[GitHub] [arrow] coryan commented on a diff in pull request #13520: ARROW-14889: [C++] GCS tests hang if testbench not installed

[GitBox <gi...@apache.org>]

coryan commented on code in PR #13520:
URL: https://github.com/apache/arrow/pull/13520#discussion_r916289559


##########
cpp/src/arrow/filesystem/gcsfs_test.cc:
##########
@@ -105,9 +106,21 @@ class GcsTestbench : public ::testing::Environment {
         error += " (exe not found)";
         continue;
       }
-
-      server_process_ = bp::child(exe_path, "-m", "testbench", "--port", port_, group_);
-      if (server_process_.valid() && server_process_.running()) break;
+      // TODO: should I be worried about lifetime of this variable?
+      bp::ipstream output;
+
+      server_process_ = bp::child(exe_path, "-m", "testbench", "--port", port_, group_,
+                                  bp::std_err > output);
+      // Wait for message: "* Restarting with"
+      auto testbench_is_running = [&output](bp::child& process) {
+        std::string line;
+        while (process.valid() && process.running() && std::getline(output, line)) {
+          if (line.find("* Restarting with") != std::string::npos) return true;
+        }
+        return false;
+      };
+
+      if (testbench_is_running(server_process_)) break;

Review Comment:
   It depends on the value of "easy".  I have not tested this, but allegedly you can use asynchronous I/O with `Boost.Process`:
   
   https://www.boost.org/doc/libs/1_79_0/doc/html/process/reference.html#header.boost.process.io_hpp
   
   The example where they use a future could be changed with a `wait_for()`:
   
   ```cc
   boost::asio::io_context ios;
   std::future<std::string> output;
   system("ls", std_out > output, ios);
   
   auto status = fut.wait_for(std::chrono::seconds(10));
   if (status == std::future_status::timeout) { /* uh oh */ }
   ```
   
   I expect you would need a thread or something to run with the `boost::asio::io_context`, something like:
   
   ```cc
     boost::asio::io_context ios;
     std::thead svc{[&] { ios.run_for(std::chrono::seconds(..)); }};
   ...
     // insert the code from above
     svc.join();
   ....
   



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