1 files changed, 162 insertions, 0 deletions
diff --git a/src/runtime/write_err_android.go b/src/runtime/write_err_android.go
new file mode 100644
index 0000000..2419fc8
--- /dev/null
+++ b/src/runtime/write_err_android.go
@@ -0,0 +1,162 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import "unsafe"
+
+var (
+	writeHeader = []byte{6 /* ANDROID_LOG_ERROR */, 'G', 'o', 0}
+	writePath   = []byte("/dev/log/main\x00")
+	writeLogd   = []byte("/dev/socket/logdw\x00")
+
+	// guarded by printlock/printunlock.
+	writeFD  uintptr
+	writeBuf [1024]byte
+	writePos int
+)
+
+// Prior to Android-L, logging was done through writes to /dev/log files implemented
+// in kernel ring buffers. In Android-L, those /dev/log files are no longer
+// accessible and logging is done through a centralized user-mode logger, logd.
+//
+// https://android.googlesource.com/platform/system/core/+/refs/tags/android-6.0.1_r78/liblog/logd_write.c
+type loggerType int32
+
+const (
+	unknown loggerType = iota
+	legacy
+	logd
+	// TODO(hakim): logging for emulator?
+)
+
+var logger loggerType
+
+func writeErr(b []byte) {
+	if logger == unknown {
+		// Use logd if /dev/socket/logdw is available.
+		if v := uintptr(access(&writeLogd[0], 0x02 /* W_OK */)); v == 0 {
+			logger = logd
+			initLogd()
+		} else {
+			logger = legacy
+			initLegacy()
+		}
+	}
+
+	// Write to stderr for command-line programs.
+	write(2, unsafe.Pointer(&b[0]), int32(len(b)))
+
+	// Log format: "<header>\x00<message m bytes>\x00"
+	//
+	// <header>
+	//   In legacy mode: "<priority 1 byte><tag n bytes>".
+	//   In logd mode: "<android_log_header_t 11 bytes><priority 1 byte><tag n bytes>"
+	//
+	// The entire log needs to be delivered in a single syscall (the NDK
+	// does this with writev). Each log is its own line, so we need to
+	// buffer writes until we see a newline.
+	var hlen int
+	switch logger {
+	case logd:
+		hlen = writeLogdHeader()
+	case legacy:
+		hlen = len(writeHeader)
+	}
+
+	dst := writeBuf[hlen:]
+	for _, v := range b {
+		if v == 0 { // android logging won't print a zero byte
+			v = '0'
+		}
+		dst[writePos] = v
+		writePos++
+		if v == '\n' || writePos == len(dst)-1 {
+			dst[writePos] = 0
+			write(writeFD, unsafe.Pointer(&writeBuf[0]), int32(hlen+writePos))
+			for i := range dst {
+				dst[i] = 0
+			}
+			writePos = 0
+		}
+	}
+}
+
+func initLegacy() {
+	// In legacy mode, logs are written to /dev/log/main
+	writeFD = uintptr(open(&writePath[0], 0x1 /* O_WRONLY */, 0))
+	if writeFD == 0 {
+		// It is hard to do anything here. Write to stderr just
+		// in case user has root on device and has run
+		//	adb shell setprop log.redirect-stdio true
+		msg := []byte("runtime: cannot open /dev/log/main\x00")
+		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
+		exit(2)
+	}
+
+	// Prepopulate the invariant header part.
+	copy(writeBuf[:len(writeHeader)], writeHeader)
+}
+
+// used in initLogdWrite but defined here to avoid heap allocation.
+var logdAddr sockaddr_un
+
+func initLogd() {
+	// In logd mode, logs are sent to the logd via a unix domain socket.
+	logdAddr.family = _AF_UNIX
+	copy(logdAddr.path[:], writeLogd)
+
+	// We are not using non-blocking I/O because writes taking this path
+	// are most likely triggered by panic, we cannot think of the advantage of
+	// non-blocking I/O for panic but see disadvantage (dropping panic message),
+	// and blocking I/O simplifies the code a lot.
+	fd := socket(_AF_UNIX, _SOCK_DGRAM|_O_CLOEXEC, 0)
+	if fd < 0 {
+		msg := []byte("runtime: cannot create a socket for logging\x00")
+		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
+		exit(2)
+	}
+
+	errno := connect(fd, unsafe.Pointer(&logdAddr), int32(unsafe.Sizeof(logdAddr)))
+	if errno < 0 {
+		msg := []byte("runtime: cannot connect to /dev/socket/logdw\x00")
+		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
+		// TODO(hakim): or should we just close fd and hope for better luck next time?
+		exit(2)
+	}
+	writeFD = uintptr(fd)
+
+	// Prepopulate invariant part of the header.
+	// The first 11 bytes will be populated later in writeLogdHeader.
+	copy(writeBuf[11:11+len(writeHeader)], writeHeader)
+}
+
+// writeLogdHeader populates the header and returns the length of the payload.
+func writeLogdHeader() int {
+	hdr := writeBuf[:11]
+
+	// The first 11 bytes of the header corresponds to android_log_header_t
+	// as defined in system/core/include/private/android_logger.h
+	//   hdr[0] log type id (unsigned char), defined in <log/log.h>
+	//   hdr[1:2] tid (uint16_t)
+	//   hdr[3:11] log_time defined in <log/log_read.h>
+	//      hdr[3:7] sec unsigned uint32, little endian.
+	//      hdr[7:11] nsec unsigned uint32, little endian.
+	hdr[0] = 0 // LOG_ID_MAIN
+	sec, nsec := walltime()
+	packUint32(hdr[3:7], uint32(sec))
+	packUint32(hdr[7:11], uint32(nsec))
+
+	// TODO(hakim):  hdr[1:2] = gettid?
+
+	return 11 + len(writeHeader)
+}
+
+func packUint32(b []byte, v uint32) {
+	// little-endian.
+	b[0] = byte(v)
+	b[1] = byte(v >> 8)
+	b[2] = byte(v >> 16)
+	b[3] = byte(v >> 24)
+}