Fix pthread_attr_getstack__main_thread.

There were two problems here:

* This would fail when run with unlimited stack, because it didn't know
  that bionic reports unlimited stacks as 8MiB.

* This would leave RLIMIT_STACK small, causing failures to exec (so the
  popen and system tests would fail).

(cherry-pick of 27a9aed819 plus the
new ScopeGuard.h from a3ad450a2e3fb6b3fe359683b247eba20896f646.)

Bug: 17394276
Change-Id: I5b92dc64ca089400223b2d9a3743e9b9d57c1bc2

libc/private/ScopeGuard.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef SCOPE_GUARD_H
+#define SCOPE_GUARD_H
+
+// TODO: include explicit std::move when it becomes available
+template<typename F>
+class ScopeGuard {
+ public:
+  ScopeGuard(F f) : f_(f), active_(true) {}
+
+  ScopeGuard(ScopeGuard&& that) : f_(that.f_), active_(that.active_) {
+    that.active_ = false;
+  }
+
+  ~ScopeGuard() {
+    if (active_) {
+      f_();
+    }
+  }
+
+  void disable() {
+    active_ = false;
+  }
+ private:
+  F f_;
+  bool active_;
+
+  ScopeGuard() = delete;
+  ScopeGuard(const ScopeGuard&) = delete;
+  ScopeGuard& operator=(const ScopeGuard&) = delete;
+};
+
+template<typename T>
+ScopeGuard<T> create_scope_guard(T f) {
+  return ScopeGuard<T>(f);
+}
+
+#endif  // SCOPE_GUARD_H