mcp: handle bad or missing params (#210)

Audit all cases where params must not be null, and enforce this using
methodInfo via a new methodFlags bitfield that parameterizes method
requirements. Write extensive conformance tests catching all the
(server-side) crashes that were possible.

We should go further and validate schema against the spec, but that is
more indirect, more complicated, and potentially slower. For now we
adopt this more explicit approach.

Still TODO in a subsequent CL: verify the client side of this with
client conformance tests.

Additionally, improve some error messages that were redundant or leaked
internal implementation details.

Fixes #195

mcp/testdata/conformance/server/bad_requests.txtar

@@ -4,10 +4,11 @@ bad requests.
 Fixed bugs:
 - No id in 'initialize' should not panic (#197).
 - No id in 'ping' should not panic (#194).
-- Notifications with IDs should not be treated like requests.
-
-TODO:
 - No params in 'initialize' should not panic (#195).
+- Notifications with IDs should not be treated like requests. (#196)
+- No params in 'logging/setLevel' should not panic.
+- No params in 'completion/complete' should not panic.
+- JSON null params should also not panic in these cases.
 
 -- prompts --
 code_review
@@ -22,6 +23,11 @@ code_review
     "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
   }
 }
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize"
+}
 {
   "jsonrpc": "2.0",
   "id": 2,
@@ -32,10 +38,21 @@ code_review
     "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
   }
 }
-{"jsonrpc":"2.0", "id": 3, "method":"notifications/initialized"}
+{"jsonrpc":"2.0", "id": 3, "method": "notifications/initialized"}
 {"jsonrpc":"2.0", "method":"ping"}
+{"jsonrpc":"2.0", "id": 4, "method": "logging/setLevel"}
+{"jsonrpc":"2.0", "id": 5, "method": "completion/complete"}
+{"jsonrpc":"2.0", "id": 4, "method": "logging/setLevel", "params": null}
 
 -- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}
 {
 	"jsonrpc": "2.0",
 	"id": 2,
@@ -59,6 +76,30 @@ code_review
 	"id": 3,
 	"error": {
 		"code": -32600,
-		"message": "JSON RPC invalid request: unexpected id for \"notifications/initialized\""
+		"message": "invalid request: unexpected id for \"notifications/initialized\""
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 4,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 4,
+	"error": {
+		"code": -32600,
+		"message": "handling 'logging/setLevel': invalid request: missing required \"params\""
 	}
 }

mcp/testdata/conformance/server/prompts.txtar

@@ -1,7 +1,8 @@
 Check behavior of a server with just prompts.
 
 Fixed bugs:
-- empty tools lists should not be returned as 'null'
+- Empty tools lists should not be returned as 'null'.
+- No params in 'prompts/get' should not panic.
 
 -- prompts --
 code_review
@@ -19,6 +20,7 @@ code_review
 }
 { "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
 { "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
+{ "jsonrpc": "2.0", "id": 5, "method": "prompts/get" }
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -63,3 +65,11 @@ code_review
 		]
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}

mcp/testdata/conformance/server/resources.txtar

@@ -2,6 +2,9 @@ Check behavior of a server with just resources.
 
 Fixed bugs:
 - A resource result holds a slice of contents, not just one.
+- No params in 'resource/read' should not panic.
+- No params in 'resources/subscribe' should not panic.
+- No params in 'resources/unsubscribe' should not panic.
 
 -- resources --
 info
@@ -39,6 +42,8 @@ info.txt
 	"roots": []
   }
 }
+{ "jsonrpc": "2.0", "id": 4, "method": "resources/read" }
+{ "jsonrpc": "2.0", "id": 5, "method": "resources/subscribe" }
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -107,3 +112,19 @@ info.txt
 		]
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 4,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}

mcp/testdata/conformance/server/tools.txtar

@@ -4,6 +4,7 @@ Fixed bugs:
 - "tools/list" can have missing params
 - "_meta" should not be nil
 - empty resource or prompts should not be returned as 'null'
+- the server should not crash when params are passed to tools/call
 
 -- tools --
 greet
@@ -22,6 +23,7 @@ greet
 { "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
 { "jsonrpc": "2.0", "id": 3, "method": "resources/list" }
 { "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
+{ "jsonrpc": "2.0", "id": 5, "method": "tools/call" }
 -- server --
 {
 	"jsonrpc": "2.0",
@@ -81,3 +83,11 @@ greet
 		"prompts": []
 	}
 }
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"error": {
+		"code": -32600,
+		"message": "invalid request: missing required \"params\""
+	}
+}