pull-local: Make multithreaded

We were blocking for easily 1/10 or 1/5 of a second in fdatasync(),
which drastically slows down the whole process.

This threading isn't quite as good as the ostree-pull command, but it
lets us avoid the dependency on libsoup everywhere, and it's simpler.

src/libotutil/ot-spawn-utils.c

@@ -56,3 +56,25 @@ ot_spawn_sync_checked (const char           *cwd,
  out:
   return ret;
 }
+
+/**
+ * ot_thread_pool_new_nproc:
+ *
+ * Like g_thread_pool_new (), but choose number of threads appropriate
+ * for CPU bound workers automatically.  Also aborts on error.
+ */
+GThreadPool *
+ot_thread_pool_new_nproc (GFunc     func,
+                          gpointer  user_data)
+{
+  long nproc_onln;
+  GThreadPool *ret;
+  GError *local_error = NULL;
+
+  nproc_onln = sysconf (_SC_NPROCESSORS_ONLN);
+  if (G_UNLIKELY (nproc_onln == -1 && errno == EINVAL))
+    nproc_onln = 2;
+  ret = g_thread_pool_new (func, user_data, (int)nproc_onln, FALSE, &local_error);
+  g_assert_no_error (local_error);
+  return ret;
+}

src/libotutil/ot-spawn-utils.h

@@ -37,6 +37,10 @@ gboolean ot_spawn_sync_checked (const char           *cwd,
                                 char                **stderr_data,
                                 GError              **error);
 
+GThreadPool * ot_thread_pool_new_nproc (GFunc     func,
+                                        gpointer  user_data);
+                                        
+
 G_END_DECLS
 
 #endif