Fix building on Android by avoiding getpwent()

[fornwall]

The getpwent() function does not link when building for Android, and user names on that platform are not interesting anyway.

This patch is part of an effort to upstream patches used by the fish package in Termux.

[floam]

I don't really like the "return 0" thing going on here - and there still should be usernames on Android, interesting or not. I'd like if you checked getpwent linkability in autoconf and provided a workaround of some sort, if practical.

[krader1961]

While this is strictly speaking correct and functionally equivalent to return false; please do the latter for clarity. Having said that, I agree with @floam that since Android is derived from Linux it presumably does utilize UIDs and we should therefore be returning something meaningful on that platform if that is possible. If everything doesn't run as UID zero, and there is no other way to map a UID to name, perhaps this should simply synthesize a user name by doing something like return sprintf(buf, "user#%d", getuid());. That is obviously just pseudo-code but you get the idea.

[fornwall]

Thanks a lot for the feedback! I've updated from return 0 to return false.

As for a fallback, a Linux user on Android isn't really a user - each installed app gets an UID assigned. Listing all UID:s is not possible without root access, and doing a ~USER type expansion does not make sense since every app is sandboxed and can't access eachother. So I think a fallback is not really possible.

As for an autoconf check if getpwent links instead of ifdef __ANDROID__: Sounds reasonable, can do that after we settle for the fallback behaviour.

[krader1961]

Okay, given that in Android OS there really isn't any such thing as usernames I'm okay with just returning false for that completer. Go ahead and make the behavior dependent on whether there is a usable getpwent() and I think you're done.

[floam]

I found this stuff interesting:

http://androidxref.com/4.2.2_r1/xref/bionic/libc/docs/OVERVIEW.TXT#231

http://androidxref.com/5.0.0_r2/xref/bionic/libc/bionic/stubs.cpp#357

Maybe we could do similar. There are some mappings defined in a header file as well.

[krader1961]

For the record I would prefer an autoconf check for whether getpwent() exists. But if that is impractical for some reason the current implementation is acceptable.

[floam]

Great, we can merge this.

[fornwall]

@krader1961 Changed it to an autoconf check instead of ifdef __ANDROID__.

I've left the Android-specific comments for now as it's the only relevant platform I know about lacking getpwent() - let me know if those comments should be removed/changed!