While this change correctly addresses the non-deterministic iteration by using a sorted slice of keys, there's a remaining logical issue in how the error handler is selected. The current logic can incorrectly choose an error handler from a less specific mounted app.

For example, if you have apps mounted at /api (with a custom error handler) and /api/v1 (without one), a request to /api/v1/users will incorrectly use the error handler from /api instead of falling back to the root app's handler.

This happens because mountedErrHandler is not reset when a more specific prefix (with no custom error handler) is found. The logic should be adjusted to ensure that only the error handler for the most specific matching prefix is considered. If that most specific prefix doesn't have a custom error handler, mountedErrHandler should be nil so that the root handler is used.

To fix this, the logic should be updated to reset mountedErrHandler when a more specific prefix is found.

Can anyone check if what gemini-code-assist says is correct? I think that if you have apps mounted at /api (with a custom error handler) and /api/v1 (without one), a request to /api/v1/users should use the error handler from the closest mount-path which would be /api instead of falling back to the root app's handler. Am i right?

Thank you for raising this important question, @duhnnie. I apologize for the confusion in my previous comment; my analysis of the desired behavior and the current code's outcome after your PR was indeed mistaken.

You are absolutely correct in your understanding:

"if you have apps mounted at /api (with a custom error handler) and /api/v1 (without one), a request to /api/v1/users should use the error handler from the closest mount-path which would be /api instead of falling back to the root app's handler."

This is the correct and expected hierarchical fallback behavior for mounted applications.

Let me clarify why your PR, with the change to iterate over app.mountFields.appListKeys (which are sorted by key length, typically ascending), actually achieves this desired behavior:

1. Iteration Order: By iterating through appListKeys sorted by length (e.g., "/", "/api", "/api/v1"), the loop processes shorter, less specific prefixes first, and then moves to longer, more specific ones.
2. mountedErrHandler and mountedPrefixParts:
   • When the loop encounters a matching prefix (e.g., "/api" for "/api/v1/users") that has an errorHandler, it updates mountedErrHandler to that handler and mountedPrefixParts to its length.
   • Crucially, when it later encounters a more specific matching prefix (e.g., "/api/v1") that does not have its own errorHandler, mountedErrHandler is not overwritten (because subApp.errorHandler is nil). However, mountedPrefixParts is updated to the length of this more specific prefix (e.g., 2 for "/api/v1").
3. The mountedPrefixParts <= parts Condition: This condition ensures that once a more specific prefix has been processed (and mountedPrefixParts updated to its length), any less specific prefixes (i.e., those with parts less than the current mountedPrefixParts) will be skipped, preventing them from overwriting the mountedErrHandler with a less specific one, unless the current mountedErrHandler is nil and a shorter prefix has a handler.

Example Trace (with EH_ROOT, EH_A for /api, and nil for /api/v1):

• Initial: mountedErrHandler = nil, mountedPrefixParts = 0
• Prefix /: Matches. EH_ROOT is set. mountedPrefixParts = 1.
• Prefix /api: Matches. EH_A is set (overwriting EH_ROOT). mountedPrefixParts = 1.
• Prefix /api/v1: Matches. subApp.errorHandler is nil. mountedErrHandler remains EH_A. mountedPrefixParts becomes 2.

Result: After the loop, mountedErrHandler will correctly be EH_A, which is the handler for the closest mounted app (/api) that has one, as you intended.

My apologies again for the misdirection. Your PR correctly implements the desired hierarchical error handler selection. The initial comment was based on an incorrect interpretation of the code's flow with the sorted keys.