[microdump] Move microdump writes to the crash ring-buffer log

So far the microdump_writer dumped the log in logcat using the default
system log. This is simple to achieve but has some drawbacks:
 1. Creates spam in the system log, pushing back other eventual useful
    messages.
 2. There is a high chance that the microdump gets lost if some log
    spam storm happens immediately after a crash and before the log
    is collected by the feedback client.
 3. Since Android L, the logger is smartly throttling messages (to
    reduce logcat spam). Throttling brekpad logs defeats the all
    point of microdumps.

This change is conceptually very simple. Replace the use of
__android_log_write() with __android_log_buf_write(), which takes
an extra bufID argument. The main drawback is that the
__android_log_buf_write is not exported in the NDK and needs to be
dynamically looked up via dlsym.
This choice has been discussed and advocated by Android owners.
See the internal bug b/21753476.

BUG=chromium:512755
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1286063003 .

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1490 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/client/linux/handler/exception_handler.cc

@@ -233,6 +233,11 @@ ExceptionHandler::ExceptionHandler(const MinidumpDescriptor& descriptor,
       !minidump_descriptor_.IsMicrodumpOnConsole())
     minidump_descriptor_.UpdatePath();
 
+#if defined(__ANDROID__)
+  if (minidump_descriptor_.IsMicrodumpOnConsole())
+    logger::initializeCrashLogWriter();
+#endif
+
   pthread_mutex_lock(&g_handler_stack_mutex_);
   if (!g_handler_stack_)
     g_handler_stack_ = new std::vector<ExceptionHandler*>;

src/client/linux/log/log.cc

@@ -31,15 +31,51 @@
 
 #if defined(__ANDROID__)
 #include <android/log.h>
+#include <dlfcn.h>
 #else
 #include "third_party/lss/linux_syscall_support.h"
 #endif
 
 namespace logger {
 
+#if defined(__ANDROID__)
+namespace {
+
+// __android_log_buf_write() is not exported in the NDK and is being used by
+// dynamic runtime linking. Its declaration is taken from Android's
+// system/core/include/log/log.h.
+using AndroidLogBufferWriteFunc = int (*)(int bufID, int prio, const char *tag,
+                                          const char *text);
+const int kAndroidCrashLogId = 4;  // From LOG_ID_CRASH in log.h.
+const char kAndroidLogTag[] = "google-breakpad";
+
+bool g_crash_log_initialized = false;
+AndroidLogBufferWriteFunc g_android_log_buf_write = nullptr;
+
+}  // namespace
+
+void initializeCrashLogWriter() {
+  if (g_crash_log_initialized)
+    return;
+  g_android_log_buf_write = reinterpret_cast<AndroidLogBufferWriteFunc>(
+      dlsym(RTLD_DEFAULT, "__android_log_buf_write"));
+  g_crash_log_initialized = true;
+}
+
+int writeToCrashLog(const char* buf) {
+  // Try writing to the crash log ring buffer. If not available, fall back to
+  // the standard log buffer.
+  if (g_android_log_buf_write) {
+    return g_android_log_buf_write(kAndroidCrashLogId, ANDROID_LOG_FATAL,
+                                   kAndroidLogTag, buf);
+  }
+  return __android_log_write(ANDROID_LOG_FATAL, kAndroidLogTag, buf);
+}
+#endif
+
 int write(const char* buf, size_t nbytes) {
 #if defined(__ANDROID__)
-  return __android_log_write(ANDROID_LOG_WARN, "google-breakpad", buf);
+  return __android_log_write(ANDROID_LOG_WARN, kAndroidLogTag, buf);
 #else
   return sys_write(2, buf, nbytes);
 #endif

src/client/linux/log/log.h

@@ -36,6 +36,20 @@ namespace logger {
 
 int write(const char* buf, size_t nbytes);
 
+// In the case of Android the log can be written to the default system log
+// (default behavior of write() above, or to the crash log (see
+// writeToCrashLog() below).
+#if defined(__ANDROID__)
+
+// The logger must be initialized in a non-compromised context.
+void initializeCrashLogWriter();
+
+// Once initialized, writeToCrashLog is safe to use in a compromised context,
+// even if the initialization failed, in which case this will silently fall
+// back on write().
+int writeToCrashLog(const char* buf);
+#endif
+
 }  // namespace logger
 
 #endif  // CLIENT_LINUX_LOG_LOG_H_

src/client/linux/microdump_writer/microdump_writer.cc

@@ -105,9 +105,11 @@ class MicrodumpWriter {
  private:
   // Writes one line to the system log.
   void LogLine(const char* msg) {
+#if defined(__ANDROID__)
+    logger::writeToCrashLog(msg);
+#else
     logger::write(msg, my_strlen(msg));
-#if !defined(__ANDROID__)
-    logger::write("\n", 1);  // Android logger appends the \n. Linux's doesn't.
+    logger::write("\n", 1);
 #endif
   }