logfiles: Introduce LogfileListConfigBuilder

In
  tpo/core/arti!462 (comment 2797697)
we decided not to do this.

However, having looked again at the way the FallbackList works, I
think there is a lot of value in making these two things (and anything
else like them[1]) as similar as possible.

[1] At least PreemptiveCircuitConfig.initial_predicted_ports and
NetworkConfig.authorities need the same treatment, and perhaps also
GuardUsage.restrictions (although there is no
GuardRestrictionBuilder).

In the irc discussion I imagined `LogfilesConfigBuilder` as opposed to
`LogfileConfigBuilder` (differing only in the `s`) which would be bad,
but we can use `List` instead.

We do *not* need to abstract away the validated version of the config.
Providing a type alias helps the derive_builder sub_builder DTRT
without needing special overrides.

I have split this commit so that we can drop it, if we conclude it's
not wanted.

crates/arti/src/logging.rs

@@ -39,14 +39,8 @@ pub struct LoggingConfig {
     /// Configuration for one or more logfiles.
     #[serde(default)]
     #[builder_field_attr(serde(default))]
-    #[builder(
-        setter(custom),
-        field(
-            type = "Option<Vec<LogfileConfigBuilder>>",
-            build = "self.build_files()?"
-        )
-    )]
-    files: Vec<LogfileConfig>,
+    #[builder(sub_builder)]
+    files: LogfileListConfig,
 }
 
 /// Return a default tracing filter value for `logging.console`.
@@ -68,20 +62,28 @@ impl LoggingConfig {
     }
 }
 
-impl LoggingConfigBuilder {
+/// Local type alias, mostly helpful for derive_builder to DTRT
+type LogfileListConfig = Vec<LogfileConfig>;
+
+#[derive(Default, Clone, Deserialize)]
+#[serde(transparent)]
+/// List of logfiles to use, being built as part of the configuration
+pub struct LogfileListConfigBuilder {
+    /// The logfiles, as overridden
+    files: Option<Vec<LogfileConfigBuilder>>,
+}
+
+impl LogfileListConfigBuilder {
     /// Add a file logger
-    pub fn append_file(&mut self, file: LogfileConfigBuilder) -> &mut Self {
+    pub fn append(&mut self, file: LogfileConfigBuilder) -> &mut Self {
         self.files
-            .get_or_insert_with(LoggingConfigBuilder::default_files)
+            .get_or_insert_with(Self::default_files)
             .push(file);
         self
     }
 
     /// Set the list of file loggers to the supplied `files`
-    pub fn set_files(
-        &mut self,
-        files: impl IntoIterator<Item = LogfileConfigBuilder>,
-    ) -> &mut Self {
+    pub fn set(&mut self, files: impl IntoIterator<Item = LogfileConfigBuilder>) -> &mut Self {
         self.files = Some(files.into_iter().collect());
         self
     }
@@ -94,7 +96,7 @@ impl LoggingConfigBuilder {
     }
 
     /// Resolve `LoggingConfigBuilder.files` to a value for `LoggingConfig.files`
-    pub(crate) fn build_files(&self) -> Result<Vec<LogfileConfig>, ConfigBuildError> {
+    pub(crate) fn build(&self) -> Result<Vec<LogfileConfig>, ConfigBuildError> {
         let default_buffer;
         let files = match &self.files {
             Some(files) => files,

doc/semver_status.md

@@ -22,6 +22,12 @@ We can delete older sections here after we bump the releases.
 
 BREAKING: Configuration of fallback directories overhauled; now uses FalllbadkDirBuilder more.
 
+### arti
+
+BREAKING: Replaced LoggingConfigBuilder::file (taking Vec) with LoggingConfigBuilder::files
+BREAKING: LoggingConfigBuilder::build() throws ConfigBuildError, not a bespoke error
+MODIFIED: LoggingConfigBuilder is now Deserialize
+
 ### tor-basic-util
 
 MODIFIED: Added `reset()` method to RetrySchedule.