Fix error when loading cert if tls is disabled (#4618)

If TLS is disabled, it should not be an error if no cert is given.

Fixes #4554.

changelog.d/4618.bugfix

@@ -0,0 +1 @@
+Fix failure to start when not TLS certificate was given even if TLS was disabled.

synapse/app/_base.py

@@ -213,12 +213,13 @@ def refresh_certificate(hs):
     Refresh the TLS certificates that Synapse is using by re-reading them from
     disk and updating the TLS context factories to use them.
     """
-    hs.config.read_certificate_from_disk()
 
     if not hs.config.has_tls_listener():
-        # nothing else to do here
+        # attempt to reload the certs for the good of the tls_fingerprints
+        hs.config.read_certificate_from_disk(require_cert_and_key=False)
         return
 
+    hs.config.read_certificate_from_disk(require_cert_and_key=True)
     hs.tls_server_context_factory = context_factory.ServerContextFactory(hs.config)
 
     if hs._listening_services:

synapse/config/tls.py

@@ -23,7 +23,7 @@ from unpaddedbase64 import encode_base64
 
 from OpenSSL import crypto
 
-from synapse.config._base import Config
+from synapse.config._base import Config, ConfigError
 
 logger = logging.getLogger(__name__)
 
@@ -45,6 +45,19 @@ class TlsConfig(Config):
 
         self.tls_certificate_file = self.abspath(config.get("tls_certificate_path"))
         self.tls_private_key_file = self.abspath(config.get("tls_private_key_path"))
+
+        if self.has_tls_listener():
+            if not self.tls_certificate_file:
+                raise ConfigError(
+                    "tls_certificate_path must be specified if TLS-enabled listeners are "
+                    "configured."
+                )
+            if not self.tls_private_key_file:
+                raise ConfigError(
+                    "tls_certificate_path must be specified if TLS-enabled listeners are "
+                    "configured."
+                )
+
         self._original_tls_fingerprints = config.get("tls_fingerprints", [])
 
         if self._original_tls_fingerprints is None:
@@ -105,26 +118,40 @@ class TlsConfig(Config):
         days_remaining = (expires_on - now).days
         return days_remaining
 
-    def read_certificate_from_disk(self):
-        """
-        Read the certificates from disk.
+    def read_certificate_from_disk(self, require_cert_and_key):
         """
-        self.tls_certificate = self.read_tls_certificate()
+        Read the certificates and private key from disk.
 
-        if self.has_tls_listener():
+        Args:
+            require_cert_and_key (bool): set to True to throw an error if the certificate
+                and key file are not given
+        """
+        if require_cert_and_key:
             self.tls_private_key = self.read_tls_private_key()
+            self.tls_certificate = self.read_tls_certificate()
+        elif self.tls_certificate_file:
+            # we only need the certificate for the tls_fingerprints. Reload it if we
+            # can, but it's not a fatal error if we can't.
+            try:
+                self.tls_certificate = self.read_tls_certificate()
+            except Exception as e:
+                logger.info(
+                    "Unable to read TLS certificate (%s). Ignoring as no "
+                    "tls listeners enabled.", e,
+                )
 
         self.tls_fingerprints = list(self._original_tls_fingerprints)
 
-        # Check that our own certificate is included in the list of fingerprints
-        # and include it if it is not.
-        x509_certificate_bytes = crypto.dump_certificate(
-            crypto.FILETYPE_ASN1, self.tls_certificate
-        )
-        sha256_fingerprint = encode_base64(sha256(x509_certificate_bytes).digest())
-        sha256_fingerprints = set(f["sha256"] for f in self.tls_fingerprints)
-        if sha256_fingerprint not in sha256_fingerprints:
-            self.tls_fingerprints.append({u"sha256": sha256_fingerprint})
+        if self.tls_certificate:
+            # Check that our own certificate is included in the list of fingerprints
+            # and include it if it is not.
+            x509_certificate_bytes = crypto.dump_certificate(
+                crypto.FILETYPE_ASN1, self.tls_certificate
+            )
+            sha256_fingerprint = encode_base64(sha256(x509_certificate_bytes).digest())
+            sha256_fingerprints = set(f["sha256"] for f in self.tls_fingerprints)
+            if sha256_fingerprint not in sha256_fingerprints:
+                self.tls_fingerprints.append({u"sha256": sha256_fingerprint})
 
     def default_config(self, config_dir_path, server_name, **kwargs):
         base_key_name = os.path.join(config_dir_path, server_name)

tests/config/test_tls.py

@@ -65,7 +65,7 @@ s4niecZKPBizL6aucT59CsunNmmb5Glq8rlAcU+1ZTZZzGYqVYhF6axB9Qg=
 
         t = TestConfig()
         t.read_config(config)
-        t.read_certificate_from_disk()
+        t.read_certificate_from_disk(require_cert_and_key=False)
 
         warnings = self.flushWarnings()
         self.assertEqual(len(warnings), 1)