cc_library warning: file has no symbols

[jayconrod]

Description of the problem / feature request / question:

On macOS, if a source file in a static cc_library doesn't define any global symbols, we see a warning like the one below:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: bazel-out/darwin_x86_64-fastbuild/bin/libempty.a(empty.o) has no symbols

This is a problem in rules_go. We use a tool in the Go toolchain (cgo) that generates some .c files that get compiled with cc_library. One of the generated files in each library frequently has no symbols, but it's hard to know ahead of time whether that's the case (the file is not empty).

It would be nice to suppress these warnings altogether. There may be a lot of libraries with cgo in a Go project, so our users may see a lot of warnings.

It looks like ranlib on macOS supports a flag -no_warnings_for_no_symbols. ar does not support this flag. The actual command that's emitting this warning looks like this:

external/local_config_cc/wrapped_ar rcs bazel-out/darwin_x86_64-fastbuild/bin/libempty.a bazel-out/darwin_x86_64-fastbuild/bin/_objs/empty/empty.o

Maybe this flag can be added in wrapped_ar.tpl?

If possible, provide a minimal example to reproduce the problem:

cc_library(
    name = "empty",
    srcs = ["empty.c"],
    linkstatic = 1,
)

empty.c is an empty file. Note that there are additional warnings if the entire library has no symbols. Those warnings don't affect rules_go, but they should probably be suppressed, too.

Environment info

• Operating System: macOS Sierra 10.12.6

• Bazel version (output of bazel info release): release 0.7.0

[robbertvanginkel]

Was this fixed with #5981 / 81b59f0 ?

[cpsauer]

It looks to me like there's still an issue outstanding here somewhere. Not sure if it's a regression since #5981/81b59f0, or an edge case that's slipping through.

[I'm seeing this warning when building a .mm (Obj-C++) file using an objc_library.]

@tetromino, I'm hoping it's okay if I tag you, since you wrote the original fix--and I don't (quickly) see where thie invocation is slipping through without -no_warning_for_no_symbols. I can see that the flag has since been moved to //tool/osx/crosstool/cc_toolchain_config.bzl, I assume as part of the general migration to toolchains.

[cpsauer]

@keith, here's the repro, per #5981:
empty_warning.zip
bazel clean && bazel build :all

Output should show the warning in all valid cases with {cc_library, objc_library}x{C, C++, Obj-C, and Objc-++}.
That is, 6 instances of
warning: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: archive library: bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cfb676c6944e/bin/<whatever>.a the table of contents is empty (no object file members in the library define global symbols)

[keith]

This warning is actually slightly different, and there doesn't appear to be a way to disable it:

https://github.com/keith/cctools/blob/bab7d4a19d480751ea4cba85d4c92b07991f2356/misc/libtool.c#L2915-L2925

[keith]

This original issue can def be closed, but we could track that one separately, or not. Theoretically we could filter the output to just ignore that, IMO it's rarely going to indicate a real issue

[cpsauer]

Good call. Figured I'd PR it up while we were discussing. How about something like #15325?

[cpsauer]

(Have tested, and it fixes all the repro cases above and the instances in my own codebase that initially brought me to this issue.)