fix: workaround curl_multi_poll returning an error on EINTR

Fixes #11647.

google/cloud/internal/curl_impl.cc

@@ -705,18 +705,32 @@ Status CurlImpl::PerformWorkUntil(absl::FunctionRef<bool()> predicate) {
 Status CurlImpl::WaitForHandles(int& repeats) {
   int const timeout_ms = 1000;
   int numfds = 0;
-#if CURL_AT_LEAST_VERSION(7, 66, 0)
-  CURLMcode result =
-      curl_multi_poll(multi_.get(), nullptr, 0, timeout_ms, &numfds);
-  TRACE_STATE() << ", numfds=" << numfds << ", result=" << result
-                << ", repeats=" << repeats;
-  if (result != CURLM_OK) return AsStatus(result, __func__);
+  CURLMcode result;
+  // Work around https://github.com/curl/curl/issues/11135 by retrying
+  // when EINTR fails poll(). Besides, before 7.84.0, poll() errors
+  // return CURLM_OK.
+  // Simply `numfds == 0 && errno == EINTR` might be a sufficient condition,
+  // but not 100% sure.
+#if _WIN32
+  result =
+        curl_multi_poll(multi_.get(), nullptr, 0, timeout_ms, &numfds);
+#elif CURL_AT_LEAST_VERSION(7, 84, 0)
+  do {
+    errno = 0;
+    result =
+        curl_multi_poll(multi_.get(), nullptr, 0, timeout_ms, &numfds);
+  } while (result == CURLM_UNRECOVERABLE_POLL && errno == EINTR);
 #else
-  CURLMcode result =
-      curl_multi_wait(multi_.get(), nullptr, 0, timeout_ms, &numfds);
+  do {
+    errno = 0;
+    result =
+        curl_multi_poll(multi_.get(), nullptr, 0, timeout_ms, &numfds);
+  } while (result == CURLM_OK && numfds == 0 && errno == EINTR);
+#endif
   TRACE_STATE() << ", numfds=" << numfds << ", result=" << result
                 << ", repeats=" << repeats;
   if (result != CURLM_OK) return AsStatus(result, __func__);
+#if !CURL_AT_LEAST_VERSION(7, 66, 0)
   // The documentation for curl_multi_wait() recommends sleeping if it returns
   // numfds == 0 more than once in a row :shrug:
   //    https://curl.haxx.se/libcurl/c/curl_multi_wait.html