Refuse a PRIVMSG or NOTICE whose body or meta has the wrong type (closes #245)

Over the HTTP API, a PRIVMSG or NOTICE body was checked only by
decoding it into a list of strings. JSON null decodes into an empty
string there, and meta was not checked at all, so a null line, or a
meta that is not an object, reached every recipient.

handlePrivmsg now answers 400 invalid request body, before anything
else, when body is present and is not an array of strings, or meta is
present and is not an object. A missing or empty body still gets
ERR_NOTEXTTOSEND. README.md says so.

Model: opus-5-5

README.md

@@ -643,8 +643,10 @@ Send a message to a channel or user. This is the primary messaging command.
   message echoed back via the queue).
 - If `to` is a bare nick, the message is a DM. The server fans out to the
   recipient and the sender (so all of the sender's clients see the DM).
-- `body` must be a non-empty array of strings; otherwise the server sends
-  ERR_NOTEXTTOSEND (412). A missing `to` gets ERR_NORECIPIENT (411).
+- A `body` that is not an array of strings, or a `meta` that is not an object,
+  gets HTTP 400 `invalid request body`, and nothing is relayed. A missing or
+  empty `body` gets ERR_NOTEXTTOSEND (412). A missing `to` gets ERR_NORECIPIENT
+  (411).
 - Only channel members can send to a channel. A non-member, a banned user, and
   on a `+m` channel a user with neither `+o` nor `+v`, get ERR_CANNOTSENDTOCHAN
   (404).
@@ -1749,17 +1751,18 @@ command). **Numeric replies**, for success and for errors, are delivered through
 the message queue (see [Numeric Reply Codes](#numeric-reply-codes-s2c-only)); a
 command that has none, such as a successful `PRIVMSG`, is answered by the HTTP
 response alone, and `PING`'s `PONG` is the HTTP response body. HTTP error codes
-(4xx/5xx) are reserved for transport-level problems: malformed JSON (400),
+(4xx/5xx) are reserved for transport-level problems: a malformed request (400),
 missing/invalid auth cookies (401), and server errors (500).
 
 **HTTP errors (transport-level only):**
 
-| Status | Error                  | When                                                    |
-| ------ | ---------------------- | ------------------------------------------------------- |
-| 400    | `invalid request body` | Malformed JSON, or a body over `MAX_MESSAGE_SIZE` bytes |
-| 400    | `command required`     | Empty command                                           |
-| 401    | `not registered`       | Missing or invalid auth cookie                          |
-| 500    | `internal error`       | Server-side failure                                     |
+| Status | Error                  | When                                                                                              |
+| ------ | ---------------------- | ------------------------------------------------------------------------------------------------- |
+| 400    | `invalid request body` | Malformed JSON, or a body over `MAX_MESSAGE_SIZE` bytes                                           |
+| 400    | `invalid request body` | A `PRIVMSG` or `NOTICE` whose `body` is not an array of strings, or whose `meta` is not an object |
+| 400    | `command required`     | Empty command                                                                                     |
+| 401    | `not registered`       | Missing or invalid auth cookie                                                                    |
+| 500    | `internal error`       | Server-side failure                                                                               |
 
 **IRC numeric error replies (delivered via message queue):**
 

internal/handlers/api.go

@@ -1141,6 +1141,16 @@ func (hdlr *Handlers) handlePrivmsg(
 	meta json.RawMessage,
 	bodyLines func() []string,
 ) {
+	if !validBodyAndMeta(body, meta) {
+		hdlr.respondError(
+			writer, request,
+			"invalid request body",
+			http.StatusBadRequest,
+		)
+
+		return
+	}
+
 	if target == "" {
 		hdlr.enqueueNumeric(
 			request.Context(), clientID,
@@ -1189,6 +1199,40 @@ func (hdlr *Handlers) handlePrivmsg(
 	)
 }
 
+// validBodyAndMeta reports whether body, when present, is an
+// array of strings and meta, when present, is an object.
+// Decoding into []string would turn a null line into "", so
+// each line is decoded as any and checked. A null body or
+// meta decodes without an error into nil.
+func validBodyAndMeta(body, meta json.RawMessage) bool {
+	if body != nil {
+		var lines []any
+
+		err := json.Unmarshal(body, &lines)
+		if err != nil || lines == nil {
+			return false
+		}
+
+		for _, line := range lines {
+			_, isString := line.(string)
+			if !isString {
+				return false
+			}
+		}
+	}
+
+	if meta != nil {
+		var fields map[string]any
+
+		err := json.Unmarshal(meta, &fields)
+		if err != nil || fields == nil {
+			return false
+		}
+	}
+
+	return true
+}
+
 // respondIRCError enqueues a numeric error reply, notifies
 // the broker, and sends HTTP 200 with {"status":"error"}.
 func (hdlr *Handlers) respondIRCError(

internal/handlers/api_test.go

@@ -8,10 +8,12 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"maps"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
 	"os"
+	"reflect"
 	"slices"
 	"strconv"
 	"strings"
@@ -1056,6 +1058,132 @@ func TestMessageMissingTo(t *testing.T) {
 	}
 }
 
