gdbclient.py: add gdb connect retry.

gdbclient.py can sometimes attempt to connect to the server before it's
ready, especially when debugging on an emulator. This CL adds
functionality to retry the connection for up to 5 seconds.

Bug: http://b/24671386
Change-Id: I65e77f918c3ea8433f992e67122cd48e3647c67b

scripts/gdbclient.py

@@ -140,7 +140,7 @@ def handle_switches(args):
 
     return (binary_file, pid, run_cmd)
 
-def generate_gdb_script(sysroot, binary_file, is64bit, port):
+def generate_gdb_script(sysroot, binary_file, is64bit, port, connect_timeout=5):
     # Generate a gdb script.
     # TODO: Detect the zygote and run 'art-on' automatically.
     root = os.environ["ANDROID_BUILD_TOP"]
@@ -167,7 +167,31 @@ def generate_gdb_script(sysroot, binary_file, is64bit, port):
     else:
         gdb_commands += "source {}\n".format(dalvik_gdb_script)
 
-    gdb_commands += "target remote :{}\n".format(port)
+    # Try to connect for a few seconds, sometimes the device gdbserver takes
+    # a little bit to come up, especially on emulators.
+    gdb_commands += """
+python
+
+def target_remote_with_retry(target, timeout_seconds):
+  import time
+  end_time = time.time() + timeout_seconds
+  while True:
+    try:
+      gdb.execute("target remote " + target)
+      return True
+    except gdb.error as e:
+      time_left = end_time - time.time()
+      if time_left < 0 or time_left > timeout_seconds:
+        print("Error: unable to connect to device.")
+        print(e)
+        return False
+      time.sleep(min(0.25, time_left))
+
+target_remote_with_retry(':{}', {})
+
+end
+""".format(port, connect_timeout)
+
     return gdb_commands