[adb client] Add "adb mdns check" command.

This command will check if the mdns daemon is available on the host
machine.

Bug: 152510294

Test: pkill -9 mdnsd; adb mdns check; mdnsd; adb mdns check;
Test: test_adb.py
Change-Id: If644678a339763817a8a7adcbdc545626d161aba

1 files changed, 11 insertions, 0 deletions

diff --git a/test_adb.py b/test_adb.py
index c872fb0f..6989e3ba 100755
--- a/test_adb.py
+++ b/test_adb.py
@@ -576,6 +576,17 @@ class PowerTest(unittest.TestCase):
         # If the power event was detected, the adb shell command should be broken very quickly.
         self.assertLess(end - start, 2)
 
+"""Use 'adb mdns check' to see if mdns discovery is available."""
+def is_adb_mdns_available():
+    with adb_server() as server_port:
+        output = subprocess.check_output(["adb", "-P", str(server_port),
+                                          "mdns", "check"]).strip()
+        return output.startswith(b"mdns daemon version")
+
+@unittest.skipIf(not is_adb_mdns_available(), "mdns feature not available")
+class MdnsTest(unittest.TestCase):
+    """Tests for adb mdns."""
+    pass
 
 def main():
     """Main entrypoint."""