pybridge: Let cockpit-beiboot find package-installed askpass helper

Teach ensure_ferny_askpass() to first check if the system has an
installed cockpit-askpass, before it creates a new temporary file.
Turn get_libexecdir() into a proper global function for that.

This fixes running cockpit-beiboot as cockpit-wsinstance system user, as
there is no existing/writable home directory, and trying to mkdir
~/.cache EPERMs.

src/cockpit/beiboot.py

@@ -34,7 +34,7 @@
 from cockpit.channel import ChannelRoutingRule
 from cockpit.channels import PackagesChannel
 from cockpit.jsonutil import JsonObject
-from cockpit.packages import Packages, PackagesLoader
+from cockpit.packages import Packages, PackagesLoader, get_libexecdir
 from cockpit.peer import Peer
 from cockpit.protocol import CockpitProblem
 from cockpit.router import Router, RoutingRule
@@ -44,6 +44,12 @@
 
 
 def ensure_ferny_askpass() -> Path:
+    # packaged case
+    askpass = Path(get_libexecdir(), 'cockpit-askpass')
+    if askpass.exists():
+        return askpass
+
+    # remote beipack case, create a temporary file
     src_path = importlib.resources.files(ferny.__name__) / 'interaction_client.py'
     src_data = src_path.read_bytes()
 

src/cockpit/packages.py

@@ -92,6 +92,21 @@ def sortify_version(version: str) -> str:
     return '.'.join(part.zfill(8) for part in version.split('.'))
 
 
+@functools.lru_cache()
+def get_libexecdir() -> str:
+    """Detect libexecdir on current machine
+
+    This only works for systems which have cockpit-ws installed.
+    """
+    for candidate in ['/usr/local/libexec', '/usr/libexec', '/usr/local/lib/cockpit', '/usr/lib/cockpit']:
+        if os.path.exists(os.path.join(candidate, 'cockpit-askpass')):
+            return candidate
+    else:
+        logger.warning('Could not detect libexecdir')
+        # give readable error messages
+        return '/nonexistent/libexec'
+
+
 # A document is a binary stream with a Content-Type, optional Content-Encoding,
 # and optional Content-Security-Policy
 class Document(NamedTuple):
@@ -281,21 +296,6 @@ class PackagesLoader:
         'path-not-exists': lambda p: not os.path.exists(p),
     }
 
-    @staticmethod
-    @functools.lru_cache()
-    def get_libexecdir() -> str:
-        """Detect libexecdir on current machine
-
-        This only works for systems which have cockpit-ws installed.
-        """
-        for candidate in ['/usr/local/libexec', '/usr/libexec', '/usr/local/lib/cockpit', '/usr/lib/cockpit']:
-            if os.path.exists(os.path.join(candidate, 'cockpit-askpass')):
-                return candidate
-        else:
-            logger.warning('Could not detect libexecdir')
-            # give readable error messages
-            return '/nonexistent/libexec'
-
     # HACK: Type narrowing over Union types is not supported in the general case,
     # but this works for the case we care about: knowing that when we pass in an
     # JsonObject, we'll get an JsonObject back.
@@ -309,7 +309,7 @@ def patch_libexecdir(cls, obj: J) -> J:
                 abs_askpass = shutil.which('cockpit-askpass')
                 if abs_askpass is not None:
                     return obj.replace('${libexecdir}/cockpit-askpass', abs_askpass)
-            return obj.replace('${libexecdir}', cls.get_libexecdir())
+            return obj.replace('${libexecdir}', get_libexecdir())
         elif isinstance(obj, dict):
             return {key: cls.patch_libexecdir(value) for key, value in obj.items()}
         elif isinstance(obj, list):