CcSkylarkApiProvider does not expose alwayslink libraries

[steeve]

Description of the problem / feature request:

CcSkylarkApiProvider should expose cc_libraryies with alwayslink = True, so that it is possible to properly add -Wl,--whole-source when we need to build the link flag ourselves, such as in rules_go.

What's the output of bazel info release?

release 0.21.0

Any other information, logs, or outputs that you want to share?

Ongoing dicussion at bazel-contrib/rules_go#1879

[jayconrod]

cc @hlopko

Just to comment on why this would be useful, Go rules like go_library allow cc_library dependencies through the cdeps attribute. The libraries are eventually passed to the linker through go_binary and go_test rules.

For some libraries, it's necessary to pass -Wl,--whole-archive and -Wl,--no-whole-archive before and after the library file. This is needed if a library contains an unreferenced object file with a constructor that has side effects.

Setting alwayslink = 1 on cc_library communicates the need for these flags to the build system, but Starlark rules can't see whether this flag is set, so rules like go_binary don't know whether to pass the flags above. We're considering passing -Wl,--whole-archive for all libraries, but we're concerned this will bloat binary size.

[hlopko]

With bazel 0.22 we'll introduce a new Starlark API that contains library_to_link which contains alwayslink field. Docs are being written. Relevant is also the issue for incompatible flag removing the CcSkylarkApiProvider: #7036.

Pedro, pls close once docs are there and there are no more comments. Thanks!

[hlopko]

I apologize, it will only be in the Bazel 0.23, the commit was submitted today: 94e5016. Thanks!

[jayconrod]

@hlopko Sorry to resurrect this old issue, but is this flag exposed anywhere in the CcInfo provider? cc.linking_context.user_link_flags sounded promising, but it appears to be empty in a simple example.

The problem is that go_library and other rules may depend on cc_libraries via a cdeps attribute. We keep track of all the libraries in the dependency tree, and we pass them to the Go linker, which invokes the C/C++ linker within go_binary. For the libraries with alwayslink = 1, we need to wrap the linker arguments with -Wl,--whole-archive.

I'm not sure how to tell if a cc_library has alwayslink = 1 set from its CcInfo provider though. Is there a way? Let me know if I should open a new issue.

[oquenchil]

You would get the libraries_to_link from the linking_context. Each library_to_link has an alwayslink field:https://docs.bazel.build/versions/master/skylark/lib/LibraryToLink.html

[jayconrod]

@oquenchil Thanks, this should work. I initially skipped over that field because I thought it was deprecated, but after reading #8118, it looks like it's only the list return value that's deprecated.