Using label path to check if file location exists - bazel

Question

Is there an easy way to get hold of a path object so I can check if a given label path exists. Say for example if path.exists("#external_project_name//:filethatmightexist.txt"):. I can see that the repository context has this. But I need to have a wrapping repository rule. Is it possible to do this in a macro or Skylark native call instead?

Answer 1

Even with a repository_rule, I had a lot of trouble with this due to what you already pointed out:
if you create a Label with a path that doesn't exist, it will cause the build to fail
But if you're willing to do a repository rule, here's a possible solution...
In this example, my rule allows specification of a default configuration if a config file is not present. The configuration can be checked into .gitignore and overridden for individual developers, but work out of the box for most cases.
I think I understand why the ctx.actions have sibling arguments now, same idea here. The trick is config_file_location is a true label, and then config_file is a string attribute. I chose BUILD arbitrarily, but since all workspaces have a top level BUILD that's public seemed legit-ish.
WORKSPACE Definition
...
workspace(name="E02_mysql_database")
json_datasource_configuration(name="E02_datasources",
config_file_location="#E02_mysql_database//:BUILD",
config_file="database.json")
The definition for json_datasource_configuration looks like this:
json_datasource_configuration = repository_rule(
attrs = {
"config_file_location": attr.label(
doc="""
Path relative to the repository root for a datasource config file.
"""),
"config_file": attr.string(
doc="""
Config file, maybe absent
"""),
"default_config": attr.string(
# better way to do this?
default="None",
doc = """
If no config is at the path, then this will be the default config.
Should look something like:
{
"datasource_name": {
"host": "<host>"
"port": <port>
"password": "<password>"
"username": "<username>"
"jdbc_connection_string": "<optional>"
}
}
There can be more than datasource configured... maybe, eventually.
""",
),
},
local = True,
implementation = _json_config_impl,
)
Then in the rule I can test for the file existence, and if not present, do other logic.
def _json_config_impl(ctx):
"""
Allows you to specify a file on disk to use for data connection.
If you pass a default
"""
config_path = ctx.path(ctx.attr.config_file_location).dirname.get_child(ctx.attr.config_file)
config = ""
if config_path.exists:
config = ctx.read(config_path)
elif ctx.attr.default_config == "None":
fail("Could not find config at %s, you must supply a default_config if this is intentional" % ctx.attr.config_file)
else:
config = ctx.attr.default_config
...
probably too late to help, but your question is the only thing I found referencing this goal. If someone knows a better way I am looking for other options. It's complicated to explain to other developers why the rule has to work the way it does.
Also note, if you change the config file, you have to clean to get the workspace to re-read the config. I haven't been able to figure out any way to fix that. glob() does not work in the workspace.