Currently opaque config blocks (config entries, and CA provider config) are
modified by PatchSliceOfMaps, making it impossible for these opaque
config sections to contain slices of maps.
In order to fix this problem, any lazy-decoding of these blocks needs to support
weak decoding of []map[string]interface{} to a struct type before
PatchSliceOfMaps is replaces. This is necessary because these config
blobs are persisted, and during an upgrade an older version of Consul
could read one of the new configuration values, which would cause an error.
To support the upgrade path, this commit first introduces the new hooks
for weak decoding of []map[string]interface{} and uses them only in the
lazy-decode paths. That way, in a future release, new style
configuration will be supported by the older version of Consul.
This decode hook has a number of advantages:
1. It no longer panics. It allows mapstructure to report the error
2. It no longer requires the user to declare which fields are slices of
structs. It can deduce that information from the 'to' value.
3. It will make it possible to preserve opaque configuration, allowing
for structured opaque config.
This hook replaces lib.TranslateKeys and has a number of advantages:
1. Primarily, aliases for fields are defined on the field itself, making
the aliases much easier to maintain, and more obvious to the reader.
2. TranslateKeys translation rules are not aware of structure. It could
very easily incorrectly translate a key on one struct that was intended
to be a translation rule for a completely different struct, leading
to very hard to debug errors. The hook removes the need for the
unexpected "translation rule is an empty string to indicate stop
traversal" special case.
3. TranslateKeys attempts to duplicate a bunch of tree traversal logic
that already exists in mapstructure. Using mapstructure for traversal
removes the need to traverse the entire structure multiple times, and
makes the behaviour more obvious to the reader.
This change is being made to enable a future change of replacing
PatchSliceOfMaps. TranslateKeys sits in between PatchSliceOfMaps and
mapstructure.Decode, so it must be converted to a hook first, before
PatchSliceOfMaps can be replaced by a decode hook.
Daniel Nephin