[bug] Bazel builds fail when the package provides no binaries

[ericriff]

Environment

$ conan --version
Conan version 2.10.1
$ bazel --version
bazel 8.0.0

Describe the bug

I'm following the tutorial from https://docs.conan.io/2/examples/tools/google/bazeltoolchain/build_simple_bazel_7x_project.html
to learn how to integrate conan into a bazel project.
Granted, the tutorial mentions it is for Bazel 7 yet I'm using 8. Maybe conan doesn't support 8?

Anyways, the build fails with

bazel --bazelrc=./conan/conan_bzl.rc build --config=conan-config //main:demo
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
WARNING: Couldn't auto load rules or symbols, because no dependency on module/repository 'rules_android' found. This will result in a failure if there's a reference to those rules or symbols.
ERROR: Traceback (most recent call last):
	File "/home/eriff/repos/examples2/examples/tools/google/bazeltoolchain/7_x/string_formatter/conan/bazelRootDir/d5c9c1c12146af1a257e11f337e7ccd9/external/+conan_extension+fmt/BUILD.bazel", line 47, column 16, in <toplevel>
		srcs = glob([
Error in glob: glob pattern 'bin/**' didn't match anything, but allow_empty is set to False (the default value of allow_empty can be set with --incompatible_disallow_empty_glob).
ERROR: /home/eriff/repos/examples2/examples/tools/google/bazeltoolchain/7_x/string_formatter/main/BUILD:1:10: Target '@@+conan_extension+fmt//:fmt' contains an error and its package is in error and referenced by '//main:demo'
ERROR: Analysis of target '//main:demo' failed; build aborted: Analysis failed
INFO: Elapsed time: 21.228s, Critical Path: 0.02s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
FAILED: 
    Fetching repository @@rules_cc++cc_configure_extension+local_config_cc; starting

It complains about the conan-generated files, particularly this part

# Filegroup library declaration
filegroup(
    name = "fmt_binaries",
    srcs = glob([
        "bin/**",
    ]),
    visibility = ["//visibility:public"],
)

Since fmt provides no binaries, that glob matches nothing and Bazel doesn't like it. Commenting out the whole block makes the build pass, but a better fix is to add allow_empty to that glob call.

# Filegroup library declaration
filegroup(
    name = "fmt_binaries",
    srcs = glob([
        "bin/**",
    ],
    allow_empty=True
    ),
    visibility = ["//visibility:public"],
)

With that fix the build passes

 bazel --bazelrc=./conan/conan_bzl.rc build --config=conan-config //main:demo
INFO: Analyzed target //main:demo (12 packages loaded, 66 targets configured).
INFO: Found 1 target...
Target //main:demo up-to-date:
  bazel-bin/main/demo
INFO: Elapsed time: 0.961s, Critical Path: 0.25s
INFO: 6 processes: 4 internal, 2 linux-sandbox.
INFO: Build completed successfully, 6 total actions

[franramirez688]

Hi @ericriff,

Thanks a lot for reporting this 😁
Yes, that would fix the error. Moreover, I just checked older bazel versions and it's also compatible 👏

Proposing #17444 to solve the issue. It'll be released in the next Conan 2.11.0.