http: run completed resolver callbacks on io executor

src/http.cpp

@@ -917,8 +917,18 @@ Resolver::add_callback(ResolverCb cb, sa_family_t family)
                                                else
                                                    cb(ec, filter(endpoints, family));
                                            });
-    else
-        cb(ec_, family == AF_UNSPEC ? endpoints_ : filter(endpoints_, family));
+    else {
+        // Always dispatch the callback on the io_context thread. Invoking it
+        // synchronously would run the caller's continuation (e.g. the whole
+        // connect chain of Request::send) on the calling thread, mutating asio
+        // objects concurrently with the io thread running the io_context.
+        asio::post(resolver_.get_executor(),
+                   [cb = std::move(cb),
+                    ec = ec_,
+                    endpoints = family == AF_UNSPEC ? endpoints_ : filter(endpoints_, family)] {
+                       cb(ec, endpoints);
+                   });
+    }
 }
 
 void

tests/test_http.cpp

@@ -8,6 +8,7 @@
 
 #include <iostream>
 #include <string>
+#include <atomic>
 #include <chrono>
 #include <condition_variable>
 
@@ -332,6 +333,36 @@ HttpTester::test_parse_url_just_slash()
     CPPUNIT_ASSERT_EQUAL(std::string("/"), parsed.target);
 }
 
+void
+HttpTester::test_completed_resolver_callback_uses_io_thread()
+{
+    asio::io_context ioContext;
+    const asio::ip::tcp::endpoint endpoint(asio::ip::address_v4::loopback(), 80);
+    dht::http::Resolver resolver(ioContext, {endpoint});
+    const auto callerThread = std::this_thread::get_id();
+    std::thread::id callbackThread;
+    asio::error_code callbackError;
+    std::vector<asio::ip::tcp::endpoint> resolvedEndpoints;
+    std::atomic_bool callbackCalled {false};
+
+    resolver.add_callback([&](const asio::error_code& ec, const auto& endpoints) {
+        callbackError = ec;
+        callbackThread = std::this_thread::get_id();
+        resolvedEndpoints = endpoints;
+        callbackCalled = true;
+    });
+
+    CPPUNIT_ASSERT(!callbackCalled);
+    std::thread ioThread([&] { ioContext.run(); });
+    ioThread.join();
+
+    CPPUNIT_ASSERT(callbackCalled);
+    CPPUNIT_ASSERT(!callbackError);
+    CPPUNIT_ASSERT(callbackThread != callerThread);
+    CPPUNIT_ASSERT_EQUAL(size_t(1), resolvedEndpoints.size());
+    CPPUNIT_ASSERT_EQUAL(endpoint, resolvedEndpoints.front());
+}
+
 void
 HttpTester::test_send_json()
 {

tests/test_http.h

@@ -39,6 +39,7 @@ class HttpTester : public CppUnit::TestFixture
     CPPUNIT_TEST(test_parse_url_complex_path_query_fragment);
     CPPUNIT_TEST(test_parse_url_empty);
     CPPUNIT_TEST(test_parse_url_just_slash);
+    CPPUNIT_TEST(test_completed_resolver_callback_uses_io_thread);
     // send
     CPPUNIT_TEST(test_send_json);
     CPPUNIT_TEST_SUITE_END();
@@ -82,6 +83,7 @@ public:
     void test_parse_url_complex_path_query_fragment();
     void test_parse_url_empty();
     void test_parse_url_just_slash();
+    void test_completed_resolver_callback_uses_io_thread();
     /**
      * Test send(json)
      */