[PATCH] Fix --disable-threading build

[Tom Tromey]

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
---
 gdb/dwarf2/cooked-index.h |  2 +-
 gdbsupport/parallel-for.h |  4 +--
 gdbsupport/thread-pool.cc |  6 ++--
 gdbsupport/thread-pool.h  | 75 ++++++++++++++++++++++++++++++++++++---
 4 files changed, 75 insertions(+), 12 deletions(-)

diff --git a/gdb/dwarf2/cooked-index.h b/gdb/dwarf2/cooked-index.h
index a460351612f..1d229586948 100644
--- a/gdb/dwarf2/cooked-index.h
+++ b/gdb/dwarf2/cooked-index.h
@@ -330,7 +330,7 @@ class cooked_index_vector : public dwarf_scanner_base
   /* A future that tracks when the 'finalize' method is done.  Note
      that the 'get' method is never called on this future, only
      'wait'.  */
-  std::future<void> m_future;
+  gdb::future<void> m_future;
 };
 
 #endif /* GDB_DWARF2_COOKED_INDEX_H */
diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h
index 713ec660306..7b6891a0dcb 100644
--- a/gdbsupport/parallel-for.h
+++ b/gdbsupport/parallel-for.h
@@ -72,7 +72,7 @@ struct par_for_accumulator
   
   /* 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 @@ struct par_for_accumulator<void>
 
 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 3674e910f1d..ddb76b691c1 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 5e203fd896c..fc35d61faf0 100644
--- a/gdbsupport/thread-pool.h
+++ b/gdbsupport/thread-pool.h
@@ -27,13 +27,69 @@
 #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.
+
+   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 +120,32 @@ class thread_pool
 
   /* 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:
-- 
2.34.1