Fix --disable-threading build

PR build/29110 points out that GDB fails to build on mingw when the
"win32" thread model is in use.  It turns out that the Fedora cross
tools using the "posix" thread model, which somehow manages to support
std::future, whereas the win32 model does not.

While looking into this, I found that the configuring with
--disable-threading will also cause a build failure.

This patch fixes this build by introducing a compatibility wrapper for
std::future.

I am not able to test the win32 thread model build, but I'm going to
ask the reporter to try this patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29110

3 files changed, 75 insertions, 11 deletions

diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h
index 713ec66..7b6891a 100644
--- a/gdbsupport/parallel-for.h
+++ b/gdbsupport/parallel-for.h
@@ -72,7 +72,7 @@ private:
   
   /* A vector of futures coming from the tasks run in the
      background.  */
-  std::vector<std::future<T>> m_futures;
+  std::vector<gdb::future<T>> m_futures;
 };
 
 /* See the generic template.  */
@@ -108,7 +108,7 @@ public:
 
 private:
 
-  std::vector<std::future<void>> m_futures;
+  std::vector<gdb::future<void>> m_futures;
 };
 
 }
diff --git a/gdbsupport/thread-pool.cc b/gdbsupport/thread-pool.cc
index 3674e91..ddb76b69 100644
--- a/gdbsupport/thread-pool.cc
+++ b/gdbsupport/thread-pool.cc
@@ -192,12 +192,13 @@ thread_pool::set_thread_count (size_t num_threads)
 #endif /* CXX_STD_THREAD */
 }
 
+#if CXX_STD_THREAD
+
 void
 thread_pool::do_post_task (std::packaged_task<void ()> &&func)
 {
   std::packaged_task<void ()> t (std::move (func));
 
-#if CXX_STD_THREAD
   if (m_thread_count != 0)
     {
       std::lock_guard<std::mutex> guard (m_tasks_mutex);
@@ -205,15 +206,12 @@ thread_pool::do_post_task (std::packaged_task<void ()> &&func)
       m_tasks_cv.notify_one ();
     }
   else
-#endif
     {
       /* Just execute it now.  */
       t ();
     }
 }
 
-#if CXX_STD_THREAD
-
 void
 thread_pool::thread_function ()
 {
diff --git a/gdbsupport/thread-pool.h b/gdbsupport/thread-pool.h
index 5e203fd..4db35ba 100644
--- a/gdbsupport/thread-pool.h
+++ b/gdbsupport/thread-pool.h
@@ -27,13 +27,70 @@
 #include <thread>
 #include <mutex>
 #include <condition_variable>
-#endif
 #include <future>
+#endif
 #include "gdbsupport/gdb_optional.h"
 
 namespace gdb
 {
 
+#if CXX_STD_THREAD
+
+/* Simply use the standard future.  */
+template<typename T>
+using future = std::future<T>;
+
+#else /* CXX_STD_THREAD */
+
+/* A compatibility wrapper for std::future.  Once <thread> and
+   <future> are available in all GCC builds -- should that ever happen
+   -- this can be removed.  GCC does not implement threading for
+   MinGW, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93687.
+
+   Meanwhile, in this mode, there are no threads.  Tasks submitted to
+   the thread pool are invoked immediately and their result is stored
+   here.  The base template here simply wraps a T and provides some
+   std::future compatibility methods.  The provided methods are chosen
+   based on what GDB needs presently.  */
+
+template<typename T>
+class future
+{
+public:
+
+  explicit future (T value)
+    : m_value (std::move (value))
+  {
+  }
+
+  future () = default;
+  future (future &&other) = default;
+  future (const future &other) = delete;
+  future &operator= (future &&other) = default;
+  future &operator= (const future &other) = delete;
+
+  void wait () const { }
+
+  T get () { return std::move (m_value); }
+
+private:
+
+  T m_value;
+};
+
+/* A specialization for void.  */
+
+template<>
+class future<void>
+{
+public:
+  void wait () const { }
+  void get () { }
+};
+
+#endif /* CXX_STD_THREAD */
+
+
 /* A thread pool.
 
    There is a single global thread pool, see g_thread_pool.  Tasks can
@@ -64,23 +121,32 @@ public:
 
   /* Post a task to the thread pool.  A future is returned, which can
      be used to wait for the result.  */
-  std::future<void> post_task (std::function<void ()> &&func)
+  future<void> post_task (std::function<void ()> &&func)
   {
+#if CXX_STD_THREAD
     std::packaged_task<void ()> task (std::move (func));
-    std::future<void> result = task.get_future ();
+    future<void> result = task.get_future ();
     do_post_task (std::packaged_task<void ()> (std::move (task)));
     return result;
+#else
+    func ();
+    return {};
+#endif /* CXX_STD_THREAD */
   }
 
   /* Post a task to the thread pool.  A future is returned, which can
      be used to wait for the result.  */
   template<typename T>
-  std::future<T> post_task (std::function<T ()> &&func)
+  future<T> post_task (std::function<T ()> &&func)
   {
+#if CXX_STD_THREAD
     std::packaged_task<T ()> task (std::move (func));
-    std::future<T> result = task.get_future ();
+    future<T> result = task.get_future ();
     do_post_task (std::packaged_task<void ()> (std::move (task)));
     return result;
+#else
+    return future<T> (func ());
+#endif /* CXX_STD_THREAD */
   }
 
 private: