attr.output cannot create directories

[pauldraper]

I would like to have a named output directory from a rule.

attr.output can create output attributes, but the outputs must always be files; they cannot be directories. That seems an artificial/unnecessary limitation.

I would like to support directories. Perhaps this could be an argument to attr.output. An argument type could be "file", "directory", or "symlink".

[chancila]

as a workaround I have done

intermediate_dir = ctx.actions.create_dir()
# emit stuff in the dir via actions
ctx.actions.symlink(output, intermediate_dir)

[pauldraper]

Clever.

[joeljeske]

I had trouble bypassing this limitation using ctx.actions.symlink. I ended up having a rule that created many directories using ctx.actions.declare_directory and then returning them using OutputGroupInfo and naming them dynamically. Then you can access each dynamic directory using filegroup[output_group].

Its frustrating to require a workaround that should be supported properly, but this does appear to work fine.

[tjgq]

Unfortunately, the ctx.actions.symlink workaround creates another problem: it causes Bazel to create a symlink from a file artifact to a directory artifact. This ought to be considered a bug in ctx.actions.symlink (it causes other parts of Bazel to become confused about whether the artifact the symlink points to is a directory or not) and I'm in the process of disallowing it.

[comius]

Keeping P4, as far as I understand treeartifacts are already hard to correctly support within the rules. As such I think it’s better not to have the same problem on the targets / external rules interface.

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