config: add convenient ConfigLayer wrapper that provides .save() method

I'm going to remove write/remove_config_value_to/from_file() functions, but I
don't want to copy layer.path.expect(..) to all callers.

cli/src/command_error.rs

@@ -22,6 +22,7 @@ use std::sync::Arc;
 
 use itertools::Itertools as _;
 use jj_lib::backend::BackendError;
+use jj_lib::config::ConfigFileSaveError;
 use jj_lib::config::ConfigGetError;
 use jj_lib::config::ConfigLoadError;
 use jj_lib::dsl_util::Diagnostics;
@@ -246,6 +247,12 @@ impl From<ConfigEnvError> for CommandError {
     }
 }
 
+impl From<ConfigFileSaveError> for CommandError {
+    fn from(err: ConfigFileSaveError) -> Self {
+        user_error(err)
+    }
+}
+
 impl From<ConfigGetError> for CommandError {
     fn from(err: ConfigGetError) -> Self {
         let hint = match &err {

cli/src/config.rs

@@ -22,6 +22,7 @@ use std::path::PathBuf;
 use std::process::Command;
 
 use itertools::Itertools;
+use jj_lib::config::ConfigFile;
 use jj_lib::config::ConfigLayer;
 use jj_lib::config::ConfigLoadError;
 use jj_lib::config::ConfigNamePathBuf;
@@ -426,42 +427,17 @@ pub fn parse_config_args(toml_strs: &[ConfigArg]) -> Result<Vec<ConfigLayer>, Co
         .try_collect()
 }
 
-fn load_config_file_or_empty(
-    source: ConfigSource,
-    path: &Path,
-) -> Result<ConfigLayer, ConfigLoadError> {
-    match ConfigLayer::load_from_file(source, path.into()) {
-        Ok(layer) => Ok(layer),
-        Err(ConfigLoadError::Read(err)) if err.error.kind() == std::io::ErrorKind::NotFound => {
-            // If config doesn't exist yet, read as empty and we'll write one.
-            let mut layer = ConfigLayer::empty(source);
-            layer.path = Some(path.into());
-            Ok(layer)
-        }
-        Err(err) => Err(err),
-    }
-}
-
-fn write_config(path: &Path, doc: &toml_edit::DocumentMut) -> Result<(), CommandError> {
-    std::fs::write(path, doc.to_string()).map_err(|err| {
-        user_error_with_message(
-            format!("Failed to write file {path}", path = path.display()),
-            err,
-        )
-    })
-}
-
 pub fn write_config_value_to_file(
     key: &ConfigNamePathBuf,
     value: toml_edit::Value,
     path: &Path,
 ) -> Result<(), CommandError> {
     // TODO: Load config layer by caller. Here we use a dummy source for now.
-    let mut layer = load_config_file_or_empty(ConfigSource::User, path)?;
+    let mut file = ConfigFile::load_or_empty(ConfigSource::User, path)?;
-    layer
+    file.set_value(key, value)
-        .set_value(key, value)
         .map_err(|err| user_error_with_message(format!("Failed to set {key}"), err))?;
-    write_config(path, &layer.data)
+    file.save()?;
+    Ok(())
 }
 
 pub fn remove_config_value_from_file(
@@ -469,14 +445,15 @@ pub fn remove_config_value_from_file(
     path: &Path,
 ) -> Result<(), CommandError> {
     // TODO: Load config layer by caller. Here we use a dummy source for now.
-    let mut layer = load_config_file_or_empty(ConfigSource::User, path)?;
+    let mut file = ConfigFile::load_or_empty(ConfigSource::User, path)?;
-    let old_value = layer
+    let old_value = file
         .delete_value(key)
         .map_err(|err| user_error_with_message(format!("Failed to unset {key}"), err))?;
     if old_value.is_none() {
         return Err(user_error(format!(r#""{key}" doesn't exist"#)));
     }
-    write_config(path, &layer.data)
+    file.save()?;
+    Ok(())
 }
 
 /// Command name and arguments specified by config.

lib/src/config.rs

@@ -18,6 +18,7 @@ use std::borrow::Borrow;
 use std::convert::Infallible;
 use std::fmt;
 use std::fs;
+use std::io;
 use std::ops::Range;
 use std::path::Path;
 use std::path::PathBuf;
@@ -58,6 +59,11 @@ pub enum ConfigLoadError {
     },
 }
 
+/// Error that can occur when saving config variables to file.
+#[derive(Debug, Error)]
+#[error("Failed to write configuration file")]
+pub struct ConfigFileSaveError(#[source] pub PathError);
+
 /// Error that can occur when looking up config variable.
 #[derive(Debug, Error)]
 pub enum ConfigGetError {
@@ -467,6 +473,71 @@ fn ensure_parent_table<'a, 'b>(
     Ok((parent_table, leaf_key))
 }
 
+/// Wrapper for file-based [`ConfigLayer`], providing convenient methods for
+/// modification.
+#[derive(Debug)]
+pub struct ConfigFile {
+    layer: ConfigLayer,
+}
+
+impl ConfigFile {
+    /// Loads TOML file from the specified `path` if exists. Returns an empty
+    /// object if the file doesn't exist.
+    pub fn load_or_empty(
+        source: ConfigSource,
+        path: impl Into<PathBuf>,
+    ) -> Result<Self, ConfigLoadError> {
+        let layer = match ConfigLayer::load_from_file(source, path.into()) {
+            Ok(layer) => layer,
+            Err(ConfigLoadError::Read(PathError { path, error }))
+                if error.kind() == io::ErrorKind::NotFound =>
+            {
+                ConfigLayer {
+                    source,
+                    path: Some(path),
+                    data: DocumentMut::new(),
+                }
+            }
+            Err(err) => return Err(err),
+        };
+        Ok(ConfigFile { layer })
+    }
+
+    /// Writes serialized data to the source file.
+    pub fn save(&self) -> Result<(), ConfigFileSaveError> {
+        fs::write(self.path(), self.layer.data.to_string())
+            .context(self.path())
+            .map_err(ConfigFileSaveError)
+    }
+
+    /// Source file path.
+    pub fn path(&self) -> &Path {
+        self.layer.path.as_ref().expect("path must be known")
+    }
+
+    /// Returns the underlying config layer.
+    pub fn layer(&self) -> &ConfigLayer {
+        &self.layer
+    }
+
+    /// See [`ConfigLayer::set_value()`].
+    pub fn set_value(
+        &mut self,
+        name: impl ToConfigNamePath,
+        new_value: impl Into<ConfigValue>,
+    ) -> Result<Option<ConfigValue>, ConfigUpdateError> {
+        self.layer.set_value(name, new_value)
+    }
+
+    /// See [`ConfigLayer::delete_value()`].
+    pub fn delete_value(
+        &mut self,
+        name: impl ToConfigNamePath,
+    ) -> Result<Option<ConfigValue>, ConfigUpdateError> {
+        self.layer.delete_value(name)
+    }
+}
+
 /// Stack of configuration layers which can be merged as needed.
 ///
 /// A [`StackedConfig`] is something like a read-only `overlayfs`. Tables and