Fixes #333 Handle parallel calls to add with osxkeychain

[beejeebus]

When docker login is called in parallel, some of the calls fail with the error The specified item already exists in the keychain.

This PR checks for this specific error in osxkeychain.Add and returns ok. If one process loses the race in this case, the desired credentials were saved by another process and we can ignore the error.

[thaJeztah]

I'm a bit on the fence if this is the right thing to do here. While the related ticket is about the same credentials being stored concurrently, it's also possible that different credentials are being stored for the same registry, in which case an error may be most appropriate.

Perhaps we could add a ErrConflict for this, and a utility to detect that, so that a client could choose to handle it;

docker-credential-helpers/credentials/error.go

Lines 100 to 111 in db1da9d

	// IsCredentialsMissingServerURL returns true if the error
	// was an errCredentialsMissingServerURL.
	func IsCredentialsMissingServerURL(err error) bool {
	var target errCredentialsMissingServerURL
	return errors.As(err, &target)
	}
	// IsCredentialsMissingServerURLMessage checks for an
	// errCredentialsMissingServerURL in the error message.
	func IsCredentialsMissingServerURLMessage(err string) bool {
	return strings.TrimSpace(err) == errCredentialsMissingServerURLMessage
	}

@laurazard any thoughts?;

[laurazard]

Apologies, took me a while to work my way here. I'll TAL.

[laurazard]

My intuition is that we should handle this a la EINTR, i.e. "things happened while your code was blocked on some call and you should handle it".

Otherwise, like @thaJeztah mentioned, we have no guarantees that the credentials saved were the desired ones by this process.

[laurazard]

@beejeebus what do you think?