Bug 961712 - Buffer (f)printf_stderr on Android. r=gal

Android logging functions append newlines to all output. Layout debugging
(and perhaps other areas) relies on being able to print partial lines, making
its output almost unreadable. This change adds manual buffering between
newlines, making the output readable again.

xpcom/glue/nsCRTGlue.cpp

@@ -299,7 +299,43 @@ vprintf_stderr(const char *fmt, va_list args)
 void
 vprintf_stderr(const char *fmt, va_list args)
 {
-  __android_log_vprint(ANDROID_LOG_INFO, "Gecko", fmt, args);
+  if (!fmt) {
+    return;
+  }
+
+  // Android's logging functions append new-lines to all output, so handle this
+  // by buffering between new-lines.
+
+  static char printf_buffer[2048];
+  static size_t printf_buffer_idx = 0;
+
+  size_t old_idx = printf_buffer_idx;
+  printf_buffer_idx +=
+    vsnprintf(printf_buffer + printf_buffer_idx,
+              sizeof(printf_buffer) - printf_buffer_idx,
+              fmt, args);
+
+  // Handle overflow. In this case, print the old buffer, the new format and
+  // clear the buffer for the next call.
+  if (printf_buffer_idx >= sizeof(printf_buffer)) {
+    printf_buffer[old_idx] = '\0';
+    printf_buffer_idx = 0;
+    __android_log_print(ANDROID_LOG_INFO, "Gecko", printf_buffer);
+    __android_log_vprint(ANDROID_LOG_INFO, "Gecko", fmt, args);
+    return;
+  }
+
+  // Print up to the last new-line, if there are any, and if there are, move
+  // the rest of the string back to the beginning.
+  char* last_newline = strrchr(printf_buffer + old_idx, '\n');
+  if (last_newline) {
+    last_newline[0] = '\0';
+    __android_log_print(ANDROID_LOG_INFO, "Gecko", printf_buffer);
+
+    char* remainder = last_newline + 1;
+    printf_buffer_idx = strlen(remainder);
+    memmove(printf_buffer, remainder, printf_buffer_idx + 1);
+  }
 }
 #else
 void