func RetrieveToken()

in internal/token.go [188:229]

• 28 lines of code
• 10 McCabe index (conditional complexity)

func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string, v url.Values, authStyle AuthStyle) (*Token, error) {
	needsAuthStyleProbe := authStyle == 0
	if needsAuthStyleProbe {
		if style, ok := lookupAuthStyle(tokenURL); ok {
			authStyle = style
			needsAuthStyleProbe = false
		} else {
			authStyle = AuthStyleInHeader // the first way we'll try
		}
	}
	req, err := newTokenRequest(tokenURL, clientID, clientSecret, v, authStyle)
	if err != nil {
		return nil, err
	}
	token, err := doTokenRoundTrip(ctx, req)
	if err != nil && needsAuthStyleProbe {
		// If we get an error, assume the server wants the
		// clientID & clientSecret in a different form.
		// See https://code.google.com/p/goauth2/issues/detail?id=31 for background.
		// In summary:
		// - Reddit only accepts client secret in the Authorization header
		// - Dropbox accepts either it in URL param or Auth header, but not both.
		// - Google only accepts URL param (not spec compliant?), not Auth header
		// - Stripe only accepts client secret in Auth header with Bearer method, not Basic
		//
		// We used to maintain a big table in this code of all the sites and which way
		// they went, but maintaining it didn't scale & got annoying.
		// So just try both ways.
		authStyle = AuthStyleInParams // the second way we'll try
		req, _ = newTokenRequest(tokenURL, clientID, clientSecret, v, authStyle)
		token, err = doTokenRoundTrip(ctx, req)
	}
	if needsAuthStyleProbe && err == nil {
		setAuthStyle(tokenURL, authStyle)
	}
	// Don't overwrite `RefreshToken` with an empty value
	// if this was a token refreshing request.
	if token != nil && token.RefreshToken == "" {
		token.RefreshToken = v.Get("refresh_token")
	}
	return token, err
}