[ericproulx]

A rescue_from :all declared in a namespace or a mounted API lost to any class handler of an enclosing scope that matched:

rescue_from ZeroDivisionError do
  error!('root', 500)
end

namespace :alpacas do
  rescue_from :all do
    error!('alpacas', 503)
  end
  get { 1 / 0 } # answered 'root'
end

A nested class handler already wins over an outer one whatever the classes are (#1405, README "Rescuing exceptions inside namespaces"). So an inner rescue_from StandardError beat the outer ZeroDivisionError, while an inner rescue_from :all, the same catch-all written differently, lost to it. The cause was ordering: Middleware::Error scanned the class handlers of every scope before it consulted :all, which is a single inherited value with no position in the chain.

Supersedes #1982. See #1975.

Change

• InheritableSetting#rescue_handlers ends the handlers of a scope that declared rescue_from :all with an :all entry. The entry is added on read, so it always follows that scope's own class handlers, however the declarations were interleaved. The catch-all therefore ranks behind its own scope's class handlers and ahead of every enclosing scope's.
• Middleware::Error stops the scan at that entry for an exception :all catches (a StandardError) and goes on to the catch-all as before, after the Grape exception checks.
  • An exception :all does not catch, such as NotImplementedError, scans past the entry and still reaches an enclosing rescue_from Exception.
  • An exception raised inside a rescue_from block is never handed to a catch-all, so redispatch ignores the entry and an enclosing scope's handler still takes it.
• The exact-match handlers (rescue_subclasses: false) are untouched. They are scanned before every subclass handler, whatever scope registered them, which predates this change and applies to nested class handlers too.

Behaviour change

The mount-order specs from #2384 (spec/grape/api/mount_and_helpers_order_spec.rb) pinned the outer class handler winning over a mounted API's :all. They now expect the mounted API's catch-all, and still give the same answer for either mount order, which is what they were written to check. UPGRADING describes the change and how to keep the old answer: register the class in the nested scope too.

Specs

spec/grape/api_spec.rb, next to the existing "rescues defined by mounted" precedence specs:

• a nested :all beats an outer class handler (fails on master);
• the nested scope's own class handlers still beat its :all;
• what :all does not catch reaches the outer rescue_from Exception, while a StandardError stops at the nested :all;
• an exception raised by the nested :all block is redispatched to the outer class handler.

Each of the last three guard specs was mutation-checked: putting the :all entry ahead of the scope's own handlers, letting redispatch stop at it, or letting it stop non-StandardErrors each fails exactly the matching spec.

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