libstore/filetransfer: Replace curl_multi_wait with curl_multi_poll and get rid of CPP

Since 7.68 libcurl already provides curl_multi_wakeup, so we can drop the hacky
pipe setup (libcurl does this internally).

src/libstore/filetransfer.cc

@@ -46,6 +46,17 @@ namespace {
 using curlSList = std::unique_ptr<::curl_slist, decltype([](::curl_slist * list) { ::curl_slist_free_all(list); })>;
 using curlMulti = std::unique_ptr<::CURLM, decltype([](::CURLM * multi) { ::curl_multi_cleanup(multi); })>;
 
+struct curlMultiError : Error
+{
+    ::CURLMcode code;
+
+    curlMultiError(::CURLMcode code)
+        : Error{"unexpected curl multi error: %s", ::curl_multi_strerror(code)}
+    {
+        assert(code != CURLM_OK);
+    }
+};
+
 } // namespace
 
 struct curlFileTransfer : public FileTransfer
@@ -362,7 +373,7 @@ struct curlFileTransfer : public FileTransfer
             return ((TransferItem *) userp)->readCallback(buffer, size, nitems);
         }
 
-#if !defined(_WIN32) && LIBCURL_VERSION_NUM >= 0x071000
+#if !defined(_WIN32)
         static int cloexec_callback(void *, curl_socket_t curlfd, curlsocktype purpose)
         {
             unix::closeOnExec(curlfd);
@@ -425,15 +436,11 @@ struct curlFileTransfer : public FileTransfer
                 ("curl/" LIBCURL_VERSION " Nix/" + nixVersion
                  + (fileTransferSettings.userAgentSuffix != "" ? " " + fileTransferSettings.userAgentSuffix.get() : ""))
                     .c_str());
-#if LIBCURL_VERSION_NUM >= 0x072b00
             curl_easy_setopt(req, CURLOPT_PIPEWAIT, 1);
-#endif
-#if LIBCURL_VERSION_NUM >= 0x072f00
             if (fileTransferSettings.enableHttp2)
                 curl_easy_setopt(req, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);
             else
                 curl_easy_setopt(req, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
-#endif
             curl_easy_setopt(req, CURLOPT_WRITEFUNCTION, TransferItem::writeCallbackWrapper);
             curl_easy_setopt(req, CURLOPT_WRITEDATA, this);
             curl_easy_setopt(req, CURLOPT_HEADERFUNCTION, TransferItem::headerCallbackWrapper);
@@ -473,10 +480,9 @@ struct curlFileTransfer : public FileTransfer
             if (settings.caFile != "")
                 curl_easy_setopt(req, CURLOPT_CAINFO, settings.caFile.get().c_str());
 
-#if !defined(_WIN32) && LIBCURL_VERSION_NUM >= 0x071000
+#if !defined(_WIN32)
             curl_easy_setopt(req, CURLOPT_SOCKOPTFUNCTION, cloexec_callback);
 #endif
-
             curl_easy_setopt(req, CURLOPT_CONNECTTIMEOUT, fileTransferSettings.connectTimeout.get());
 
             curl_easy_setopt(req, CURLOPT_LOW_SPEED_LIMIT, 1L);
@@ -708,13 +714,6 @@ struct curlFileTransfer : public FileTransfer
 
     Sync<State> state_;
 
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-    /* We can't use a std::condition_variable to wake up the curl
-       thread, because it only monitors file descriptors. So use a
-       pipe instead. */
-    Pipe wakeupPipe;
-#endif
-
     std::thread workerThread;
 
     curlFileTransfer()
@@ -725,38 +724,33 @@ struct curlFileTransfer : public FileTransfer
 
         curlm = curlMulti(curl_multi_init());
 
