Refactor Certificate Expiry Sensor (#32066)

* Cert Expiry refactor

* Unused parameter

* Reduce delay

* Deprecate 'name' config

* Use config entry unique_id

* Fix logic bugs found with tests

* Rewrite tests to use config flow core interfaces, validate created sensors

* Update strings

* Minor consistency fix

* Review fixes, complete test coverage

* Move error handling to helper

* Subclass exceptions

* Better tests

* Use first object reference

* Fix docstring

homeassistant/components/cert_expiry/helper.py

@@ -1,12 +1,19 @@
 """Helper functions for the Cert Expiry platform."""
+from datetime import datetime
 import socket
 import ssl
 
 from .const import TIMEOUT
+from .errors import (
+    ConnectionRefused,
+    ConnectionTimeout,
+    ResolveFailed,
+    ValidationFailure,
+)
 
 
 def get_cert(host, port):
-    """Get the ssl certificate for the host and port combination."""
+    """Get the certificate for the host and port combination."""
     ctx = ssl.create_default_context()
     address = (host, port)
     with socket.create_connection(address, timeout=TIMEOUT) as sock:
@@ -14,3 +21,24 @@ def get_cert(host, port):
             # pylint disable: https://github.com/PyCQA/pylint/issues/3166
             cert = ssock.getpeercert()  # pylint: disable=no-member
             return cert
+
+
+async def get_cert_time_to_expiry(hass, hostname, port):
+    """Return the certificate's time to expiry in days."""
+    try:
+        cert = await hass.async_add_executor_job(get_cert, hostname, port)
+    except socket.gaierror:
+        raise ResolveFailed(f"Cannot resolve hostname: {hostname}")
+    except socket.timeout:
+        raise ConnectionTimeout(f"Connection timeout with server: {hostname}:{port}")
+    except ConnectionRefusedError:
+        raise ConnectionRefused(f"Connection refused by server: {hostname}:{port}")
+    except ssl.CertificateError as err:
+        raise ValidationFailure(err.verify_message)
+    except ssl.SSLError as err:
+        raise ValidationFailure(err.args[0])
+
+    ts_seconds = ssl.cert_time_to_seconds(cert["notAfter"])
+    timestamp = datetime.fromtimestamp(ts_seconds)
+    expiry = timestamp - datetime.today()
+    return expiry.days