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internal/mcp: avoid JSON nulls on the wire

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Add a test that checks for JSON null on the wire, and fix two categories
of bugs leading to null:

1. In many places, we could pass typed nil where we shouldn't have. Add
   an orZero helper to avoid typed nills.
2. When listing roots, we should avoid nil.

Fixes golang/go#74105

Change-Id: I061420487dbfcbf9016cb94b39bd72270e871c0e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682675
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Rob Findley 2025-06-18 16:52:23 +00:00 • committed by Robert Findley
commit 927070053a
1 changed files with 104 additions and 21 deletions
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24
mcp/shared.go
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@ -25,8 +25,7 @@ import (
// A MethodHandler handles MCP messages.
// For methods, exactly one of the return values must be nil.
// For notifications, both must be nil.
type MethodHandler[S Session] func(
ctx context.Context, _ S, method string, params Params) (result Result, err error)
type MethodHandler[S Session] func(ctx context.Context, _ S, method string, params Params) (result Result, err error)
// A methodHandler is a MethodHandler[Session] for some session.
// We need to give up type safety here, or we will end up with a type cycle somewhere
@ -73,6 +72,15 @@ func defaultSendingMethodHandler[S Session](ctx context.Context, session S, meth
return res, nil
}
// Helper methods to avoid typed nil.
func orZero[T any, P *U, U any](p P) T {
if p == nil {
var zero T
return zero
}
return any(p).(T)
}
func handleNotify[S Session](ctx context.Context, session S, method string, params Params) error {
mh := session.sendingMethodHandler().(MethodHandler[S])
_, err := mh(ctx, session, method, params)
@ -141,8 +149,13 @@ type methodInfo struct {
// A typedMethodHandler is like a MethodHandler, but with type information.
type typedMethodHandler[S Session, P Params, R Result] func(context.Context, S, P) (R, error)
type paramsPtr[T any] interface {
*T
Params
}
// newMethodInfo creates a methodInfo from a typedMethodHandler.
func newMethodInfo[S Session, P Params, R Result](d typedMethodHandler[S, P, R]) methodInfo {
func newMethodInfo[S Session, P paramsPtr[T], R Result, T any](d typedMethodHandler[S, P, R]) methodInfo {
return methodInfo{
unmarshalParams: func(m json.RawMessage) (Params, error) {
var p P
@ -151,9 +164,12 @@ func newMethodInfo[S Session, P Params, R Result](d typedMethodHandler[S, P, R])
return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
}
}
return p, nil
return orZero[Params](p), nil
},
handleMethod: MethodHandler[S](func(ctx context.Context, session S, _ string, params Params) (Result, error) {
if params == nil {
return d(ctx, session, nil)
}
return d(ctx, session, params.(P))
}),
// newResult is used on the send side, to construct the value to unmarshal the result into.