os: hangs in os.(*Process).blockUntilWaitable on android-amd64-emu

[bcmills]

I've only seen one log like this so far, but it contains three of the same apparent hang:
2020-05-11T22:38:32-8c1db77/android-amd64-emu

This was at CL 232862, and may be related to that change (CC @ianlancetaylor).

Tentatively marking as release-blocker, at least until we better understand whether it is a bug in the builder or in the standard library.

[toothrot]

/cc @dmitshur

[ianlancetaylor]

I don't think this is a hang in blockUntilWaitable. In each case the program that is hanging is misc/android/go_android_exec. That is a fairly simple program that runs adb and waits for it to complete. While waiting for it to complete, go_android_exec will be sleeping in blockUntilWaitable. In this case, it appears that adb does nothing; specifically adb wait-for-device does nothing. Then eventually the test harness times out and kills go_android_exec, and we see the kind of stack traces shown in the log. That log does not show any successful tests; all tests fail with the same error. The problem has not recurred. So I think this was simply a case of the Android device not responding. Closing.