[networking] Ignore invalid proxies in env (#7704)

Authored by: coletdjnz

yt_dlp/networking/common.py

@@ -262,9 +262,13 @@ class RequestHandler(abc.ABC):
                 # Skip proxy scheme checks
                 continue
 
-            # Scheme-less proxies are not supported
-            if urllib.request._parse_proxy(proxy_url)[0] is None:
-                raise UnsupportedRequest(f'Proxy "{proxy_url}" missing scheme')
+            try:
+                if urllib.request._parse_proxy(proxy_url)[0] is None:
+                    # Scheme-less proxies are not supported
+                    raise UnsupportedRequest(f'Proxy "{proxy_url}" missing scheme')
+            except ValueError as e:
+                # parse_proxy may raise on some invalid proxy urls such as "/a/b/c"
+                raise UnsupportedRequest(f'Invalid proxy url "{proxy_url}": {e}')
 
             scheme = urllib.parse.urlparse(proxy_url).scheme.lower()
             if scheme not in self._SUPPORTED_PROXY_SCHEMES:

yt_dlp/utils/networking.py

@@ -98,7 +98,13 @@ def clean_proxies(proxies: dict, headers: HTTPHeaderDict):
             continue
         if proxy_url is not None:
             # Ensure proxies without a scheme are http.
-            proxy_scheme = urllib.request._parse_proxy(proxy_url)[0]
+            try:
+                proxy_scheme = urllib.request._parse_proxy(proxy_url)[0]
+            except ValueError:
+                # Ignore invalid proxy URLs. Sometimes these may be introduced through environment
+                # variables unrelated to proxy settings - e.g. Colab `COLAB_LANGUAGE_SERVER_PROXY`.
+                # If the proxy is going to be used, the Request Handler proxy validation will handle it.
+                continue
             if proxy_scheme is None:
                 proxies[proxy_key] = 'http://' + remove_start(proxy_url, '//')