app/internal/log: add synchronized logger on Android

By default, the standard library log package outputs to stderr.
However, stderr is redirected through a pipe to the Android logger,
so recent writes may be not have been sent when os.Exit is called.

The log.Fatal family of functions does just that: write to the log
and call os.Exit.

To ensure all messages are sent, register a synchronized logger at
startup.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/log/log_android.go

@@ -19,13 +19,39 @@ import (
 	"unsafe"
 )
 
+// 1024 is the truncation limit from android/log.h, plus a \n.
+const logLineLimit = 1024
+
+var logTag = C.CString("gio")
+
 func init() {
 	// Android's logcat already includes timestamps.
 	log.SetFlags(log.Flags() &^ log.LstdFlags)
+	log.SetOutput(new(androidLogWriter))
+
+	// Redirect stdout and stderr to the Android logger.
 	logFd(os.Stdout.Fd())
 	logFd(os.Stderr.Fd())
 }
 
+type androidLogWriter struct {
+	// buf has room for the maximum log line, plus a terminating '\0'.
+	buf [logLineLimit + 1]byte
+}
+
+func (w *androidLogWriter) Write(data []byte) (int, error) {
+	// Truncate the buffer, leaving space for the '\0'.
+	if max := len(w.buf) - 1; len(data) > max {
+		data = data[:max]
+	}
+	buf := w.buf[:len(data)+1]
+	copy(buf, data)
+	// Terminating '\0'.
+	buf[len(data)] = 0
+	C.__android_log_write(C.ANDROID_LOG_INFO, logTag, (*C.char)(unsafe.Pointer(&buf[0])))
+	return len(data), nil
+}
+
 func logFd(fd uintptr) {
 	r, w, err := os.Pipe()
 	if err != nil {
@@ -35,10 +61,7 @@ func logFd(fd uintptr) {
 		panic(err)
 	}
 	go func() {
-		tag := C.CString("gio")
-		defer C.free(unsafe.Pointer(tag))
-		// 1024 is the truncation limit from android/log.h, plus a \n.
-		lineBuf := bufio.NewReaderSize(r, 1024)
+		lineBuf := bufio.NewReaderSize(r, logLineLimit)
 		// The buffer to pass to C, including the terminating '\0'.
 		buf := make([]byte, lineBuf.Size()+1)
 		cbuf := (*C.char)(unsafe.Pointer(&buf[0]))
@@ -49,7 +72,7 @@ func logFd(fd uintptr) {
 			}
 			copy(buf, line)
 			buf[len(line)] = 0
-			C.__android_log_write(C.ANDROID_LOG_INFO, tag, cbuf)
+			C.__android_log_write(C.ANDROID_LOG_INFO, logTag, cbuf)
 		}
 		// The garbage collector doesn't know that w's fd was dup'ed.
 		// Avoid finalizing w, and thereby avoid its finalizer closing its fd.