sambacc: add a URL opener type

Add a URLOpener based on python's urllib, but with non-HTTP(s) protocols
disabled. The URLOpener can be extended later with other remote
storage protocols.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

sambacc/url_opener.py

@@ -0,0 +1,88 @@
+#
+# sambacc: a samba container configuration tool
+# Copyright (C) 2023  John Mulligan
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>
+#
+
+import errno
+import http
+import typing
+import urllib.error
+import urllib.request
+
+from .opener import SchemeNotSupported
+
+
+class _UnknownHandler(urllib.request.BaseHandler):
+    def unknown_open(self, req: urllib.request.Request) -> None:
+        raise SchemeNotSupported(req.full_url)
+
+
+class URLOpener:
+    """An Opener type used for fetching remote resources named in
+    a pseudo-URL (URI-like) style.
+    By default works like urllib.urlopen but only for HTTP(S).
+
+    Example:
+    >>> uo = URLOpener()
+    >>> res = uo.open("http://abc.example.org/foo/x.json")
+    >>> res.read()
+    """
+
+    # this list is similar to the defaults found in build_opener
+    # but only for http/https handlers. No FTP, etc.
+    _handlers = [
+        urllib.request.ProxyHandler,
+        urllib.request.HTTPHandler,
+        urllib.request.HTTPDefaultErrorHandler,
+        urllib.request.HTTPRedirectHandler,
+        urllib.request.HTTPErrorProcessor,
+        urllib.request.HTTPSHandler,
+        _UnknownHandler,
+    ]
+
+    def __init__(self) -> None:
+        self._opener = urllib.request.OpenerDirector()
+        for handler in self._handlers:
+            self._opener.add_handler(handler())
+
+    def open(self, url: str) -> typing.IO:
+        try:
+            return self._opener.open(url)
+        except ValueError as err:
+            # too bad urllib doesn't use a specific subclass of ValueError here
+            if "unknown url type" in str(err):
+                raise SchemeNotSupported(url) from err
+            raise
+        except urllib.error.HTTPError as err:
+            _map_errno(err)
+            raise
+
+
+_EMAP = {
+    http.HTTPStatus.NOT_FOUND.value: errno.ENOENT,
+    http.HTTPStatus.UNAUTHORIZED.value: errno.EPERM,
+}
+
+
+def _map_errno(err: urllib.error.HTTPError) -> None:
+    """While HTTPError is an OSError, it often doesn't have an errno set.
+    Since our callers care about the errno, do a best effort mapping of
+    some HTTP statuses to errnos.
+    """
+    if getattr(err, "errno", None) is not None:
+        return
+    status = int(getattr(err, "status", -1))
+    setattr(err, "errno", _EMAP.get(status, None))