+// TestMessageInvalidBodyOrMeta checks that a PRIVMSG or
+// NOTICE, to a channel or a nick, whose body is not an array
+// of strings or whose meta is not an object gets 400 and
+// reaches nobody.
+func TestMessageInvalidBodyOrMeta(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name   string
+		fields map[string]any
+	}{
+		{"null line", map[string]any{bodyKey: []any{"hi", nil}}},
+		{"number line", map[string]any{bodyKey: []any{"hi", 1}}},
+		{"object line", map[string]any{bodyKey: []any{map[string]any{}}}},
+		{"null body", map[string]any{bodyKey: nil}},
+		{"object body", map[string]any{bodyKey: map[string]any{}}},
+		{"null meta", map[string]any{bodyKey: []string{"hi"}, metaKey: nil}},
+		{"array meta", map[string]any{bodyKey: []string{"hi"}, metaKey: []string{}}},
+		{"string meta", map[string]any{bodyKey: []string{"hi"}, metaKey: "hi"}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			const channel = "#badmsg"
+
+			tserver := newTestServer(t)
+			aliceToken := tserver.createSession("alice_badmsg")
+			bobToken := tserver.createSession("bob_badmsg")
+
+			tserver.sendCommand(aliceToken, map[string]any{
+				commandKey: joinCmd, toKey: channel,
+			})
+			tserver.sendCommand(bobToken, map[string]any{
+				commandKey: joinCmd, toKey: channel,
+			})
+
+			_, bobLastID := tserver.pollMessages(bobToken, 0)
+
+			for _, command := range []string{privmsgCmd, irc.CmdNotice} {
+				for _, target := range []string{channel, "bob_badmsg"} {
+					request := map[string]any{
+						commandKey: command, toKey: target,
+					}
+					maps.Copy(request, tc.fields)
+
+					status, result := tserver.sendCommand(
+						aliceToken, request,
+					)
+					if status != http.StatusBadRequest ||
+						result["error"] != "invalid request body" {
+						t.Errorf(
+							"%s to %s: expected 400 invalid "+
+								"request body, got %d: %v",
+							command, target, status, result,
+						)
+					}
+				}
+			}
+
+			msgs, _ := tserver.pollMessages(bobToken, bobLastID)
+			if len(msgs) != 0 {
+				t.Fatalf("expected no messages, bob got %v", msgs)
+			}
+		})
+	}
+}
+
+// TestMessageRelaysBodyAndMeta checks that a PRIVMSG or
+// NOTICE, to a channel or a nick, reaches its recipient with
+// body and meta as sent.
+func TestMessageRelaysBodyAndMeta(t *testing.T) {
+	t.Parallel()
+
+	const channel = "#relaymsg"
+
+	tserver := newTestServer(t)
+	aliceToken := tserver.createSession("alice_relaymsg")
+	bobToken := tserver.createSession("bob_relaymsg")
+
+	tserver.sendCommand(aliceToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+	tserver.sendCommand(bobToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+
+	_, bobLastID := tserver.pollMessages(bobToken, 0)
+
+	body := []any{"line one", ""}
+	meta := map[string]any{"sig": "abc", "alg": "ed25519"}
+
+	for _, command := range []string{privmsgCmd, irc.CmdNotice} {
+		for _, target := range []string{channel, "bob_relaymsg"} {
+			status, result := tserver.sendCommand(
+				aliceToken,
+				map[string]any{
+					commandKey: command, toKey: target,
+					bodyKey: body, metaKey: meta,
+				},
+			)
+			if status != http.StatusOK {
+				t.Fatalf(
+					"%s to %s: expected 200, got %d: %v",
+					command, target, status, result,
+				)
+			}
+
+			var msgs []map[string]any
+
+			msgs, bobLastID = tserver.pollMessages(
+				bobToken, bobLastID,
+			)
+			if len(msgs) != 1 ||
+				msgs[0][commandKey] != command ||
+				!reflect.DeepEqual(msgs[0][bodyKey], body) ||
+				!reflect.DeepEqual(msgs[0][metaKey], meta) {
+				t.Fatalf(
+					"%s to %s: bob got %v", command, target, msgs,
+				)
+			}
+		}
+	}
+}
+
 func TestNonMemberCannotSend(t *testing.T) {
 	t.Parallel()