Enhancement request: Warn if stamp = True is used with no other input files

[mmorearty]

Description of the problem / feature request:

It is a feature, not a bug, of build-stamping (genrule stamp = True) that the genrule will only be re-executed if one of the input files other than bazel-out/volatile-status.txt has changed.

This is a good feature; but it is subtle, and seems to be something that a lot of people miss. One example, apparently from within Google: See step 3 under https://github.com/bazelbuild/rules_nodejs/blob/de5393f683b9a73d69d023ca0ffce8ed5d39fcfd/README.md#stamping

Bazel could help with this. If Bazel encounters a genrule which includes stamp = True, but has no srcs at all, it would make sense to omit an error message (or at least a warning; but I think an error is appropriate).

Feature requests: what underlying problem are you trying to solve with this feature?

People mis-using stamp = True, assuming that a genrule with stamp = True and no input files can be used to generate an intermediate output file that can then be used by other targets.

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

WORKSPACE: empty file

BUILD:

# main.out will contain main.txt plus bazel-out/volatile-status.txt.
# Since the below rule `mystamp` is incorrectly written, this will always
# end up writing the same volatile-status.txt to main.out even main.in
# changes.
genrule(
    name = "main",
    output_to_bindir = True,
    srcs = ["main.in", "mystamp"],
    outs = ["main.out"],
    cmd = "cat $(SRCS) > $@",
)

# This is an incorrectly written rule, because it has `stamp = True`, but
# no `srcs`, so it will not behave as the author intended. It will only
# run once.
genrule(
    name = "mystamp",
    output_to_bindir = True,
    outs = ["mystamp.txt"],
    stamp = True,
    cmd = "cat bazel-out/volatile-status.txt > $@",
)

main.in:

this is main.txt

To repro the problem:

• bazel build :main, then look at bazel-bin/main.out
• Edit main.in
• bazel build :main again, and then look at the new bazel-bin/main.out
• Notice that the BUILD_TIMESTAMP line of bazel-bin/main.out has not changed! It would be great if Bazel had given me an error in this case, since I wrote the mystamp rule incorrectly.

What operating system are you running Bazel on?

Ubuntu Linux 14.04

What's the output of bazel info release?

release 0.11.1

Have you found anything relevant by searching the web?

Searched bazelbuild/bazel/issues for is:open stamp

[alexeagle]

@buchgr related, the issue @mmorearty points out in rules_nodejs is because I couldn't find a way to apply a stamp to arbitrary rules (I'd rather have it on my *_package rules).
In my experimentation, stamp only gathers the volatile-status.txt input for genrule and other native rules. Can you suggest an alternative?

[ulfjack]

@alexeagle - are you saying that there's no way to add stamp inputs for Skylark actions? That's independent of this feature request?

[alexeagle]

@ulfjack I haven't found a way to do it yet.

Also the only resource I could find on stamping has an empty-srcs[] on the genrule too: https://www.kchodorow.com/blog/2017/03/27/stamping-your-builds/

[alexeagle]

See discussion in https://groups.google.com/d/msgid/bazel-discuss/CAP4t0yjHEy%2BuHtTq%3Dw4j3-%2BX%2B95XSrmiHoceuxRPe5h_GQ9g6w%40mail.gmail.com?utm_medium=email&utm_source=footer

[github-actions]

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[github-actions]

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