improve error message when SETCREDS overwrites git-annex config

That is not allowed, so it's not a bug in git-annex when it happens and
instead tell the special remote developer how it's messed up.

Note that currently only Remote.External can overwrite the parsed remote
config with a PassedThrough value. PassedThrough values are otherwise
only generated for configs that are not parsed by the remote config
parser.

Sponsored-by: Joshua Antonishen

Annex/SpecialRemote/Config.hs

@@ -206,13 +206,23 @@ getRemoteConfigValue :: HasCallStack => Typeable v => RemoteConfigField -> Parse
 getRemoteConfigValue f (ParsedRemoteConfig m _) = case M.lookup f m of
 	Just (RemoteConfigValue v) -> case cast v of
 		Just v' -> Just v'
-		Nothing -> error $ unwords
+		Nothing -> case cast v :: Maybe PassedThrough of
-			[ "getRemoteConfigValue"
+			-- Handle the case where an external special remote
-			, fromProposedAccepted f
+			-- tries to SETCONFIG a value belonging to git-annex,
-			, "found value of unexpected type"
+			-- resulting in a PassedThrough type being stored.
-			, show (typeOf v) ++ "."
+			Just _ -> error $ unwords
-			, "This is a bug in git-annex!"
+				[ "Special remote config "
-			]
+				, fromProposedAccepted f
+				, "has been overwritten by SETCONFIG."
+				, "This is not supported."
+				]
+			Nothing -> error $ unwords
+				[ "getRemoteConfigValue"
+				, fromProposedAccepted f
+				, "found value of unexpected type"
+				, show (typeOf v) ++ "."
+				, "This is a bug in git-annex!"
+				]
 	Nothing -> Nothing
 
 {- Gets all fields that remoteConfigRestPassthrough matched. -}

doc/todo/encrypt_only_the_credentials/comment_8_7ec2d0de7c0b90f05475b86148982197._comment

@@ -0,0 +1,24 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 8"""
+ date="2025-09-16T16:57:11Z"
+ content="""
+SETCONFIG is limited to setting the external program's configuration,
+not to reaching inside git-annex and setting its own configuration.
+The docs say that, but could perhaps be more clear.
+
+I have improved the error message.
+
+git-annex sets up encryption for the remote based on the encryption= and
+encryptonlycreds= settings before it ever starts up the external program.
+That would need to change in order to support this.
+
+But I'm also doubtful it would be a good idea to support SETCONFIG
+of any of the things git-annex uses for encryption, chunking, etc.
+It's essentially monkey-patching git-annex from the external program.
+Some changes to git-annex's configs could lead to very unexpected behavior.
+
+If you really need the ability to turn on onlyencryptcreds by default
+with your special remote, there will need to be some other way implemented
+to do it. Please open a new todo about that.
+"""]]