chore: import upstream snapshot with attribution

internal/ghmcp/oauth.go

@@ -0,0 +1,133 @@
+package ghmcp
+
+import (
+	"context"
+	"crypto/rand"
+	"fmt"
+	"log/slog"
+
+	"github.com/github/github-mcp-server/internal/oauth"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+// sessionPrompter adapts an MCP server session to oauth.Prompter, presenting
+// authorization prompts to the user via elicitation. Keeping the prompt on the
+// MCP control channel (rather than a tool result) keeps the authorization URL
+// and any session-bound state out of the model's context.
+type sessionPrompter struct {
+	session *mcp.ServerSession
+}
+
+// elicitationCaps returns the client's declared elicitation capabilities, or nil
+// if the client did not advertise any.
+func (p *sessionPrompter) elicitationCaps() *mcp.ElicitationCapabilities {
+	params := p.session.InitializeParams()
+	if params == nil || params.Capabilities == nil {
+		return nil
+	}
+	return params.Capabilities.Elicitation
+}
+
+// CanPromptURL reports whether the client supports URL-mode elicitation.
+func (p *sessionPrompter) CanPromptURL() bool {
+	caps := p.elicitationCaps()
+	return caps != nil && caps.URL != nil
+}
+
+// PromptURL presents the authorization URL via URL-mode elicitation and blocks
+// until the user acknowledges, declines, or ctx is done.
+func (p *sessionPrompter) PromptURL(ctx context.Context, prompt oauth.Prompt) error {
+	res, err := p.session.Elicit(ctx, &mcp.ElicitParams{
+		Mode:          "url",
+		Message:       prompt.Message,
+		URL:           prompt.URL,
+		ElicitationID: rand.Text(),
+	})
+	if err != nil {
+		// The client advertised URL elicitation but the request itself failed:
+		// classify it as undeliverable (not a user decision) so the flow can fall
+		// back to a channel that needs no client capability.
+		return fmt.Errorf("%w: %w", oauth.ErrPromptUnavailable, err)
+	}
+	if res.Action != "accept" {
+		return oauth.ErrPromptDeclined
+	}
+	return nil
+}
+
+// CanPromptForm reports whether the client supports form-mode elicitation. The
+// SDK treats a client that advertises neither form nor URL capabilities as
+// supporting forms, for backward compatibility, so we mirror that here.
+func (p *sessionPrompter) CanPromptForm() bool {
+	caps := p.elicitationCaps()
+	if caps == nil {
+		return false
+	}
+	return caps.Form != nil || caps.URL == nil
+}
+
+// PromptForm presents a textual acknowledgement (used to display a device code
+// when URL elicitation is unavailable) and blocks until the user responds.
+func (p *sessionPrompter) PromptForm(ctx context.Context, prompt oauth.Prompt) error {
+	res, err := p.session.Elicit(ctx, &mcp.ElicitParams{
+		Mode:    "form",
+		Message: prompt.Message,
+	})
+	if err != nil {
+		// As with PromptURL, a delivery failure is undeliverable rather than a
+		// decline, so the flow can fall back instead of aborting.
+		return fmt.Errorf("%w: %w", oauth.ErrPromptUnavailable, err)
+	}
+	if res.Action != "accept" {
+		return oauth.ErrPromptDeclined
+	}
+	return nil
+}
+
+// oauthAuthenticator is the subset of *oauth.Manager that the middleware needs.
+// Depending on the interface (rather than the concrete manager) lets the
+// middleware be exercised with a deterministic fake, since driving the real
+// manager to its branches would require standing up live GitHub flows.
+type oauthAuthenticator interface {
+	HasToken() bool
+	Authenticate(ctx context.Context, prompter oauth.Prompter) (*oauth.Outcome, error)
+}
+
+// createOAuthMiddleware returns receiving middleware that authorizes the session
+// lazily, on the first tool call. Authorization is deferred until here (rather
+// than at startup) because the prompts depend on an initialized session whose
+// elicitation capabilities are known.
+//
+// When a token is already available the call proceeds untouched. Otherwise the
+// flow runs: secure channels (browser, URL elicitation) block until the token
+// arrives and then the call proceeds; the last-resort channel returns the
+// instruction to the user as a tool result and asks them to retry.
+func createOAuthMiddleware(mgr oauthAuthenticator, logger *slog.Logger) func(next mcp.MethodHandler) mcp.MethodHandler {
+	return func(next mcp.MethodHandler) mcp.MethodHandler {
+		return func(ctx context.Context, method string, request mcp.Request) (mcp.Result, error) {
+			if method != "tools/call" || mgr.HasToken() {
+				return next(ctx, method, request)
+			}
+
+			callReq, ok := request.(*mcp.CallToolRequest)
+			if !ok {
+				return next(ctx, method, request)
+			}
+
+			outcome, err := mgr.Authenticate(ctx, &sessionPrompter{session: callReq.Session})
+			if err != nil {
+				return nil, fmt.Errorf("github authorization failed: %w", err)
+			}
+			if outcome != nil && outcome.UserAction != nil {
+				logger.Info("surfacing github authorization instructions to user")
+				return &mcp.CallToolResult{
+					Content: []mcp.Content{&mcp.TextContent{Text: outcome.UserAction.Message}},
+				}, nil
+			}
+			return next(ctx, method, request)
+		}
+	}
+}
+
+// ensure sessionPrompter satisfies the Prompter contract.
+var _ oauth.Prompter = (*sessionPrompter)(nil)