config: rework the way the meta is populated

This patch reworks the way the meta is populated. This is done to
incrementally populate the metadata, instead of set the metadata at the
end of reading data from the source. This allows to get the correct meta
in cases where getting data from the source failed.

Follow-up #8789

NO_DOC=feature not yet released
NO_TEST=tested in EE
NO_CHANGELOG=feature not yet released

src/box/lua/config/init.lua

@@ -80,13 +80,18 @@ function methods._alert(self, alert)
     table.insert(self._alerts, alert)
 end
 
+function methods._meta(self, source_name, key, value)
+    local data = self._metadata[source_name] or {}
+    data[key] = value
+    self._metadata[source_name] = data
+end
+
 function methods._register_source(self, source)
     assert(type(source) == 'table')
     assert(source.name ~= nil)
     assert(source.type == 'instance' or source.type == 'cluster')
     assert(source.sync ~= nil)
     assert(source.get ~= nil)
-    assert(source.meta ~= nil)
     table.insert(self._sources, source)
 end
 
@@ -134,7 +139,6 @@ function methods._collect(self, opts)
 
     -- For error reporting.
     local source_info = {}
-    self._meta = {}
 
     for _, source in ipairs(self._sources) do
         -- Gather config values.
@@ -146,7 +150,6 @@ function methods._collect(self, opts)
         if sync_source == source.name or sync_source == 'all' then
             source.sync(self, iconfig)
         end
-        self._meta[source.name] = source.meta()
 
         -- Validate configurations gathered from the sources.
         if source.type == 'instance' then
@@ -285,6 +288,7 @@ function methods._reload_noexc(self, opts)
     broadcast(self)
 
     self._alerts = {}
+    self._metadata = {}
     local ok, err = pcall(self._collect, self, opts)
     if ok then
         ok, err = pcall(self._apply, self)
@@ -321,7 +325,7 @@ function methods.info(self)
     selfcheck(self, 'info')
     return {
         alerts = self._alerts,
-        meta = self._meta,
+        meta = self._metadata,
         status = self._status,
     }
 end
@@ -343,7 +347,7 @@ local function new()
         -- Track situations when something is going wrong.
         _alerts = {},
         -- Metadata from sources.
-        _meta = {},
+        _metadata = {},
         -- Current status.
         _status = 'uninitialized',
     }, mt)

src/box/lua/config/source/env.lua

@@ -26,10 +26,6 @@ local function get()
     return values
 end
 
-local function meta()
-    return nil
-end
-
 return {
     name = 'env',
     -- The type is either 'instance' or 'cluster'.
@@ -40,6 +36,4 @@ return {
     --
     -- source.get()
     get = get,
-    -- Metadata of the current configuration from the source.
-    meta = meta,
 }

src/box/lua/config/source/file.lua

@@ -75,10 +75,6 @@ local function get()
     return values
 end
 
-local function meta()
-    return nil
-end
-
 return {
     name = 'file',
     -- The type is either 'instance' or 'cluster'.
@@ -89,6 +85,4 @@ return {
     --
     -- source.get()
     get = get,
-    -- Metadata of the current configuration from the source.
-    meta = meta,
 }