[GSoC] Git Blog 4

[ZheNing Hu <adlternative@gmail.com>]

My fourth week blog finished:
The web version is here:
https://adlternative.github.io/GSOC-Git-Blog-3/

## Week4: Trouble is a friend

At the beginning of this week , since my previous code
broke some Github CI tests , I tried to solve these bugs
related to the atom `%(raw)` . The most confusing thing
is that some bugs may pass the tests of your local machine,
but fail to pass in the CI of GitHub .

E.g. I need to add the `GPG` prerequisites to the test like this:

```sh
test_expect_success GPG 'basic atom: refs/tags/signed-empty raw' '
git cat-file tag refs/tags/signed-empty >expected &&
git for-each-ref --format="%(raw)" refs/tags/signed-empty >actual &&
sanitize_pgp <expected >expected.clean &&
sanitize_pgp <actual >actual.clean &&
echo "" >>expected.clean &&
test_cmp expected.clean actual.clean
'
```

Otherwise, some operating systems that do not contain GnuPG
may not be able to perform related tests.

In addition, some scripts like `printf "%b" "a\0b\0c" >blob1` will
be truncated at the first NUL on a 32-bit machine, but it performs
well on 64-bit machines, and NUL is normally stored in the file.
This made me think that Git's file decompression had an error on
a 32-bit machine before I used Ubuntu32's docker container to
clone the git repository and In-depth analysis of bugs... In the end,
I used `printf "a\0b\0c"` to make 32-bit machines not truncated
in NUL. Is there a better way to write binary data onto a file than
`printf` and `echo`?

Since I am a newbie to docker, I would like to know if there is any
way to run the Git's Github CI program remotely or locally?

In the second half of this week, I tried to make `cat-file` reuse the
logic of `ref-filter`. I have to say that this is a very difficult process.
"rebase -i" again and again to repair the content of previous commits.
squeeze commits, split commits, modify commit messages... Finally, I
submitted the patches to the Git mailing list in
[[PATCH 0/8] [GSOC][RFC] cat-file: reuse `ref-filter`
logic](https://lore.kernel.org/git/pull.980.git.1623496458.gitgitgadget@gmail.com/).
Now `cat-file` has learned most of the atoms in `ref-filter`. I am very
happy to be able to make git support richer functions through my own code.

Regrettably, `git cat-file --batch --batch-all-objects` seems to take up
a huge amount of memory on a large repo such as git.git, and it will
be killed by Linux's oom. This is mainly because we will make a large
number of copies of the object's raw data. The original `git cat-file`
uses `read_object_file()` or `stream_blob()` to output the object's
raw data, but in `ref-filter`, we have to use `v->s` to copy the object's
data, it is difficult to eliminate `v->s` and print the output directly to the
final output buffer. Because we may have atoms like `%(if)`, `%(else)`
that need to use buffers on the stack to build the final output string
layer by layer, or the `cmp_ref_sorting()` needs to use `v->s` to
compare two refs. In short, it is very difficult for `ref-filter` to reduce
copy overhead. I even thought about using the string pool API
`memintern()` to replace `xmemdupz()`, but it seems that the effect
is not obvious. A large number of objects' data will still reside in memory,
so this may not be a good method.

Anyway, stay confident. I can solve these difficult problems with
the help of mentors and reviewers. `:)`