Re: [PATCH 1/2] [GSOC] cat-file: fix --batch report changed-type bug

Jeff King [off-list ref] 于2021年6月3日周四 上午4:01写道：

I don't see any inaccuracies there. I do think we could explain it a bit
more succinctly. I'll give my attempt, and then you can pick and choose
which parts to include between ours. :)

Yes, you wrote it more concisely. I don't need to write out the details of
those variables. I just need to tell the reader the introduction of
skip_object_info will break the assumption of --batch that we have found
the object type.

  Subject: cat-file: handle trivial --batch format with --batch-all-objects

  The --batch code to print an object assumes we found out the type of
  the object from calling oid_object_info_extended(). This is true for
  the default format, but even in a custom format, we manually modify
  the object_info struct to ask for the type.

  This assumption was broken by 845de33a5b (cat-file: avoid noop calls
  to sha1_object_info_extended, 2016-05-18). That commit skips the call
  to oid_object_info_extended() entirely when --batch-all-objects is in
  use, and the custom format does not include any placeholders that
  require calling it.

  This results in an error when we try to confirm that the type didn't
  change:

    $ git cat-file --batch=batman --batch-all-objects
    batman
    fatal: object 000023961a0c02d6e21dc51ea3484ff71abf1c74 changed type!?

  and also has other subtle effects (e.g., we'd fail to stream a blob,
  since we don't realize it's a blog in the first place).

s/blog/blob/

  We can fix this by flipping the order of the setup. The check for "do
  we need to get the object info" must come _after_ we've decided
  whether we need to look up the type.

-Peff

Thanks.
--
ZheNing Hu