From 7216c76654bbb57bc0ebc27498de7eda247f8ffb Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <github@rvanderhoff.org.uk>
Date: Tue, 17 Oct 2017 14:46:17 +0100
Subject: [PATCH] Improve error handling for missing files (#2551)

`os.path.exists` doesn't allow us to distinguish between permissions errors and
the path actually not existing, which repeatedly confuses people. It also means
that we try to overwrite existing key files, which is super-confusing. (cf
issues #2455, #2379). Use os.stat instead.

Also, don't recomemnd the the use of --generate-config, which screws everything
up if you're using debian (cf #2455).
---
 synapse/config/_base.py | 36 ++++++++++++++++++++++++++----------
 synapse/config/key.py   |  8 ++++----
 synapse/config/tls.py   |  6 +++---
 3 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index 1ab5593c6..fa105bce7 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -81,22 +81,38 @@ class Config(object):
     def abspath(file_path):
         return os.path.abspath(file_path) if file_path else file_path
 
+    @classmethod
+    def path_exists(cls, file_path):
+        """Check if a file exists
+
+        Unlike os.path.exists, this throws an exception if there is an error
+        checking if the file exists (for example, if there is a perms error on
+        the parent dir).
+
+        Returns:
+            bool: True if the file exists; False if not.
+        """
+        try:
+            os.stat(file_path)
+            return True
+        except OSError as e:
+            if e.errno != errno.ENOENT:
+                raise e
+            return False
+
     @classmethod
     def check_file(cls, file_path, config_name):
         if file_path is None:
             raise ConfigError(
                 "Missing config for %s."
-                " You must specify a path for the config file. You can "
-                "do this with the -c or --config-path option. "
-                "Adding --generate-config along with --server-name "
-                "<server name> will generate a config file at the given path."
                 % (config_name,)
             )
-        if not os.path.exists(file_path):
+        try:
+            os.stat(file_path)
+        except OSError as e:
             raise ConfigError(
-                "File %s config for %s doesn't exist."
-                " Try running again with --generate-config"
-                % (file_path, config_name,)
+                "Error accessing file '%s' (config for %s): %s"
+                % (file_path, config_name, e.strerror)
             )
         return cls.abspath(file_path)
 
@@ -248,7 +264,7 @@ class Config(object):
                     " -c CONFIG-FILE\""
                 )
             (config_path,) = config_files
-            if not os.path.exists(config_path):
+            if not cls.path_exists(config_path):
                 if config_args.keys_directory:
                     config_dir_path = config_args.keys_directory
                 else:
@@ -261,7 +277,7 @@ class Config(object):
                         "Must specify a server_name to a generate config for."
                         " Pass -H server.name."
                     )
-                if not os.path.exists(config_dir_path):
+                if not cls.path_exists(config_dir_path):
                     os.makedirs(config_dir_path)
                 with open(config_path, "wb") as config_file:
                     config_bytes, config = obj.generate_config(
diff --git a/synapse/config/key.py b/synapse/config/key.py
index 6ee643793..4b8fc063d 100644
--- a/synapse/config/key.py
+++ b/synapse/config/key.py
@@ -118,10 +118,9 @@ class KeyConfig(Config):
         signing_keys = self.read_file(signing_key_path, "signing_key")
         try:
             return read_signing_keys(signing_keys.splitlines(True))
-        except Exception:
+        except Exception as e:
             raise ConfigError(
-                "Error reading signing_key."
-                " Try running again with --generate-config"
+                "Error reading signing_key: %s" % (str(e))
             )
 
     def read_old_signing_keys(self, old_signing_keys):
@@ -141,7 +140,8 @@ class KeyConfig(Config):
 
     def generate_files(self, config):
         signing_key_path = config["signing_key_path"]
-        if not os.path.exists(signing_key_path):
+
+        if not self.path_exists(signing_key_path):
             with open(signing_key_path, "w") as signing_key_file:
                 key_id = "a_" + random_string(4)
                 write_signing_keys(
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index e081840a8..247f18f45 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -126,7 +126,7 @@ class TlsConfig(Config):
         tls_private_key_path = config["tls_private_key_path"]
         tls_dh_params_path = config["tls_dh_params_path"]
 
-        if not os.path.exists(tls_private_key_path):
+        if not self.path_exists(tls_private_key_path):
             with open(tls_private_key_path, "w") as private_key_file:
                 tls_private_key = crypto.PKey()
                 tls_private_key.generate_key(crypto.TYPE_RSA, 2048)
@@ -141,7 +141,7 @@ class TlsConfig(Config):
                     crypto.FILETYPE_PEM, private_key_pem
                 )
 
-        if not os.path.exists(tls_certificate_path):
+        if not self.path_exists(tls_certificate_path):
             with open(tls_certificate_path, "w") as certificate_file:
                 cert = crypto.X509()
                 subject = cert.get_subject()
@@ -159,7 +159,7 @@ class TlsConfig(Config):
 
                 certificate_file.write(cert_pem)
 
-        if not os.path.exists(tls_dh_params_path):
+        if not self.path_exists(tls_dh_params_path):
             if GENERATE_DH_PARAMS:
                 subprocess.check_call([
                     "openssl", "dhparam",