Multiple Hits on Select

[CuckooEXE]

Hi all! :) Hope this is the right place to post, if not please lmk where I should!
A bit of background: I'm responsible for migrating my team to Bazel from our current Makefile hell, and am slowly picking up on the ecosystem. Right now, we have a make docs target, that uses the PROJECTS environment variable to selectively pass in a colon-delimited list of project names (that get mapped to their respective directories). The docs target, will then copy the appropriate documentation files into the source directory of our documentation generator, and build it. That way when we deliver Product A and Product C to our engineers, we only package the documentation for those two projects and, and not Product B, with PROJECTS=ProjectA:ProjectC make docs.

I am starting to understand the role of select and config_setting, but am having a hard time getting it to fit right into my team's needs. I have it working where I can select one or all projects to build with select, but I'd like to be able to pick an arbitrary list through a command-line option.

Here is the repository that demonstrates what I'm trying to do (it's very contrived, but it's a simple example that mirrors my goal.
Right now, I can do bazel build //example:ConditionalAggregate --//example:FilesToInclude=FileB which does what you expect, and not using --//example:FilesToInclude will include them all with the way I generate the default, but I want to do bazel build //example:ConditionalAggregate --//example:FilesToInclude=FileB,FileA . But I get an error:

$ bazel version                                                                                                                                                                          1 ↵
Build label: 8.4.2-homebrew
Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer
Build time: Tue Jan 01 00:00:00 1980 (315532800)
Build timestamp: 315532800
Build timestamp as int: 315532800

$ bazel build //example:ConditionalAggregate --//example:FilesToInclude=FileB,FileA
ERROR: /tmp/Projects/bazelExample/example/BUILD:63:14: Illegal ambiguous match on configurable attribute "srcs" in //example:ConditionalAggregate:
//example:FileAIncluded
//example:FileBIncluded
Multiple matches are not allowed unless one is unambiguously more specialized or they resolve to the same value. See https://bazel.build/reference/be/functions#select.
ERROR: Analysis of target '//example:ConditionalAggregate' failed; build aborted
INFO: Elapsed time: 0.063s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

[mattnworb]

Have you considered modeling the docs site for each project just as separate targets?

[CuckooEXE]

We want the docs site to be unified when possible so we don't deliver many docs packages when we could just do one.

[CuckooEXE]

I ended up asking on the Slack at the same time (had no idea both of these would be responded to so quickly!) Here is the solution

aggregate_txt(
    name = "ConditionalAggregate",
    srcs = select({
        ":FileAIncluded": [":FileA"],
        "//conditions:default": [],
    }) + select({
        ":FileBIncluded": [":FileB"],
        "//conditions:default": [],
    }) + select({
        ":FileCIncluded": [":FileC"],
        "//conditions:default": [],
    }),
)