WIP: config/schema: collect annotations in :pairs()

Sometimes we need to know somethings about parent nodes during
processing of a current node.

The ideal solution would be pre-process schema in `schema.new()` and
saturate schema nodes with extra information. For example, we can add a
`parent` field. Or merge annotations from the root node down to the
current one and store in the current node.

However, we have shared schema nodes between instance_config and
cluster_config. I'm in a doubt whether the copying in appropriate places
would be error-prone.

The next commit needs the annotations from the parent nodes in
`<schema>:pairs()`, so this functionality is added to this particular
method.

TBD: Consider the alternative approach with precomputing on
`schema.new()` and a copying function for a schema.

TBD: Add a test.

TBD: Part of #xxxx

NO_DOC=the schema module is internal, at least now
NO_CHANGELOG=see NO_DOC

Author: Totktonada

src/box/lua/config/utils/schema.lua

@@ -102,6 +102,31 @@ local function walkthrough_assert_table(ctx, schema, data)
         schema.type, type(data))
 end
 
+-- XXX
+local function walkthrough_add_annotations(ctx, schema)
+    assert(ctx.annotations ~= nil)
+    local non_annotation_keys = {
+        type = true,
+        fields = true,
+        key = true,
+        value = true,
+        items = true,
+    }
+    local frame = table.copy(ctx.annotations[#ctx.annotations] or {})
+    for k, v in pairs(schema) do
+        if not non_annotation_keys[k] then
+            frame[k] = v
+        end
+    end
+    table.insert(ctx.annotations, frame)
+end
+
+-- XXX
+local function walkthrough_remove_annotations(ctx)
+    assert(ctx.annotations ~= nil)
+    table.remove(ctx.annotations)
+end
+
 -- }}} Walkthrough helpers
 
 -- {{{ Scalar definitions
@@ -1345,13 +1370,18 @@ end
 -- {{{ <schema object>:pairs()
 
 local function schema_pairs_append_node(schema, ctx)
+    walkthrough_add_annotations(ctx, schema)
     table.insert(ctx.acc, {
         path = table.copy(ctx.path),
         schema = schema,
+        annotations = ctx.annotations[#ctx.annotations]
     })
+    walkthrough_remove_annotations(ctx)
 end
 
 local function schema_pairs_impl(schema, ctx)
+    walkthrough_add_annotations(ctx, schema)
+
     if is_scalar(schema) then
         schema_pairs_append_node(schema, ctx)
     elseif schema.type == 'record' then
@@ -1367,6 +1397,8 @@ local function schema_pairs_impl(schema, ctx)
     else
         assert(false)
     end
+
+    walkthrough_remove_annotations(ctx)
 end
 
 -- Walk over the schema and return scalar, array and map schema
@@ -1377,12 +1409,18 @@ end
 -- for _, w in schema:pairs() do
 --     local path = w.path
 --     local schema = w.schema
+--     local annotations = w.annotations
 --     <...>
 -- end
 --
+-- w.annotations contains all the annotations merged from the root
+-- schema node down to the given one. If the same annotation is
+-- present in an ancestor node and in an descendant node, the
+-- latter is preferred.
+--
 -- TODO: Rewrite it without collecting a list beforehand.
 function methods.pairs(self)
-    local ctx = walkthrough_start(self, {acc = {}})
+    local ctx = walkthrough_start(self, {acc = {}, annotations = {}})
     schema_pairs_impl(rawget(self, 'schema'), ctx)
     return fun.iter(ctx.acc)
 end