Expose `ErrorCodes` constants (errno)

[tvolkert]

Problem Statement

There are times in code when you catch an IOException and want to perform conditional logic based on the osError.errorCode within the exception. However, for any given semantic error code (e.g. EISDIR), its value may differ on various platforms. Complicating the problem is that the different platforms have disjoint sets of semantic errno values.

Proposed Solution

If dart:io exposed the set of semantic errno values that is common to all platforms, it would solve the 90% use case. I propose we create an ErrorCodes class with static const int values for this set of errno values.

Looking at the errno values from Linux, Mac OS, and Windows, it looks like the common set would yield the following: https://gist.github.com/tvolkert/e59f531df15ee54edb71c9d2fba6fe71

Alternatives Considered

• We could provide these values as an enum, but we want developers to be able to compare the IOException.osError.errorCode int value against these values, so an enum doesn't fit the use case as well as static constants.
• We could provide all errno values for all platforms (rather than just exposing the common set and abstracting away the platform), but that seems gratuitous and not tailored towards solving a real problem.
• We could do nothing and let individual libraries worry about this, but given that we already expose the OSError and its errorCode, it seems like we should provide something like this in dart:io itself.

[tvolkert]

@zanderso @johnmccutchan

[tvolkert]

FYI, in the meantime, I did this manually in https://github.com/google/file.dart/blob/master/lib/src/interface/error_codes.dart

[natebosch]

should these be lower camel cased? I can see an argument for uppercase for consistency with how you'd see these in man errno...

[tvolkert]

I could go either way...

[cbatson]

This sure would be nice. I find myself checking the error code in situations where, for example, it's acceptable that a file does not exist but other errors should be thrown / reported. And, of course, checking that the file exists() beforehand is a race condition.