Use ipv4 host if ipv6 is not available (#497)

mesop/cli/cli.py

@@ -23,6 +23,7 @@
 from mesop.server.logging import log_startup
 from mesop.server.server import configure_flask_app
 from mesop.server.static_file_serving import configure_static_file_serving
+from mesop.utils.host_util import get_default_host
 from mesop.utils.runfiles import get_runfile_location
 
 FLAGS = flags.FLAGS
@@ -152,7 +153,7 @@ def main(argv: Sequence[str]):
     log_startup(port=port())
     logging.getLogger("werkzeug").setLevel(logging.WARN)
 
-  flask_app.run(host="::", port=port(), use_reloader=False)
+  flask_app.run(host=get_default_host(), port=port(), use_reloader=False)
 
 
 if __name__ == "__main__":

mesop/colab/colab_run.py

@@ -8,6 +8,7 @@
 from mesop.server.logging import log_startup
 from mesop.server.server import configure_flask_app
 from mesop.server.static_file_serving import configure_static_file_serving
+from mesop.utils.host_util import get_default_host
 
 
 def colab_run(*, port: int = 32123, prod_mode: bool = False):
@@ -43,7 +44,7 @@ def colab_run(*, port: int = 32123, prod_mode: bool = False):
   log_startup(port=port)
 
   def run_flask_app():
-    flask_app.run(host="::", port=port, use_reloader=False)
+    flask_app.run(host=get_default_host(), port=port, use_reloader=False)
 
   # Launch Flask in background thread so we don't hog up the main thread
   # for regular Colab usage.

mesop/server/wsgi_app.py

@@ -10,6 +10,7 @@
 from mesop.server.logging import log_startup
 from mesop.server.server import configure_flask_app
 from mesop.server.static_file_serving import configure_static_file_serving
+from mesop.utils.host_util import get_default_host
 
 
 class App:
@@ -20,7 +21,9 @@ def __init__(self, flask_app: Flask):
 
   def run(self):
     log_startup(port=port())
-    self._flask_app.run(host="::", port=port(), use_reloader=False)
+    self._flask_app.run(
+      host=get_default_host(), port=port(), use_reloader=False
+    )
 
 
 def create_app(

mesop/utils/host_util.py

@@ -0,0 +1,42 @@
+import socket
+
+
+def get_default_host() -> str:
+  """
+  Returns the default host (which is externally accessible)
+  based on the availability of IPv6.
+  """
+  if has_ipv6():
+    return "::"
+  return "0.0.0.0"
+
+
+# Context: https://github.com/urllib3/urllib3/pull/611
+def has_ipv6():
+  """Returns True if the system can bind an IPv6 address."""
+  # First, check if Python was compiled with IPv6 support
+  if not socket.has_ipv6:
+    return False
+
+  sock = None
+  try:
+    # Attempt to create an IPv6 socket
+    sock = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
+
+    # Try to bind to the IPv6 unspecified address
+    # Note: cannot use IPv6 loopback address: "::1"
+    # because it doesn't work on Colab, likely due to
+    # some networking configuration w/ the container.
+    #
+    # Using port 0 lets the OS choose an available port
+    sock.bind(("::", 0))
+
+    # If we get here, the bind was successful
+    return True
+  except Exception:
+    # An exception means IPv6 is not supported or cannot be used
+    return False
+  finally:
+    # Ensure the socket is closed, even if an exception occurred
+    if sock:
+      sock.close()