Merge bitcoin-core#245: type-context.h: Extent cancel_mutex lock to prevent theoretical race

ef96a5b doc: Comment cleanups after bitcoin-core#240 (Ryan Ofsky)
e0f1cd7 type-context.h: Extent cancel_mutex lock to prevent theoretical race (Ryan Ofsky)

Pull request description:

  This is a followup to bitcoin-core#240 that fixes a theoretical race condition in that PR pointed out by janb bitcoin/bitcoin#34422 (comment). Details are in the commit message. There is also an additional commit fixing up some documentation added in that PR

ACKs for top commit:
  Sjors:
    ACK ef96a5b

Tree-SHA512: 9d3c1585b7e1862a3b2bada48f72c389cf5d60cc72aa348c83fe87fb8f05287971216fc561a4334140df4a8d2bcc28f5256dd733ff3fb4ec08aa72e57d136963

Author: ryanofsky

include/mp/type-context.h

@@ -123,7 +123,7 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
                             // it. So in addition to locking the mutex, the
                             // execution thread always checks request_canceled
                             // as well before accessing the structs.
-                            Lock{cancel_mutex};
+                            Lock cancel_lock{cancel_mutex};
                             server_context.request_canceled = true;
                         };
                         // Update requests_threads map if not canceled.
@@ -210,7 +210,7 @@ auto PassField(Priority<1>, TypeList<>, ServerContext& server_context, const Fn&
     // connection is destroyed. (By default Cap'n Proto does not cancel requests
     // on disconnect, since it's possible clients might want to make requests
     // and immediately disconnect without waiting for results, but not want the
-    // the requests to be canceled.)
+    // requests to be canceled.)
     return server.m_context.connection->m_canceler.wrap(kj::mv(result));
 }
 } // namespace mp

include/mp/util.h

@@ -331,7 +331,7 @@ void CancelMonitor::promiseDestroyed(CancelProbe& probe)
     // theory this method could be called when a promise was fulfilled or
     // rejected rather than canceled, but it's safe to assume that's not the
     // case because the CancelMonitor class is meant to be used inside code
-    // fulfilling or rejecting the promise and destroyed before doing these.
+    // fulfilling or rejecting the promise and destroyed before doing so.
     assert(m_probe == &probe);
     m_canceled = true;
     if (m_on_cancel) m_on_cancel();