write_source_files.md

write_source_files provides a workaround for the restriction that bazel build cannot write to the source tree.

Read more about the philosophy of writing to the source tree: https://blog.aspect.build/bazel-can-write-to-the-source-folder

Usage

load("@aspect_bazel_lib//lib:write_source_files.bzl", "write_source_files")

write_source_files(
    name = "write_foobar",
    files = {
        "foobar.json": "//some/generated:file",
    },
)

To update the source file, run:

bazel run //:write_foobar

The generated diff_test will fail if the file is out of date and print out instructions on how to update it.

If the file does not exist, Bazel will fail at analysis time and print out instructions on how to create it.

You can declare a tree of generated source file targets:

load("@aspect_bazel_lib//lib:write_source_files.bzl", "write_source_files")

write_source_files(
    name = "write_all",
    additional_update_targets = [
        # Other write_source_files targets to run when this target is run
        "//a/b/c:write_foo",
        "//a/b:write_bar",
    ]
)

And update them with a single run:

bazel run //:write_all

When a file is out of date, you can leave a suggestion to run a target further up in the tree by specifying suggested_update_target. For example,

write_source_files(
    name = "write_foo",
    files = {
        "foo.json": ":generated-foo",
    },
    suggested_update_target = "//:write_all"
)

A test failure from foo.json being out of date will yield the following message:

//a/b:c:foo.json is out of date. To update this and other generated files, run:

    bazel run //:write_all

To update *only* this file, run:

    bazel run //a/b/c:write_foo

You can also add a more customized error message using the diff_test_failure_message argument:

write_source_file(
    name = "write_foo",
    out_file = "foo.json",
    in_file = ":generated-foo",
    diff_test_failure_message = "Failed to build Foo; please run {{TARGET}} to update."
)

A test failure from foo.json being out of date will then yield:

Failed to build Foo; please run //a/b/c:write_foo to update.

If you have many write_source_files targets that you want to update as a group, we recommend wrapping write_source_files in a macro that defaults suggested_update_target to the umbrella update target.

NOTE: If you run formatters or linters on your codebase, it is advised that you exclude/ignore the outputs of this rule from those formatters/linters so as to avoid causing collisions and failing tests.

WriteSourceFileInfo

WriteSourceFileInfo(executable)

Provider for write_source_file targets

FIELDS

Name	Description
executable	Executable that updates the source files

write_source_file

write_source_file(name, in_file, out_file, executable, additional_update_targets,
                  suggested_update_target, diff_test, diff_test_failure_message,
                  file_missing_failure_message, check_that_out_file_exists, kwargs)

Write a file or directory to the source tree.

By default, a diff_test target ("{name}_test") is generated that ensure the source tree file or directory to be written to is up to date and the rule also checks that the source tree file or directory to be written to exists. To disable the exists check and up-to-date test set diff_test to False.

PARAMETERS

Name	Description	Default Value
name	Name of the runnable target that creates or updates the source tree file or directory.	none
in_file	File or directory to use as the desired content to write to out_file. This is typically a file or directory output of another target. If in_file is a directory then entire directory contents are copied.	None
out_file	The file or directory to write to in the source tree. The output file or directory must be within the same containing Bazel package as this target if check_that_out_file_exists is True. See check_that_out_file_exists docstring for more info.	None
executable	Whether source tree file or files within the source tree directory written should be made executable.	False
additional_update_targets	List of other write_source_files or write_source_file targets to call in the same run.	[]
suggested_update_target	Label of the write_source_files or write_source_file target to suggest running when files are out of date.	None
diff_test	Test that the source tree file or directory exist and is up to date.	True
diff_test_failure_message	Text to print when the diff test fails, with templating options for relevant targets. Substitutions are performed on the failure message, with the following substitutions being available: {{DEFAULT_MESSAGE}}: Prints the default error message, listing the target(s) that may be run to update the file(s). {{TARGET}}: The target to update the individual file that does not match in the diff test. {{SUGGESTED_UPDATE_TARGET}}: The suggested_update_target if specified.	"{{DEFAULT_MESSAGE}}"
file_missing_failure_message	Text to print when the output file is missing. Subject to the same substitutions as diff_test_failure_message.	"{{DEFAULT_MESSAGE}}"
check_that_out_file_exists	Test that the output file exists and print a helpful error message if it doesn't. If True, the output file or directory must be in the same containing Bazel package as the target since the underlying mechanism for this check is limited to files in the same Bazel package.	True
kwargs	Other common named parameters such as tags or visibility	none

RETURNS

Name of the generated test target if requested, otherwise None.

write_source_files

write_source_files(name, files, executable, additional_update_targets, suggested_update_target,
                   diff_test, diff_test_failure_message, file_missing_failure_message,
                   check_that_out_file_exists, kwargs)

Write one or more files and/or directories to the source tree.

By default, diff_test targets are generated that ensure the source tree files and/or directories to be written to are up to date and the rule also checks that all source tree files and/or directories to be written to exist. To disable the exists check and up-to-date tests set diff_test to False.

PARAMETERS

Name	Description	Default Value
name	Name of the runnable target that creates or updates the source tree files and/or directories.	none
files	A dict where the keys are files or directories in the source tree to write to and the values are labels pointing to the desired content, typically file or directory outputs of other targets. Destination files and directories must be within the same containing Bazel package as this target if check_that_out_file_exists is True. See check_that_out_file_exists docstring for more info.	{}
executable	Whether source tree files written should be made executable. This applies to all source tree files written by this target. This attribute is not propagated to additional_update_targets. To set different executable permissions on different source tree files use multiple write_source_files targets.	False
additional_update_targets	List of other write_source_files or write_source_file targets to call in the same run.	[]
suggested_update_target	Label of the write_source_files or write_source_file target to suggest running when files are out of date.	None
diff_test	Test that the source tree files and/or directories exist and are up to date.	True
diff_test_failure_message	Text to print when the diff test fails, with templating options for relevant targets. Substitutions are performed on the failure message, with the following substitutions being available: {{DEFAULT_MESSAGE}}: Prints the default error message, listing the target(s) that may be run to update the file(s). {{TARGET}}: The target to update the individual file that does not match in the diff test. {{SUGGESTED_UPDATE_TARGET}}: The suggested_update_target if specified, or the target which will update all of the files which do not match.	"{{DEFAULT_MESSAGE}}"
file_missing_failure_message	Text to print when the output file is missing. Subject to the same substitutions as diff_test_failure_message.	"{{DEFAULT_MESSAGE}}"
check_that_out_file_exists	Test that each output file exists and print a helpful error message if it doesn't. If True, destination files and directories must be in the same containing Bazel package as the target since the underlying mechanism for this check is limited to files in the same Bazel package.	True
kwargs	Other common named parameters such as tags or visibility	none