dkim_sign.lua: make base and domain optional

When splitting the config across multiple files, we should not
raise an error if the base or domain section is missing from
the file that is currently being processed.

Let's also show which file is currently being processed.

assets/policy-extras/dkim_sign.lua

@@ -19,7 +19,7 @@ local SignatureSigningPolicy =
 local SigningAlgo = typing.enum('SigningAlgo', 'sha256', 'ed25519')
 
 local DkimSignConfig = Record('DkimSignConfig', {
-  base = Record('DkimSignConfig.Base', {
+  base = Option(Record('DkimSignConfig.Base', {
     vault_mount = Option(String),
     vault_path_prefix = Option(String),
     additional_signatures = Option(List(String)),
@@ -29,9 +29,9 @@ local DkimSignConfig = Record('DkimSignConfig', {
     header_canonicalization = Option(String),
     body_canonicalization = Option(String),
     over_sign = Option(Bool),
-  }),
+  })),
 
-  domain = typing.map(
+  domain = Option(typing.map(
     String,
     Record('DkimSignConfig.Domain', {
       selector = Option(String),
@@ -43,7 +43,7 @@ local DkimSignConfig = Record('DkimSignConfig', {
       body_canonicalization = Option(String),
       over_sign = Option(Bool),
     })
-  ),
+  )),
 
   signature = Option(typing.map(
     String,
@@ -129,7 +129,11 @@ domain = "myesp.com"
 ]]
 
 local function load_dkim_data_from_file(file_name, target)
-  local data = DkimSignConfig(utils.load_json_or_toml_file(file_name))
+  local raw_data = utils.load_json_or_toml_file(file_name)
+  local is_ok, data = pcall(DkimSignConfig, raw_data)
+  if not is_ok then
+    error(string.format("reading data from file '%s': %s'", file_name, data))
+  end
 
   if data.base then
     utils.merge_into(data.base, target.base)

docs/changelog/main.md

@@ -40,3 +40,7 @@
   `0ns` for sessions for which we had not observed the session opening. Now we
   will assume a start time time of the first record observed for a session, so
   that some sense of relative time can be gleaned from the trace output.
+
+* When using the dkim helper and splitting the configuration
+  across multiple files, a missing `domain` or `base` configuration section in
+  any individual file would raise a validation error.