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Demonstration of Bazel runfiles bug

This repository demonstrates a potential bug. bazelbuild/bazel#12033. The bug is a race condition in runfiles tree creation, possibly Windows ONLY.

Tested on:

  • Windows 10
  • Long paths and symlinks enabled for the user running this.
  • Bazel 3.4.1
  • See .bazelrc for option that forces Bazel to create runfiles tree on Windows.

Details of the bug

If:

  1. a rule depends on other rules to create its runfiles,
  2. The dependency creates a directory, and
  3. Due to dependencies of the dependencies taking time to build, the directory that should end up in runfiles takes a while to build.

Then: It is possible for the runfiles tree to have a file symlink instead of a directory symlink.

Expected behavior: The runfiles tree should always be created after all runfiles dependencies are ready, and directories should be represented by directory symlinks in the runfiles tree.

Actual behavior: The runfiles tree is created before runfiles dependencies are ready, leading to an incorrect file symlink to a directory.

This happens because Bazel seems to create the runfiles tree before the directory creating action is run. When Bazel does this, it uses build-runfiles.exe on Windows. This parses the runfiles manifest and creates symlinks. Symlinks are created here. Since this executable is invoked too early, the blaze_util::IsDirectoryW check returns false, so a file symlink is created. This is allowed by Windows as symlinks may point to non-existent files. This is probably why the bug was not caught earlier.

How to use this repository to see the bug.

  1. Clone the repo
  2. Read through the BUILD file and dummy.bzl to understand what is going on. Basically, user is just a simple batch script target that prints out the runfiles tree. It depends on mydummy which produces a directory (mydummy\lib) that is part of user's runfiles. This directory itself depends on i_sleep, which just sleeps to make the race condition obvious.
  3. It can be useful to use the Process Monitor tools from sys-internals, with the following filters to see the commands executed. The green filters are what I've added and the red ones are defaults.

procmon filters

It is very important to bazel clean between runs, otherwise the directory will already exist, the race will not manifest, and everything will work correctly.

Bug happens

bazel clean
bazel run :user

Output

INFO: Analyzed target //:user (4 packages loaded, 8 targets configured).
INFO: Found 1 target...
Target //:user up-to-date:
  bazel-bin/user.bat
INFO: Elapsed time: 12.811s, Critical Path: 9.72s
INFO: 2 processes: 2 local.
INFO: Build completed successfully, 6 total actions
INFO: Build completed successfully, 6 total actions
------------CURRENT DIR--------------
 Volume in drive C is system
 Volume Serial Number is AA07-0727

 Directory of c:\users\nikhilm\_bazel~1\tvcdbfus\execroot\__main__\bazel-~1\x64_wi~1\bin\userba~2.run\__main__

09/01/2020  10:40 AM    <DIR>          .
09/01/2020  10:40 AM    <DIR>          ..
09/01/2020  10:40 AM    <DIR>          mydummy
09/01/2020  10:40 AM    <SYMLINK>      user.bat [\\?\C:\users\nikhilm\_bazel_nikhilm\tvcdbfus\execroot\__main__\bazel-out\x64_windows-fastbuild\bin\user.bat]
               1 File(s)              0 bytes
               3 Dir(s)  842,240,598,016 bytes free
------------mydummy DIR--------------
 Volume in drive C is system
 Volume Serial Number is AA07-0727

 Directory of c:\users\nikhilm\_bazel~1\tvcdbfus\execroot\__main__\bazel-~1\x64_wi~1\bin\userba~2.run\__main__\mydummy

09/01/2020  10:40 AM    <DIR>          .
09/01/2020  10:40 AM    <DIR>          ..
09/01/2020  10:40 AM    <SYMLINK>      lib [\\?\C:\users\nikhilm\_bazel_nikhilm\tvcdbfus\execroot\__main__\bazel-out\x64_windows-fastbuild\bin\mydummy\lib]
               1 File(s)              0 bytes
               2 Dir(s)  842,240,598,016 bytes free

Observe how it says SYMLINK.

Also notice how build-runfiles.exe ran before java.exe.

bad ordering bad detail

Bug doesn't happen

If we remove the delay, the directory creation action can sneak in before runfiles are created and things work. To do this, comment out the i_sleep dependency in dummy in the BUILD file.

dummy_dir(
    name = "mydummy",
    # sleep_on = ":i_sleep",
)
bazel clean
bazel run :user

Output:

INFO: Analyzed target //:user (4 packages loaded, 7 targets configured).
INFO: Found 1 target...
Target //:user up-to-date:
  bazel-bin/user.bat
INFO: Elapsed time: 3.092s, Critical Path: 2.16s
INFO: 1 process: 1 local.
INFO: Build completed successfully, 5 total actions
INFO: Build completed successfully, 5 total actions
------------CURRENT DIR--------------
 Volume in drive C is system
 Volume Serial Number is AA07-0727

 Directory of c:\users\nikhilm\_bazel~1\tvcdbfus\execroot\__main__\bazel-~1\x64_wi~1\bin\userba~2.run\__main__

09/01/2020  10:46 AM    <DIR>          .
09/01/2020  10:46 AM    <DIR>          ..
09/01/2020  10:46 AM    <DIR>          mydummy
09/01/2020  10:46 AM    <SYMLINK>      user.bat [\\?\C:\users\nikhilm\_bazel_nikhilm\tvcdbfus\execroot\__main__\bazel-out\x64_windows-fastbuild\bin\user.bat]
               1 File(s)              0 bytes
               3 Dir(s)  842,238,795,776 bytes free
------------mydummy DIR--------------
 Volume in drive C is system
 Volume Serial Number is AA07-0727

 Directory of c:\users\nikhilm\_bazel~1\tvcdbfus\execroot\__main__\bazel-~1\x64_wi~1\bin\userba~2.run\__main__\mydummy

09/01/2020  10:46 AM    <DIR>          .
09/01/2020  10:46 AM    <DIR>          ..
09/01/2020  10:46 AM    <SYMLINKD>     lib [\\?\C:\users\nikhilm\_bazel_nikhilm\tvcdbfus\execroot\__main__\bazel-out\x64_windows-fastbuild\bin\mydummy\lib]
               0 File(s)              0 bytes
               3 Dir(s)  842,238,795,776 bytes free

Observe how it says SYMLINKD!

java.exe ran before runfiles.exe.

good good details

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Repository to demonstrate a Bazel bug https://github.com/bazelbuild/bazel/issues/12033

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