internal/gensupport: SendRequestWithRetry sometimes spuriously makes requests when its context was canceled

[lhchavez]

Environment details

• Programming language: Go (v1.7+)
• OS: Any
• Language runtime version: v1.7+
• Package version: v0.61.0

Steps to reproduce

1. Call SendRequestWithRetry with a canceled context
2. The underlying network call sometimes still happens. This is due to Go legally being allowed to visit either of the branches in the following snippet of code if both are satisfied:

   google-api-go-client/internal/gensupport/send.go

   Lines 91 to 100 in 8fed2c8

   	select {
   	case <-ctx.Done():
   	// If we got an error, and the context has been canceled,
   	// the context's error is probably more useful.
   	if err == nil {
   	err = ctx.Err()
   	}
   	return resp, err
   	case <-time.After(pause):
   	}

Checking the context being done afterwards makes the issue go away.

Motivation

Some Cloud APIs expect callers to cancel the context if they want to cancel the operation, like
storage.(*ObjectHandle).NewWriter. The spuriousness of SendRequestWithRetry sometimes causes spooky things to happen, like uploading a 0-byte object to GCS, where no object was desired or expected!