Use of ctx.info_file and ctx.version_file in rule implementations

[mvukov]

Description of the problem / feature request:

At the moment information in ctx.info_file and ctx.version_file cannot be used for rule implementation.

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

See for instance bazelbuild/rules_python#451. Therein, I would like to use git info, routed via workspace status script, to make an output file name of a Python wheel. Since I cannot read a file from context, at the moment I process ctx.info_file and ctx.version_file in a separate Python script and then dump a wheel version to a .txt file. The new py_wheel_push rule in that PR consumes my .txt file and then assembles the correct file name. However, it is desirable that py_wheel rule already assembles the correct wheel name, which at the moment is not possible if I want to use info from workspace status -- e.g. a wheel version in py_wheel rule as version = "{FOO}", where I expect FOO to be picked up from the workspace status.

Are there any plans to enable use of the workspace status in rule implementations?

What operating system are you running Bazel on?

Ubuntu 18.04

What's the output of bazel info release?

release 2.0.0

Have you found anything relevant by searching the web?

The only thing I could find is that Bazel context cannot read files, and that's for good reasons as far as I understand.

[gregestren]

Where are ctx.info_file and ctx.version_file in the Bazel docs?

[mvukov]

Unfortunately, they are not documented. See #9363 and #11422. FWIW, I had to dig into rules_docker repo to learn how to access the workspace status.

[aiuto]

+1 to documenting these.

[tetromino]

Clarification for future readers: the problem is that the content of ctx.info_file and version_file cannot be used in a rule implementation.

[brandjon]

We can't read file content prior to execution. But perhaps workspace status information can be (or is currently?) exposed in some alternate API.

[tetromino]

See also #13300 requesting better documentation for reading files in Starlark.

[comius]

Assigning to @buildbreaker2021, who's currently working on this functionality, to review if there's need to update documentation and/or if this issue can be resolved.

[buildbreaker2021]

There is an API already implemented(although private still) which will support use-cases like these.

bazel/src/main/java/com/google/devtools/build/lib/starlarkbuildapi/StarlarkActionFactoryApi.java

Lines 849 to 926 in 8828015

	@StarlarkMethod(
	name = "transform_version_file",
	documented = false,
	doc =
	"Similar to <a href=\"#transform_info_file\">transform_info_file</a>, but transforms <a"
	+ " href='https://bazel.build/rules/lib/builtins/ctx#version_file'>ctx.version_file</a>.",
	parameters = {
	@Param(
	name = "transform_func",
	doc = TRANSFORM_VERSION_FUNC_DOC,
	allowedTypes = {@ParamType(type = StarlarkFunction.class)},
	positional = false,
	named = true),
	@Param(
	name = "template",
	doc = TEMPLATE_DOC,
	allowedTypes = {@ParamType(type = FileApi.class)},
	positional = false,
	named = true),
	@Param(
	name = "output_file_name",
	doc = VERSION_OUTPUT_DOC,
	allowedTypes = {@ParamType(type = String.class)},
	positional = false,
	named = true),
	},
	useStarlarkThread = true)
	FileApi transformVersionFile(
	Object transformFuncObject,
	Object templateObject,
	String outputFileName,
	StarlarkThread thread)
	throws InterruptedException, EvalException;
	@StarlarkMethod(
	name = "transform_info_file",
	documented = false,
	doc =
	"Transforms <a"
	+ " href='https://bazel.build/rules/lib/builtins/ctx#info_file'>ctx.info_file</a> to"
	+ " a language-consumable file and writes its contents to <code>output</code>. Keys"
	+ " and values are transformed by calling the <code>transform_func</code> Starlark"
	+ " method, and the output file format is generated according to the"
	+ " <code>template</code>. Use this call to create an action in an auxiliary rule."
	+ " Create a single target for an auxiliary rule which is then used as an implicit"
	+ " dependency for the main rule. Main rule is the one which needs the transformed"
	+ " <code>info_file</code>. The auxiliary rule should also declare"
	+ " <code>output</code> file to which transformed content is written. The"
	+ " <code>output</code> file is then provided to the dependant main rule. This will"
	+ " avoid action conflicts and duplicated file generations.",
	parameters = {
	@Param(
	name = "transform_func",
	doc = TRANSFORM_INFO_FUNC_DOC,
	allowedTypes = {@ParamType(type = StarlarkFunction.class)},
	positional = false,
	named = true),
	@Param(
	name = "template",
	doc = TEMPLATE_DOC,
	allowedTypes = {@ParamType(type = FileApi.class)},
	positional = false,
	named = true),
	@Param(
	name = "output_file_name",
	doc = INFO_OUTPUT_DOC,
	allowedTypes = {@ParamType(type = String.class)},
	positional = false,
	named = true),
	},
	useStarlarkThread = true)
	FileApi transformInfoFile(
	Object transformFuncObject,
	Object templateObject,
	String outputFileName,
	StarlarkThread thread)
	throws InterruptedException, EvalException;
	}