Dependence on git hash and status

[Onath Claridge]

In order to synchronize build information across different processes, we have a generated C file that contains a small amount of git metadata -- the git hash, and an indication of whether the status is clean. Is there a way to make this work? My first thought is to write a genrule that will always rebuild the generated file when it's needed, but I don't see a way to do that.

[Austin Schuh]

The following should give you a pretty good starting point.  Feedback is welcome though if you find issues with it :)

# Generate a cc file with the git revision in a string.

genrule(

  name = 'git_version',

  srcs = glob(['.git/HEAD', '.git/refs/**']),

  outs = [ 

    'git_rev_autogen.cc',

  ],  

  cmd = 'echo "char git_rev_autogen[] = \\"$$(git rev-parse HEAD)\\";"' +

        ' > $(location git_rev_autogen.cc)',

)

cc_library(

  name = 'git_rev_autogen',

  srcs = [ 

    ':git_version',

  ],  

)

Austin

On Tue, May 26, 2015 at 3:33 PM, Onath Claridge <clar...@google.com> wrote:

In order to synchronize build information across different processes, we have a generated C file that contains a small amount of git metadata -- the git hash, and an indication of whether the status is clean. Is there a way to make this work? My first thought is to write a genrule that will always rebuild the generated file when it's needed, but I don't see a way to do that.

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[Jonathan Nieder]

You'll also want .git/packed-refs in srcs.

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[Austin Schuh]

Thanks!  Greatly appreciated.

[Onath Claridge]

Two problems so far... First, a genrule executes in a Bazel-owned environment that isn't a Git repo. It's possible to work around this by writing a script to generate the file and tracing symlinks back to the script's actual location. But then there's the second problem -- the output of the glob() is empty. glob only matches files within the current package, and .git doesn't lie inside of any package.

Is there a file in .git that indicates whether there are any uncommitted modifications? It seems like git wouldn't necessarily know that if you've made changes without running any git commands.

[Austin Schuh]

I placed that in a BUILD file in the root of the repo, so the .git folder is then in the root package.

[Onath Claridge]

Interesting, my Bazel doesn't seem to pay attention to a BUILD file in the root directory. Maybe my version is too old. It does look like I can point to .git with a symlink, though, and glob will recognize the files!

This gets me rebuilds when I check in a change, but this approach doesn't seem to catch uncommitted changes. I'm not too familiar with the contents of .git, but for the reason mentioned before, I'm suspicious whether .git can know that at all.

I'm also curious whether Bazel makes this complicated for a good reason. :)

[Lukács T. Berki]

We should really have support for this, except, well, that we should really have other features, too and so far, this has always drawn the short straw. The internal version has a command line option that points to a script that generates this data, and then this is converted to a language-specific format (e.g. properties for Java, a .h file for C++).

The only problem is that we'd have to figure out what to do with the language-specific workspace status logic (that creates e.g. Java properties files / C++ headers files from {stable,volatile}-status.txt .  I think the right approach is to provide language-specific identifier transformation (e.g. GIT_HASH -> #define GIT_HASH in C++ and bazel.git.hash in Java). We already have some infrastructure for this (inherited from the internal version)

On Wed, May 27, 2015 at 12:33 AM, Onath Claridge <clar...@google.com> wrote:

In order to synchronize build information across different processes, we have a generated C file that contains a small amount of git metadata -- the git hash, and an indication of whether the status is clean. Is there a way to make this work? My first thought is to write a genrule that will always rebuild the generated file when it's needed, but I don't see a way to do that.

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[Onath Claridge]

Austin and Jonathan - thanks for your suggestions!

Lukács - does the internal version provide a way to tell whether there are uncommitted changes? We want to compile in the output of `git status --porcelain | grep -v '^??' | wc -l`.

[Onath Claridge]

From hanwen's internal reply: "For the open-source version,  I don't see problems running "git status". I think it's fast enough."

I filed https://github.com/google/bazel/issues/216.

In the meantime, we'll use a wrapper around the Bazel binary to handle this.