proposal: os,os/user: stop special-casing Android and iOS in UserHomeDir and user.Current

[bcmills]

The android GOOS is generally a special case of linux, and the ios GOOS is generally a special case of darwin.

However, the behavior of os.UserHomeDir on android and ios deviates from the standard behavior for linux and darwin.

• android was special-cased to return / in https://go.dev/cl/139717.
  • The commit message and code-review comments do not explain why, as far as I can tell, and no code comment was added explaining it either.
  • Its path was changed to /sdcard in https://go.dev/cl/169540 for os: The result of UserHomeDir and User.HomeDir is inconsistent. #31070, to match an existing special case in os/user.
    • That special case was added in https://go.dev/cl/37960, with the explanation “return a dummy user instead of failing”. However, it did not explain why failing is not the correct behavior in that configuration, as I would expect it to be — it seems like the code was changed to conform to an erroneous test, instead of changing the test to conform to the previously-correct(?) code.
• ios was special-cased to return / in https://go.dev/cl/141798.
  • The explanation given was that “The UserHomeDir test succeeds on the builder, but not when run
    manually where HOME is set to the host $HOME.”
  • To me, that seems like a bug in the exec wrapper, not os.UserHomeDir — and, indeed, later (in https://go.dev/cl/167938) the wrapper was changed to avoid propagating $HOME to the device.

A month after the two special cases were added, the signature of UserHomeDir was changed to include an error return value (#28562). It seems to me that the special cases should have been removed at that point, but they weren't, and I think that was an oversight.

I propose that we add a GODEBUG guard for those special cases (mostly for a smoother version-based transition per #56986) and default UserHomeDir and os/user.Current on android and ios to be consistent with linux and darwin respectively as of the next Go version.

(CC @golang/android @golang/ios)

[bcmills]

This proposal was motivated by https://go.dev/cl/520262 (for #57638), in which it is turning out to be more difficult than it ought to be to add tests for UserCacheDir and UserConfigDir because of lingering bugs on android and possibly ios.

[dmitshur]

android was special-cased to return / in https://go.dev/cl/139717.
The commit message and code-review comments do not explain why, as far as I can tell, and no code comment was added explaining it either.

I'll note that code review comments did include the unanswered question "Or would /data/local/tmp be better?", which adds a little context on how "/" was chosen.

[bcmills]

That does add some context on how "/" was chosen, but unfortunately doesn't shed much light on why a special case was thought to be needed at all. 😅