Windows: Bazel cannot share cpp cache between two projects with different workspace name

[meteorcloudy]

If you try to share cache between two projects with different workspace name, when the cache is hit for a C++ compilation, the header check will fail because the header paths printed by MSVC's /showInclude option are absolute paths, which contains the workspace name.

You can reproduce with:
https://github.com/meteorcloudy/my_tests/tree/master/cpp_cache_with_different_workspace_name

$ cd A
$ bazel build --disk_cache=C:/src/tmp/bazel_disk_cache :hello
$ cd ../B
$ bazel build --disk_cache=C:/src/tmp/bazel_disk_cache :hello
Starting local Bazel server and connecting to it...
INFO: Invocation ID: e98dabd3-b255-4c34-86e5-ab5070b346f3
INFO: Analyzed target //:hello (13 packages loaded, 72 targets configured).
INFO: Found 1 target...
ERROR: C:/tools/msys64/home/pcloudy/workspace/my_tests/cpp_cache_with_different_workspace_name/b/BUILD:1:1: undeclared inclusion(s) in rule '//:hello':
this rule is missing dependency declarations for the following files included by 'hello.cc':
  'C:/users/pcloudy/_bazel_pcloudy/7tppxn5m/execroot/a/hello.h'
Target //:hello failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 10.570s, Critical Path: 0.09s
INFO: 0 processes.
FAILED: Build did NOT complete successfully

Note the error message when we build //:hello in B,
'C:/users/pcloudy/_bazel_pcloudy/7tppxn5m/execroot/a/hello.h' is from project A.

This is the root cause of https://buildkite.com/bazel/bazel-federation/builds/19#ee3795da-1088-4c27-b81e-2636e30e3587

FYI @fweikert @laszlocsomor

[meteorcloudy]

We had a similar bug before because output base was different. #6847 (comment)
That issue was fixed at a1aa5c1, but it doesn't take different workspace names into consideration.

[meteorcloudy]

So the question is, should we allow sharing cache between two projects with different workspace name?

[laszlocsomor]

Thanks for the report!

So the question is, should we allow sharing cache between two projects with different workspace name?

Why wouldn't we?

[meteorcloudy]

Why wouldn't we?

Exactly, I don't have any good reason to object it.
Then, I might know how to fix this, will send a change today.

[fweikert]

Nice, thank you Yun!