Use async safe logging in signal handlers.

Test: Boot on bullhead.
Test: Ran the libbacktrace tests on bullhead.
Test: Added a temporary log message in the signal handler, and ran the
Test: backtrace tests.
Change-Id: I0a6888c9f311af2c8cc7fbb4929315911bd2bb3c
diff --git a/libbacktrace/BacktraceAsyncSafeLog.h b/libbacktrace/BacktraceAsyncSafeLog.h
new file mode 100644
index 0000000..14f51be
--- /dev/null
+++ b/libbacktrace/BacktraceAsyncSafeLog.h
@@ -0,0 +1,41 @@
+/*
+ * 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 _LIBBACKTRACE_BACKTRACE_ASYNC_SAFE_LOG_H
+#define _LIBBACKTRACE_BACKTRACE_ASYNC_SAFE_LOG_H
+
+#if defined(__ANDROID__)
+
+#include <async_safe/log.h>
+
+// Logging macros for use in signal handler, only available on target.
+#define BACK_ASYNC_SAFE_LOGW(format, ...)                                                     \
+  async_safe_format_log(ANDROID_LOG_WARN, "libbacktrace", "%s: " format, __PRETTY_FUNCTION__, \
+                        ##__VA_ARGS__)
+
+#define BACK_ASYNC_SAFE_LOGE(format, ...)                                                      \
+  async_safe_format_log(ANDROID_LOG_ERROR, "libbacktrace", "%s: " format, __PRETTY_FUNCTION__, \
+                        ##__VA_ARGS__)
+
+#else
+
+#define BACK_ASYNC_SAFE_LOGW(format, ...)
+
+#define BACK_ASYNC_SAFE_LOGE(format, ...)
+
+#endif
+
+#endif  // _LIBBACKTRACE_BACKTRACE_ASYNC_SAFE_LOG_H