Add support for stable Func keys in App Engine second gen

[sbuss]

Fixes #122

[zombiezen]

Question about requirements: does this need to preserve compatibility with a previous scheme for encoding function names? If not, then may I suggesting using the value of Frame.Function from runtime.CallersFrames (example)? Seems more stable than reverse engineering file paths, and has the added benefit of not requiring a new exported symbol.

[sbuss]

does this need to preserve compatibility with a previous scheme for encoding function names?

Unfortunately, yes. The scheme is described here: #122 (comment)

[zombiezen]

Gotcha, that was the context I was missing. I'll review with that in mind.

[zombiezen]

I think you're on the right track here now that I understand the issue more fully. Since we are codifying something that is kinda fragile to begin with, I want to make sure we proceed carefully.

[broady]

Since we are codifying something that is kinda fragile to begin with

+1, and more generally, we should be very relaxed with any guarantee for this package. It's inherently flaky, and people should know that going in.

[sbuss]

It's inherently flaky, and people should know that going in.

How should I document that? @broady

[broady]

How should I document that? @broady

Package-level doc, maybe? I think it's in the docs on cloud.google.com somewhere, but fairly hidden.

[zombiezen]

LGTM, please wait for @broady's approval.

[sbuss]

Package-level doc, maybe? I think it's in the docs on cloud.google.com somewhere, but fairly hidden.

@broady WDYT of ac8a57f