we're going to be eventually maintaining on the side?

Correct. The current plan is that when the transition code is fully implemented, we'll read object IDs from the user in whatever algorithms we support, translate them into the default algorithm, and then use them.

-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204
Previous: Ævar Arnfjörð BjarmasonNext: brian m. carlson
Message 40 of 56 in “Hash function transition part 16”
  1. 00/31 Hash function transition part 16brian m. carlson, Feb 12, 2019
  2. 01/31 t/lib-submodule-update: use appropriate length constantbrian m. carlson, Feb 12, 2019
  3. 03/31 pack-bitmap: convert struct stored_bitmap to object_idbrian m. carlson, Feb 12, 2019
  4. 02/31 pack-bitmap: make bitmap header handling hash agnosticbrian m. carlson, Feb 12, 2019
  5. 05/31 pack-bitmap: switch hard-coded constants to the_hash_algobrian m. carlson, Feb 12, 2019
  6. Ævar Arnfjörð BjarmasonFeb 12, 2019
  7. 04/31 pack-bitmap: replace sha1_to_hexbrian m. carlson, Feb 12, 2019
  8. Jeff KingFeb 12, 2019
  9. brian m. carlsonFeb 13, 2019
  10. Jeff KingFeb 14, 2019
  11. 08/31 notes: make hash size independentbrian m. carlson, Feb 12, 2019
  12. Eric SunshineFeb 12, 2019
  13. brian m. carlsonFeb 12, 2019
  14. 09/31 notes: replace sha1_to_hexbrian m. carlson, Feb 12, 2019
  15. 11/31 builtin/name-rev: make hash-size independentbrian m. carlson, Feb 12, 2019
  16. 06/31 submodule: avoid hard-coded constantsbrian m. carlson, Feb 12, 2019
  17. 10/31 object-store: rename and expand packed_git's sha1 memberbrian m. carlson, Feb 12, 2019
  18. Eric SunshineFeb 12, 2019
  19. brian m. carlsonFeb 14, 2019
  20. 14/31 builtin/am: make hash size independentbrian m. carlson, Feb 12, 2019
  21. 15/31 builtin/pull: make hash-size independentbrian m. carlson, Feb 12, 2019
  22. Eric SunshineFeb 12, 2019
  23. 13/31 fast-import: replace sha1_to_hexbrian m. carlson, Feb 12, 2019
  24. 17/31 http-backend: allow 64-character hex namesbrian m. carlson, Feb 12, 2019
  25. 16/31 http-push: convert to use the_hash_algobrian m. carlson, Feb 12, 2019
  26. 12/31 fast-import: make hash-size independentbrian m. carlson, Feb 12, 2019
  27. Eric SunshineFeb 12, 2019
  28. brian m. carlsonFeb 12, 2019
  29. 18/31 http-push: remove remaining uses of sha1_to_hexbrian m. carlson, Feb 12, 2019
  30. 20/31 http: replace hard-coded constant with the_hash_algobrian m. carlson, Feb 12, 2019
  31. 22/31 http: replace sha1_to_hexbrian m. carlson, Feb 12, 2019
  32. 21/31 http: compute hash of downloaded objects using the_hash_algobrian m. carlson, Feb 12, 2019
  33. 23/31 remote-curl: make hash size independentbrian m. carlson, Feb 12, 2019
  34. Ævar Arnfjörð BjarmasonFeb 12, 2019
  35. 19/31 http-walker: replace sha1_to_hexbrian m. carlson, Feb 12, 2019
  36. Eric SunshineFeb 12, 2019
  37. 29/31 read-cache: read data in a hash-independent waybrian m. carlson, Feb 12, 2019
  38. 28/31 dir: make untracked cache extension hash size independentbrian m. carlson, Feb 12, 2019
  39. Ævar Arnfjörð BjarmasonFeb 12, 2019
  40. brian m. carlsonFeb 13, 2019
  41. 25/31 archive: convert struct archiver_args to object_idbrian m. carlson, Feb 12, 2019
  42. 26/31 refspec: make hash size independentbrian m. carlson, Feb 12, 2019
  43. 31/31 gitweb: make hash size independentbrian m. carlson, Feb 12, 2019
  44. Ævar Arnfjörð BjarmasonFeb 12, 2019
  45. 30/31 Git.pm: make hash size independentbrian m. carlson, Feb 12, 2019
  46. Ævar Arnfjörð BjarmasonFeb 12, 2019
  47. brian m. carlsonFeb 18, 2019
  48. Ævar Arnfjörð BjarmasonFeb 18, 2019
  49. 24/31 archive-tar: make hash size independentbrian m. carlson, Feb 12, 2019
  50. René ScharfeFeb 12, 2019
  51. René ScharfeFeb 12, 2019
  52. brian m. carlsonFeb 13, 2019
  53. 27/31 builtin/difftool: use parse_oid_hexbrian m. carlson, Feb 12, 2019
  54. Eric SunshineFeb 12, 2019
  55. 07/31 notes-merge: switch to use the_hash_algobrian m. carlson, Feb 12, 2019
  56. Ævar Arnfjörð BjarmasonFeb 12, 2019

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