logging: Add better logging on IPC server-side failures

Motivation: when trying to add a new unit test ran into a confusing error
"(remote):0: failed: remote exception: Called null capability" caused by
forgetting to make a FooInterface.initThreadMap call during initialization and
these logging prints would have made it easier to see where the error was
coming from and debug.

Author: ryanofsky

include/mp/proxy-types.h

@@ -719,6 +719,13 @@ kj::Promise<void> serverInvoke(Server& server, CallContext& call_context, Fn fn)
             .then([&server, req](CallContext call_context) {
                 server.m_context.loop->log() << "IPC server send response #" << req << " " << TypeName<Results>()
                                                  << " " << LogEscape(call_context.getResults().toString(), server.m_context.loop->m_log_opts.max_chars);
+            }, [&server, req](::kj::Exception&& e) {
+                // Call failed for some reason. Cap'n Proto will try to send
+                // this this error to the client as well, but it is good to log
+                // the failure early here and include the request number.
+                server.m_context.loop->log() << "IPC server error request #" << req << " " << TypeName<Results>()
+                                                 << " " << kj::str("kj::Exception: ", e).cStr();
+                return kj::mv(e);
             });
     } catch (const std::exception& e) {
         server.m_context.loop->log() << "IPC server unhandled exception: " << e.what();

include/mp/type-context.h

@@ -158,6 +158,25 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
                     << "IPC server error request #" << req << ", missing thread to execute request";
                 throw std::runtime_error("invalid thread handle");
             }
+        }, [&server, req](::kj::Exception&& e) {
+            // Detect when getLocalServer call fails. The serverInvoke function
+            // will print the full exception object after this, but it is worth
+            // printing an additional message here because a failure here is
+            // probably caused by failing to pass a ThreadMap object from the
+            // server to the client before this call. Specifically what happens
+            // if there was not a prior IPC call returning a ThreadMap, is that
+            // the client's Connection::m_thread_map member will be null when
+            // the client is making this IPC, and when it uses
+            // m_thread_map.makeThreadRequest() as part of this IPC, the
+            // makeThread request will appear to succeed due to Cap'n Proto's
+            // request pipelining, but actually return a null Thread::Client
+            // capability, and when that capability is passed to getLocalServer
+            // it will fail with an error here: "(remote):0: failed: remote
+            // exception: Called null capability" that's pretty opaque, so good
+            // to print a more specific message.
+            server.m_context.loop->log()
+                << "IPC server error request #" << req << " CapabilityServerSet<Thread>::getLocalServer call failed, did you forget to provide a ThreadMap to the client prior to this IPC call?";
+            return kj::mv(e);
         })
         // Wait for the invocation to finish before returning to the caller.
         .then([invoke_wait = kj::mv(future.promise)]() mutable { return kj::mv(invoke_wait); });