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internal/mcp: simplify session type params

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Write a separate constraint for ClientSession | ServerSession,
and change the pointerness of the type parameter.

There is still a dynamic cast, but overall I think this is a bit
simpler:

- The exported types have the more readable constraint "Session"
  instead of "ClientSession | ServerSession".

- We can put the unexported methods in Session instead of needing a
  separate interface (previously "session").

- This CL enables a simplified treatment of sending middleware, in
  the next CL.

Change-Id: I41184fe93ea201d1da29dbfba26ac59f93771277
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676175
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
This commit is contained in:
Jonathan Amsterdam 2025-05-25 07:35:56 -04:00
commit 19d58e6955
1 changed files with 67 additions and 72 deletions
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12
design/design.md
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@ -416,11 +416,11 @@ We provide a mechanism to add MCP-level middleware on the both the client and se
// For methods, a MethodHandler must return either an XXResult struct pointer and a nil error, or
// nil with a non-nil error.
// For notifications, a MethodHandler must return nil, nil.
type MethodHandler[S ClientSession | ServerSession] func(
ctx context.Context, _ *S, method string, params any) (result any, err error)
type MethodHandler[S Session] func(
ctx context.Context, _ *S, method string, params Params) (result Result, err error)
// Middleware is a function from MethodHandlers to MethodHandlers.
type Middleware[S ClientSession | ServerSession] func(MethodHandler[S]) MethodHandler[S]
type Middleware[S Session] func(MethodHandler[S]) MethodHandler[S]
// AddMiddleware wraps the client/server's current method handler using the provided
// middleware. Middleware is applied from right to left, so that the first one
@ -428,14 +428,14 @@ type Middleware[S ClientSession | ServerSession] func(MethodHandler[S]) MethodHa
//
// For example, AddMiddleware(m1, m2, m3) augments the server method handler as
// m1(m2(m3(handler))).
func (c *Client) AddMiddleware(middleware ...Middleware[ClientSession])
func (s *Server) AddMiddleware(middleware ...Middleware[ServerSession])
func (c *Client) AddMiddleware(middleware ...Middleware[*ClientSession])
func (s *Server) AddMiddleware(middleware ...Middleware[*ServerSession])
```
As an example, this code adds server-side logging:
```go
func withLogging(h mcp.MethodHandler[ServerSession]) mcp.MethodHandler[ServerSession]{
func withLogging(h mcp.MethodHandler[*ServerSession]) mcp.MethodHandler[*ServerSession]{
return func(ctx context.Context, s *mcp.ServerSession, method string, params any) (res any, err error) {
log.Printf("request: %s %v", method, params)
defer func() { log.Printf("response: %v, %v", res, err) }()