android: fix logs forwarding latency

Instead of executing an adb shell command for each log line, spawn one
adb shell per android device and write one log command per log to its
stdin. This considerably reduce the forwarding latency.

avatar/pandora_server.py

@@ -20,10 +20,10 @@
 import grpc
 import grpc.aio
 import logging
-import os
 import portpicker
 import re
 import shlex
+import subprocess
 import threading
 import types
 
@@ -118,6 +118,7 @@ class AndroidPandoraServer(PandoraServer[AndroidDevice]):
     _port: int
     _logger: logging.Logger
     _handler: logging.Handler
+    _adb_shell: subprocess.Popen[bytes]
 
     def start(self) -> PandoraClient:
         """Sets up and starts the Pandora server on the Android device."""
@@ -140,27 +141,36 @@ def start(self) -> PandoraClient:
 
         # Forward all logging to ADB logs
         adb = self.device.adb
+        self._adb_shell = subprocess.Popen(['adb', '-s', adb.serial, 'shell'], stdin=subprocess.PIPE)
+
+        # This regex match all ANSI escape sequences (colors, style, ..).
+        ansi_escape = re.compile(r'\x1B(?:[@-Z\\-_]|\[[0-?]*[ -/]*[@-~])')
 
         class AdbLoggingHandler(logging.Handler):
+            LOGGING_TO_ANDROID_LEVELS = {
+                logging.FATAL: 'f',
+                logging.ERROR: 'e',
+                logging.WARN: 'w',
+                logging.INFO: 'i',
+                logging.DEBUG: 'd',
+                logging.NOTSET: 'd',
+            }
+
+            def __init__(self, adb_shell: subprocess.Popen[bytes]) -> None:
+                self.adb_shell = adb_shell
+
             def emit(self, record: logging.LogRecord) -> None:
                 if record.levelno <= logging.DEBUG:
                     return
-                ansi_escape = re.compile(r'\x1B(?:[@-Z\\-_]|\[[0-?]*[ -/]*[@-~])')
-                msg = self.format(record)
-                msg = ansi_escape.sub('', msg)
-                level = {
-                    logging.FATAL: 'f',
-                    logging.ERROR: 'e',
-                    logging.WARN: 'w',
-                    logging.INFO: 'i',
-                    logging.DEBUG: 'd',
-                    logging.NOTSET: 'd',
-                }[record.levelno]
-                for msg in msg.splitlines():
-                    os.system(f'adb -s {adb.serial} shell "log -t Avatar -p {level} {shlex.quote(msg)}"')
+                # Format and remove all ANSI escape sequences.
+                msg = ansi_escape.sub('', self.format(record))
+                level = AdbLoggingHandler.LOGGING_TO_ANDROID_LEVELS[record.levelno]
+                assert self.adb_shell.stdin
+                self.adb_shell.stdin.write(f'log -t Avatar -p {level} {shlex.quote(msg)}\n'.encode('utf-8'))
+                self.adb_shell.stdin.flush()
 
         self._logger = logging.getLogger()
-        self._handler = AdbLoggingHandler()
+        self._handler = AdbLoggingHandler(self._adb_shell)
         self._logger.addHandler(self._handler)
 
         return PandoraClient(f'localhost:{self._port}', 'android')
@@ -176,6 +186,9 @@ def stop(self) -> None:
 
         # Remove ADB logging handler
         self._logger.removeHandler(self._handler)
+        assert self._adb_shell.stdin
+        self._adb_shell.stdin.close()
+        self._adb_shell.wait()
 
         self.device.adb.forward(['--remove', f'tcp:{self._port}'])  # type: ignore
         self._instrumentation.join()