glob(allow_empty=False) evaluated unconditionally; cannot be guarded by select

[rbeasley-avgo]

Description of the bug:

BUILD files' glob expressions seem to be evaluated unconditionally, preventing users from selecting between different glob patterns based on a build condition. In light of the --incompatible_disallow_empty_glob flag flip, builds may now error when ostensibly inapplicable globs don't match anything.

In our case, we have instances where we choose between glob patterns based on the host operating system. (Our case involves repository rules, where the outputs fetched by the repo rule impl may vary by platform.) As a result of the flag flip, I'll need to scrub all instances of glob and inject an allow_empty=True as needed, which can be misleading to users/readers. (That said, I totally agree w/ the flag flip. We really would like allow_empty=False to be the default.)

If this is working as intended, then I'd like to suggest this become a documentation bug. (If that's the case, then my gut feeling is that this limitation should be noted at https://bazel.build/rules/lib/globals/build#glob .)

Which category does this issue belong to?

Core, Configurability, Loading API, Documentation

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Example from Linux or macOS:

$ touch MODULE.bazel
$ cat >BUILD.bazel <<"EOF"
genrule(
  name = "demo",
  outs = ["demo.out"],
  srcs = select({
    "@platforms//os:windows": glob(["*.windows"]),
    "//conditions:default": glob(["*.bazel"]),
  }),
  cmd = "echo $(SRCS); touch $@",
)
EOF
$ bazelisk build :demo
...
ERROR: Traceback (most recent call last):
        File "/.../BUILD.bazel", line 5, column 35, in <toplevel>
                "@platforms//os:windows": glob(["*.windows"]),
Error in glob: glob pattern '*.windows' 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).
$ USE_BAZEL_VERSION=last_green bazelisk --output_user_root=$XDG_CACHE_HOME/bazel build :demo
2025/02/21 17:13:57 Using unreleased version at commit 80a9971055c90670effbf4f1fec53347bd0f0e86
2025/02/21 17:13:57 Downloading https://storage.googleapis.com/bazel-builds/artifacts/centos7/80a9971055c90670effbf4f1fec53347bd0f0e86/bazel...
Extracting Bazel installation...
Starting local Bazel server (no_version) and connecting to it...
INFO: Invocation ID: a33f1811-b015-49bd-8e70-6ce480274903
ERROR: Traceback (most recent call last):
        File "/.../BUILD.bazel", line 5, column 35, in <toplevel>
                "@platforms//os:windows": glob(["*.windows"]),
Error in glob: glob pattern '*.windows' 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).

Commenting out the @platforms//os:windows condition, which should be a no-op when invoked on Linux/macOS, yields a successful build.

Which operating system are you running Bazel on?

Linux

What is the output of bazel info release?

release 8.1.0 or development version (see above)

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

No response

Have you found anything relevant by searching the web?

• [bazel.build] Problem with /reference/be/functions: effect of glob(allow_empty = False) #20377 (comment)

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

No response