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2019-10-24-file-visibility.md

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created last updated status reviewers title authors
2019-10-24
2019-10-30
Implemented
laurentlb
Visibility for source files
aehlig

Abstract

We propose that source files used in a BUILD file do not implicitly inherit the public visibility of the package.

Background

Bazel uses labels as a uniform name space for all dependencies, regardless of their type. Therefore, labels must also exist for source files being depended upon. To simplify writing BUILD files, these are added implicitly. More precisely, after evaluating a BUILD file, targets are created for all not-yet defined labels in the same package, assuming they are source files. These implicitly defined targets inherit the default_visibility of the package.

Example and description of the problem

Consider the following BUILD file.

package(default_visibility=["//visibility:public"])

exports_files(["app.1"])

cc_binary(
  name = "app",
  srcs = ["main.c", "util.c", "util.h"],
)

After evaluating the BUILD file, the only declared targets are :app and :app.1, both inheriting the default_visibility. Moreover, :app depends on on the targets :main.c, :util.c, and :util.h in the same package. As these targets are not declared, it is assumed that they are source files, and they are added implicitly as targets, as if there were an additional exports_files(["main.c", "util.c", "util.h"]); those implicitly added targets also inherit the default_visibility. So, just from the fact that those files are used, they become publicly visible, even though the intention might have been to only expose the application and its manual page. As a result of this, every package can depend on, e.g., the file util.c directly, making it hard for the package to evolve this internal interface without breaking other packages.

It is the usage of a file that exposes it with the default_visibility. A file testtool.c never used in the BUILD file of its package cannot be used by other packages either. But any use, even in a private target exposes it. E.g., adding the following to said BUILD file will still publicly expose the testtool.c even for non-test targets (as opposed to the private test-only binary generated from that file).

cc_binary(
  name = "testtool",
  srcs = ["testtool.c"],
  testonly = True,
  visibility = ["//visibility:private"],
)

This semantics, that a use of a file in a private target exposes that very file with the default visibility is often considered counterintuitive and has led to unintended dependencies in the past.

Proposal

We propose that implicitly added targets are added with visibility //visibility:private regardless of the default_visibility of the package. If a source file should be exposed more broadly, an explict exports_files would have to be added.

Backward-compatibility

The change is not backwards compatible. Therefore, it will be guarded by a flag --incompatible_no_implicit_file_export.