Avoid submitting empty tasks in parallel_for_each

I found that parallel_for_each would submit empty tasks to the thread
pool.  For example, this can happen if the number of tasks is smaller
than the number of available threads.  In the DWARF reader, this
resulted in the cooked index containing empty sub-indices.  This patch
arranges to instead shrink the result vector and process the trailing
entries in the calling thread.

1 files changed, 39 insertions, 0 deletions

diff --git a/gdb/unittests/parallel-for-selftests.c b/gdb/unittests/parallel-for-selftests.c
index 3162db18df1..15a095ae62b 100644
--- a/gdb/unittests/parallel-for-selftests.c
+++ b/gdb/unittests/parallel-for-selftests.c
@@ -149,6 +149,45 @@ TEST (int n_threads)
   SELF_CHECK (counter == NUMBER);
 
 #undef NUMBER
+
+  /* Check that if there are fewer tasks than threads, then we won't
+     end up with a null result.  */
+  std::vector<std::unique_ptr<int>> intresults;
+  std::atomic<bool> any_empty_tasks (false);
+
+  FOR_EACH (1, 0, 1,
+	    [&] (int start, int end)
+	      {
+		if (start == end)
+		  any_empty_tasks = true;
+		return std::unique_ptr<int> (new int (end - start));
+	      });
+  SELF_CHECK (!any_empty_tasks);
+  SELF_CHECK (std::all_of (intresults.begin (),
+			   intresults.end (),
+			   [] (const std::unique_ptr<int> &entry)
+			     {
+			       return entry != nullptr;
+			     }));
+
+  /* The same but using the task size parameter.  */
+  intresults.clear ();
+  any_empty_tasks = false;
+  FOR_EACH (1, 0, 1,
+	    [&] (int start, int end)
+	      {
+		if (start == end)
+		  any_empty_tasks = true;
+		return std::unique_ptr<int> (new int (end - start));
+	      },
+	    task_size_one);
+  SELF_CHECK (!any_empty_tasks);
+  SELF_CHECK (std::all_of (intresults.begin (),
+			   intresults.end (),
+			   [] (const std::unique_ptr<int> &entry)
+			     {
+			       return entry != nullptr;
+			     }));
 }
 
 #endif /* FOR_EACH */