Bug 1111407 - Allow sending a copy of all stderr to a file to get around logcat overflow. r=ehsan

xpcom/glue/nsCRTGlue.cpp

@@ -21,6 +21,7 @@
 
 #ifdef ANDROID
 #include <android/log.h>
+#include <unistd.h>
 #endif
 
 const char*
@@ -293,6 +294,29 @@ set_stderr_callback(StderrCallback aCallback)
   sStderrCallback = aCallback;
 }
 
+#if defined(ANDROID) && !defined(RELEASE_BUILD)
+static FILE* sStderrCopy = nullptr;
+
+void
+stderr_to_file(const char* aFmt, va_list aArgs)
+{
+  vfprintf(sStderrCopy, aFmt, aArgs);
+}
+
+void
+copy_stderr_to_file(const char* aFile)
+{
+  if (sStderrCopy) {
+    return;
+  }
+  char* buf = (char*)malloc(strlen(aFile) + 16);
+  sprintf(buf, "%s.%u", aFile, (uint32_t)getpid());
+  sStderrCopy = fopen(buf, "w");
+  free(buf);
+  set_stderr_callback(stderr_to_file);
+}
+#endif
+
 #ifdef HAVE_VA_COPY
 #define VARARGS_ASSIGN(foo, bar)        VA_COPY(foo,bar)
 #elif defined(HAVE_VA_LIST_AS_ARRAY)

xpcom/glue/nsDebug.h

@@ -440,6 +440,33 @@ void fprintf_stderr(FILE* aFile, const char* aFmt, ...) MOZ_FORMAT_PRINTF(2, 3);
 // advanced performance debugging and display/layers visualization.
 void set_stderr_callback(StderrCallback aCallback);
 
+#if defined(ANDROID) && !defined(RELEASE_BUILD)
+// Call this if you want a copy of stderr logging sent to a file. This is
+// useful to get around logcat overflow problems on android devices, which use
+// a circular logcat buffer and can intermittently drop messages if there's too
+// much spew.
+//
+// This is intended for local debugging only, DO NOT USE IN PRODUCTION CODE.
+// (This is ifndef RELEASE_BUILD to catch uses of it that accidentally get
+// checked in). Using this will also prevent the profiler from getting a copy of
+// the stderr messages which it uses for various visualization features.
+//
+// This function can be called from any thread, but if it is called multiple
+// times all invocations must be on the same thread. Invocations after the
+// first one are ignored, so you can safely put it inside a loop, for example.
+// Once this is called there is no way to turn it off; all stderr output from
+// that point forward will go to the file. Note that the output is subject to
+// buffering so make sure you have enough output to flush the messages you care
+// about before you terminate the process.
+//
+// The file passed in should be writable, so on Android devices a path like
+// "/data/local/tmp/blah" is a good one to use as it is world-writable and will
+// work even in B2G child processes which have reduced privileges. Note that the
+// actual file created will have the PID appended to the path you pass in, so
+// that on B2G the output from each process goes to a separate file.
+void copy_stderr_to_file(const char* aFile);
+#endif
+
 #ifdef __cplusplus
 }
 #endif