Read subprocess pipes in background

Async use of wxExecute requires that user code periodically checks the
streams and reads their content. This is particularly important on
platforms with tiny buffers, such as… Windows.

src/subprocess.cpp

@@ -34,6 +34,7 @@
 #include <wx/filename.h>
 #include <wx/process.h>
 #include <wx/mstream.h>
+#include <wx/timer.h>
 #include <wx/txtstrm.h>
 
 
@@ -44,34 +45,52 @@ namespace
 class Process : public wxProcess
 {
 public:
-    Process() = default;
+    Process() : m_timer(this)
+    {
+    }
+
+    void watch_pipes()
+    {
+        Bind(wxEVT_TIMER, &Process::on_timer, this);
+        m_timer.Start(100/*ms*/);
+    }
 
     subprocess::Output make_output(int status)
     {
-        return {status, extract_stream(GetInputStream()), extract_stream(GetErrorStream())};
+        read_available_output();
+        return {status, m_stdout.str(), m_stderr.str()};
     }
 
 protected:
-    std::string extract_stream(wxInputStream *s)
+    void read_available_output(std::ostringstream& oss, wxInputStream *s)
     {
         if (!s || !s->CanRead())
-            return std::string();
+            return;
 
-        // the stream could be already at EOF or in wxSTREAM_BROKEN_PIPE state
-        s->Reset();
-        std::ostringstream oss;
         char buffer[4096];
         while (true)
         {
             s->Read(buffer, sizeof(buffer));
             size_t bytesRead = s->LastRead();
             if (!bytesRead)
-                break; // EOF
+                break; // EOF or temporarily no more data
             oss.write(buffer, bytesRead);
         }
+    }
 
-        return oss.str();
+    void read_available_output()
+    {
+        read_available_output(m_stdout, GetInputStream());
+        read_available_output(m_stderr, GetErrorStream());
+    }
+
+    void on_timer(wxTimerEvent&)
+    {
+        read_available_output();
     }
+
+    wxTimer m_timer;
+    std::ostringstream m_stdout, m_stderr;
 };
 
 
@@ -197,6 +216,8 @@ dispatch::future<Output> Runner::do_run_async(Arguments&& argv)
                 wxLogTrace("poedit.execute", "  failed to launch child process(%d): %s", (int)retval, argv.pretty_print());
                 BOOST_THROW_EXCEPTION(Exception(wxString::Format(_("Cannot execute program: %s"), argv.pretty_print())));
             }
+
+            process->watch_pipes();
         }
         catch (...)
         {