time: loadZoneFile() doesn't forward error returned while reading zoneinfo file.

[ksaket]

Loading ZoneInfo accesses disk which can fail for various reasons. The method time.loadZoneFile() does not expose these errors to time.LoadLocation(). Hence, it reports a generic "unknown time zone" error as an effect to such failures.

https://golang.org/src/time/zoneinfo_unix.go

@dsymonds: who is aware of this problem.

[dsymonds]

I think this is an issue because loadZoneFile is swallowing error details too much. The most common failure mode for any particular attempt in that function is to get a file not found, which is fine to suppress, but other kinds of errors should be reported back so that callers to time.LoadLocation can discover why it's failing.

Here's my proposal:

• if loadZoneFile finds a file it can successfully load, it reports no error (current behaviour)
• if no file can be loaded, and there's an error that does not satisfy os.IsNotExist, report that error (new behaviour)
• if all the errors satisfy os.IsNotExist, report the generic message (current behaviour)

@rsc, does that sound okay to you?

[dsymonds]

https://go-review.googlesource.com/3984 out for review.