HttpServerConnection: Prefer error codes over Boost exceptions

When run within a coroutine, exceptions on Windows may influence
bad behaviour here. Instead, we'll check for the error code
and extract the message from memory. In contrast to exceptions
which are stored on the stack frame and then return, this costs
a little more memory but simplifies the logic.

This doesn't fix the linked issue, but is related to the analysis.

refs #7431

lib/remote/httpserverconnection.cpp

@@ -78,20 +78,13 @@ void HttpServerConnection::Disconnect()
 			Log(LogInformation, "HttpServerConnection")
 				<< "HTTP client disconnected (from " << m_PeerAddress << ")";
 
-			try {
-				m_Stream->next_layer().async_shutdown(yc);
-			} catch (...) {
-			}
+			boost::system::error_code ec;
 
-			try {
-				m_Stream->lowest_layer().shutdown(m_Stream->lowest_layer().shutdown_both);
-			} catch (...) {
-			}
+			m_Stream->next_layer().async_shutdown(yc[ec]);
 
-			try {
-				m_Stream->lowest_layer().cancel();
-			} catch (...) {
-			}
+			m_Stream->lowest_layer().shutdown(m_Stream->lowest_layer().shutdown_both, ec);
+
+			m_Stream->lowest_layer().cancel(ec);
 
 			m_CheckLivenessTimer.cancel();
 
@@ -148,16 +141,18 @@ bool EnsureValidHeaders(
 	bool httpError = true;
 
 	try {
-		try {
-			http::async_read_header(stream, buf, parser, yc);
-		} catch (const boost::system::system_error& ex) {
+		boost::system::error_code ec;
+
+		http::async_read_header(stream, buf, parser, yc[ec]);
+
+		if (ec) {
 			/**
 			 * Unfortunately there's no way to tell an HTTP protocol error
 			 * from an error on a lower layer:
 			 *
 			 * <https://github.com/boostorg/beast/issues/643>
 			 */
-			throw std::invalid_argument(ex.what());
+			throw std::invalid_argument(ec.message());
 		}
 
 		httpError = false;
@@ -185,8 +180,10 @@ bool EnsureValidHeaders(
 
 		response.set(http::field::connection, "close");
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		boost::system::error_code ec;
+
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 
 		return false;
 	}
@@ -208,8 +205,10 @@ void HandleExpect100(
 
 		response.result(http::status::continue_);
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		boost::system::error_code ec;
+
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 	}
 }
 
@@ -252,8 +251,10 @@ bool HandleAccessControl(
 					response.set(http::field::content_length, response.body().size());
 					response.set(http::field::connection, "close");
 
-					http::async_write(stream, response, yc);
-					stream.async_flush(yc);
+					boost::system::error_code ec;
+
+					http::async_write(stream, response, yc[ec]);
+					stream.async_flush(yc[ec]);
 
 					return false;
 				}
@@ -281,8 +282,10 @@ bool EnsureAcceptHeader(
 		response.set(http::field::content_length, response.body().size());
 		response.set(http::field::connection, "close");
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		boost::system::error_code ec;
+
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 
 		return false;
 	}
@@ -320,8 +323,10 @@ bool EnsureAuthenticatedUser(
 			response.set(http::field::content_length, response.body().size());
 		}
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		boost::system::error_code ec;
+
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 
 		return false;
 	}
@@ -378,9 +383,11 @@ bool EnsureValidBody(
 		parser.body_limit(maxSize);
 	}
 
-	try {
-		http::async_read(stream, buf, parser, yc);
-	} catch (const boost::system::system_error& ex) {
+	boost::system::error_code ec;
+
+	http::async_read(stream, buf, parser, yc[ec]);
+
+	if (ec) {
 		/**
 		 * Unfortunately there's no way to tell an HTTP protocol error
 		 * from an error on a lower layer:
@@ -393,18 +400,18 @@ bool EnsureValidBody(
 		if (parser.get()[http::field::accept] == "application/json") {
 			HttpUtility::SendJsonBody(response, nullptr, new Dictionary({
 				{ "error", 400 },
-				{ "status", String("Bad Request: ") + ex.what() }
+				{ "status", String("Bad Request: ") + ec.message() }
 			}));
 		} else {
 			response.set(http::field::content_type, "text/html");
-			response.body() = String("<h1>Bad Request</h1><p><pre>") + ex.what() + "</pre></p>";
+			response.body() = String("<h1>Bad Request</h1><p><pre>") + ec.message() + "</pre></p>";
 			response.set(http::field::content_length, response.body().size());
 		}
 
 		response.set(http::field::connection, "close");
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 
 		return false;
 	}
@@ -438,8 +445,10 @@ bool ProcessRequest(
 
 		HttpUtility::SendJsonError(response, nullptr, 500, "Unhandled exception" , DiagnosticInformation(ex));
 
-		http::async_write(stream, response, yc);
-		stream.async_flush(yc);
+		boost::system::error_code ec;
+
+		http::async_write(stream, response, yc[ec]);
+		stream.async_flush(yc[ec]);
 
 		return true;
 	}
@@ -448,8 +457,10 @@ bool ProcessRequest(
 		return false;
 	}
 
-	http::async_write(stream, response, yc);
-	stream.async_flush(yc);
+	boost::system::error_code ec;
+
+	http::async_write(stream, response, yc[ec]);
+	stream.async_flush(yc[ec]);
 
 	return true;
 }