typing: refine error handling for _dynamic fields

Better distinguish between unknown and mismatched types for dynamic
fields, and show a better error message for them.

refs: #211

assets/policy-extras/listener_domains.lua

@@ -14,9 +14,11 @@ local function is_listener_domain_option(name, value)
   local p = { [name] = value }
   local status, err = pcall(kumo.make_listener_domain, p)
   if not status then
+    local err = typing.extract_deserialize_error(err)
     if tostring(err):find 'invalid type' then
-      error(err, 3)
+      return false, err
     end
+    return false, nil
   end
   return status
 end
@@ -332,6 +334,20 @@ log_oob = false
     ),
     { relay_from = { '10.0.0.0/24' }, relay_to = false }
   )
+
+  local status, err = pcall(
+    parse_toml_data,
+    [[
+# Define a per-listener configuration
+[listener."127.0.0.1:25"."*.example.com"]
+log_arf = "yes"
+  ]]
+  )
+  assert(not status)
+  utils.assert_matches(
+    err,
+    'ListenerConfig: invalid value for field \'log_arf\'\n.*invalid type: string "yes", expected a boolean'
+  )
 end
 
 return mod

assets/policy-extras/queue.lua

@@ -75,9 +75,11 @@ local function is_queue_config_option(name, value)
   local p = { [name] = value }
   local status, err = pcall(kumo.make_queue_config, p)
   if not status then
+    local err = typing.extract_deserialize_error(err)
     if tostring(err):find 'invalid type' then
-      error(err, 3)
+      return false, err
     end
+    return false, nil
   end
   return status
 end

assets/policy-extras/typing.lua

@@ -176,7 +176,10 @@ end
 -- rust code in the embedding application.  The value in that
 -- case must be a function that accepts the key and value
 -- and returns a boolean to indicate whether that combination
--- is acceptable.
+-- is acceptable. If the function returns `false, nil` then
+-- that signals that the field is unknown. Otherwise it should
+-- return `false, err` with the second element of the tuple
+-- containing the error about the mismatched type.
 --
 -- { _dynamic = function(key, value) return true end }
 function mod.record(name, fields)
@@ -204,9 +207,20 @@ function mod.record(name, fields)
     local field_def = ty.fields[k]
     if not field_def then
       local dyn_validate = ty.fields._dynamic
-      if dyn_validate and dyn_validate(k, v) then
-        rawset(t, k, v)
-        return
+      if dyn_validate then
+        local status, err = dyn_validate(k, v)
+        if status then
+          rawset(t, k, v)
+          return
+        end
+        if err then
+          TypeError
+            :new(
+              string.format("%s: invalid value for field '%s'", ty.name, k),
+              err
+            )
+            :raise()
+        end
       end
 
       TypeError:new(string.format("%s: unknown field '%s'", ty.name, k))
@@ -499,6 +513,16 @@ function mod.option(target_type)
   return make_simple_ctor(ty)
 end
 
+function mod.extract_deserialize_error(err)
+  local err = tostring(err)
+  local re = kumo.regex.compile 'deserialize error: (.*), while processing'
+  local cap = re:captures(err)
+  if cap then
+    return cap[1]
+  end
+  return err
+end
+
 function mod:test()
   local utils = require 'policy-extras.policy_utils'