pw_rpc: Prevent starting calls with channel ID 0

- Channel ID 0 (Channel::kUnassignedChannelID) cannot be used for calls,
  and any packets with channel ID 0 are already rejected by RPC
  endpoints. Add a PW_CHECK() that crashes if code attempts to start a
  call on channel 0.
- Add a test for invoking a call on an unknown channel. No test is added
  for starting calls on channel 0, since pw_unit_test does not support
  testing that assertions failed.

Change-Id: Idc4a21925694c6300daa507c160fbf6822a93244
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/127036
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

3 files changed, 22 insertions, 3 deletions

diff --git a/pw_rpc/call.cc b/pw_rpc/call.cc
index ed16c1f4c..e96fd793f 100644
--- a/pw_rpc/call.cc
+++ b/pw_rpc/call.cc
@@ -62,6 +62,10 @@ Call::Call(LockedEndpoint& endpoint_ref,
                                ? kClientStreamActive
                                : kClientStreamInactive),
       properties_(properties) {
+  PW_CHECK_UINT_NE(channel_id,
+                   Channel::kUnassignedChannelId,
+                   "Calls cannot be created with channel ID 0 "
+                   "(Channel::kUnassignedChannelId)");
   endpoint().RegisterCall(*this);
 }
 
diff --git a/pw_rpc/docs.rst b/pw_rpc/docs.rst
index 3e8bf6b1f..133f41310 100644
--- a/pw_rpc/docs.rst
+++ b/pw_rpc/docs.rst
@@ -927,7 +927,8 @@ Example
   // Generated clients are namespaced with their proto library.
   using EchoClient = pw_rpc::nanopb::EchoService::Client;
 
-  // RPC channel ID on which to make client calls.
+  // RPC channel ID on which to make client calls. RPC calls cannot be made on
+  // channel 0 (Channel::kUnassignedChannelId).
   constexpr uint32_t kDefaultChannelId = 1;
 
   pw::rpc::NanopbUnaryReceiver<pw_rpc_EchoMessage> echo_call;
@@ -1191,8 +1192,8 @@ instantiate as:
   ``MethodRequestType<RpcMethod>``/``MethodResponseType<RpcMethod>``. Raw has
   them both set as ``void``. In reality, they are ``pw::ConstByteSpan``. Any
   helper/trait that wants to use this types for raw methods should do a custom
-  implemenation that copies the bytes under the span instead of copying just the
-  span.
+  implementation that copies the bytes under the span instead of copying just
+  the span.
 
 Client Synchronous Call wrappers
 ================================
diff --git a/pw_rpc/raw/client_reader_writer_test.cc b/pw_rpc/raw/client_reader_writer_test.cc
index 7b4184f0b..98b394679 100644
--- a/pw_rpc/raw/client_reader_writer_test.cc
+++ b/pw_rpc/raw/client_reader_writer_test.cc
@@ -14,6 +14,8 @@
 
 #include "pw_rpc/raw/client_reader_writer.h"
 
+#include <optional>
+
 #include "gtest/gtest.h"
 #include "pw_rpc/raw/client_testing.h"
 #include "pw_rpc/writer.h"
@@ -310,6 +312,18 @@ TEST(RawUnaryReceiver, Move_ActiveToActive) {
   EXPECT_TRUE(active_call_2.active());
 }
 
+TEST(RawUnaryReceiver, InvalidChannelId) {
+  RawClientTestContext ctx;
+  std::optional<Status> error;
+
+  RawUnaryReceiver call = TestService::TestUnaryRpc(
+      ctx.client(), 1290341, {}, {}, [&error](Status status) {
+        error = status;
+      });
+  EXPECT_FALSE(call.active());
+  EXPECT_EQ(error, Status::Unavailable());
+}
+
 TEST(RawClientReader, NoClientStream_OutOfScope_SilentlyCloses) {
   RawClientTestContext ctx;