[common] implement '"user-agent": "@BROWSER"' (#7947)

support automatically fetching the 'User-Agent' header
of a specific installed browser, e.g. "@firefox"

docs/configuration.rst

@@ -696,12 +696,19 @@ Default
     * ``"Patreon/72.2.28 (Android; Android 14; Scale/2.10)"``: ``patreon``
     * ``"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/LATEST.0.0.0 Safari/537.36"``: ``instagram``
     * ``"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:LATEST) Gecko/20100101 Firefox/LATEST"``: otherwise
+Example
+    * ``"curl/8.14.1"``
+    * ``"browser"``
+    * ``"@chrome"``
 Description
     User-Agent header value used for HTTP requests.
 
     Setting this value to ``"browser"`` will try to automatically detect
     and use the ``User-Agent`` header of the system's default browser.
 
+    Setting this value to ``"@BROWSER"``, e.g. ``"@chrome"``, will try to automatically detect
+    and use the ``User-Agent`` header of this installed browser.
+
 
 extractor.*.browser
 -------------------

gallery_dl/extractor/common.py

@@ -464,7 +464,9 @@ class Extractor():
         if custom_ua is None or custom_ua == "auto":
             pass
         elif custom_ua == "browser":
-            headers["User-Agent"] = _browser_useragent()
+            headers["User-Agent"] = _browser_useragent(None)
+        elif custom_ua[0] == "@":
+            headers["User-Agent"] = _browser_useragent(custom_ua[1:])
         elif self.useragent is Extractor.useragent and not self.browser or \
                 custom_ua is not config.get(("extractor",), "user-agent"):
             headers["User-Agent"] = custom_ua
@@ -1042,19 +1044,20 @@ def _build_requests_adapter(
     return adapter
 
 
-@cache.cache(maxage=86400)
-def _browser_useragent():
+@cache.cache(maxage=86400, keyarg=0)
+def _browser_useragent(name):
     """Get User-Agent header from default browser"""
     import webbrowser
-    import socket
+    browser = webbrowser.get(name)
 
+    import socket
     server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
     server.bind(("127.0.0.1", 0))
     server.listen(1)
 
     host, port = server.getsockname()
-    webbrowser.open(f"http://{host}:{port}/user-agent")
+    browser.open(f"http://{host}:{port}/user-agent")
 
     client = server.accept()[0]
     server.close()