-#if LIBCURL_VERSION_NUM >= 0x072b00 // Multiplex requires >= 7.43.0
         curl_multi_setopt(curlm.get(), CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);
-#endif
-#if LIBCURL_VERSION_NUM >= 0x071e00 // Max connections requires >= 7.30.0
         curl_multi_setopt(curlm.get(), CURLMOPT_MAX_TOTAL_CONNECTIONS, fileTransferSettings.httpConnections.get());
-#endif
-
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-        wakeupPipe.create();
-        fcntl(wakeupPipe.readSide.get(), F_SETFL, O_NONBLOCK);
-#endif
 
         workerThread = std::thread([&]() { workerThreadEntry(); });
     }
 
     ~curlFileTransfer()
     {
-        stopWorkerThread();
-
+        try {
+            stopWorkerThread();
+        } catch (...) {
+            ignoreExceptionInDestructor();
+        }
         workerThread.join();
     }
 
     void stopWorkerThread()
     {
         /* Signal the worker thread to exit. */
-        {
-            auto state(state_.lock());
-            state->quit();
-        }
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-        writeFull(wakeupPipe.writeSide.get(), " ", false);
-#endif
+        state_.lock()->quit();
+        wakeupMulti();
+    }
+
+    void wakeupMulti()
+    {
+        if (auto ec = ::curl_multi_wakeup(curlm.get()))
+            throw curlMultiError(ec);
     }
 
     void workerThreadMain()
@@ -805,13 +799,6 @@ struct curlFileTransfer : public FileTransfer
             }
 
             /* Wait for activity, including wakeup events. */
-            int numfds = 0;
-            struct curl_waitfd extraFDs[1];
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-            extraFDs[0].fd = wakeupPipe.readSide.get();
-            extraFDs[0].events = CURL_WAIT_POLLIN;
-            extraFDs[0].revents = 0;
-#endif
             long maxSleepTimeMs = items.empty() ? 10000 : 100;
             auto sleepTimeMs = nextWakeup != std::chrono::steady_clock::time_point()
                                    ? std::max(
@@ -820,23 +807,14 @@ struct curlFileTransfer : public FileTransfer
                                              nextWakeup - std::chrono::steady_clock::now())
                                              .count())
                                    : maxSleepTimeMs;
-            vomit("download thread waiting for %d ms", sleepTimeMs);
-            mc = curl_multi_wait(curlm.get(), extraFDs, 1, sleepTimeMs, &numfds);
+
+            int numfds = 0;
+            mc = curl_multi_poll(curlm.get(), nullptr, 0, sleepTimeMs, &numfds);
             if (mc != CURLM_OK)
-                throw nix::Error("unexpected error from curl_multi_wait(): %s", curl_multi_strerror(mc));
+                throw curlMultiError(mc);
 
             nextWakeup = std::chrono::steady_clock::time_point();
 
-            /* Add new curl requests from the incoming requests queue,
-               except for requests that are embargoed (waiting for a
-               retry timeout to expire). */
-            if (extraFDs[0].revents & CURL_WAIT_POLLIN) {
-                char buf[1024];
-                auto res = read(extraFDs[0].fd, buf, sizeof(buf));
-                if (res == -1 && errno != EINTR)
-                    throw SysError("reading curl wakeup socket");
-            }
-
             std::vector<std::shared_ptr<TransferItem>> incoming;
             auto now = std::chrono::steady_clock::now();
 
@@ -910,10 +888,8 @@ struct curlFileTransfer : public FileTransfer
                 throw nix::Error("cannot enqueue download request because the download thread is shutting down");
             state->incoming.push(item);
         }
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-        writeFull(wakeupPipe.writeSide.get(), " ");
-#endif
 
+        wakeupMulti();
         return ItemHandle(static_cast<Item &>(*item));
     }
 
@@ -933,9 +909,7 @@ struct curlFileTransfer : public FileTransfer
     {
         auto state(state_.lock());
         state->unpause.push_back(std::move(item));
-#ifndef _WIN32 // TODO need graceful async exit support on Windows?
-        writeFull(wakeupPipe.writeSide.get(), " ");
-#endif
+        wakeupMulti();
     }
 
     void unpauseTransfer(ItemHandle handle) override