Implement extra-config option

AGENTS.md

@@ -4,6 +4,8 @@
 
 `config-example.toml` in the repo root is the canonical config file. It is symlinked into `$XDG_CONFIG_HOME/agent-sandbox/config.toml` on the host. When editing it, remember that changes take effect immediately for all sandbox invocations.
 
+The config file may set `extra-config = "<absolute path>"` to layer a second file on top using the same merge rules (scalars replace, vecs append, profiles merge by name). A missing extra file is silently skipped; nesting is not supported (the extra file cannot itself set `extra-config`).
+
 ## Build and test
 
 - `cargo fmt` and `cargo clippy` must pass before every commit.

config-example.toml

@@ -1,6 +1,12 @@
 # Layered settings: CLI > active profile > globals. `--profile` selects the
 # profile, otherwise `default-profile` below is used. Vec fields append across
 # layers; scalars replace.
+#
+# `extra-config` optionally points to a second file layered on top of this one
+# using the same rules (scalars replace, vecs append, profiles merge by name).
+# Missing extra-config files are silently skipped; nesting is not supported.
+
+extra-config = "~/.config/agent-sandbox/extra.toml"
 
 whitelist = true
 # blacklist = true

src/config.rs

@@ -292,6 +292,8 @@ pub struct FileConfig {
     pub profile: HashMap<String, Options>,
     #[serde(rename = "default-profile", default)]
     pub default_profile: Option<String>,
+    #[serde(rename = "extra-config", default)]
+    pub extra_config: Option<PathBuf>,
     // Collects unrecognized keys; deny_unknown_fields is incompatible with flatten.
     #[serde(flatten)]
     _unknown: HashMap<String, toml::Value>,
@@ -299,6 +301,30 @@ pub struct FileConfig {
 
 impl FileConfig {
     pub fn load(path: &Path) -> Result<Self, SandboxError> {
+        Self::load_file(path)?.load_extra()
+    }
+
+    fn load_extra(self) -> Result<Self, SandboxError> {
+        let Some(ref extra_path) = self.extra_config else {
+            return Ok(self);
+        };
+        let resolved = expand_and_require_absolute(extra_path)?;
+        let extra = match Self::load_file(&resolved) {
+            Ok(c) => c,
+            Err(SandboxError::ConfigRead { source, .. })
+                if source.kind() == std::io::ErrorKind::NotFound =>
+            {
+                return Ok(self);
+            }
+            Err(e) => return Err(e),
+        };
+        if extra.extra_config.is_some() {
+            return Err(SandboxError::NestedExtraConfig(resolved));
+        }
+        Ok(self.merge_with(extra))
+    }
+
+    fn load_file(path: &Path) -> Result<Self, SandboxError> {
         let contents = std::fs::read_to_string(path).map_err(|e| SandboxError::ConfigRead {
             path: path.to_path_buf(),
             source: e,
@@ -312,6 +338,24 @@ impl FileConfig {
         })
     }
 
+    fn merge_with(self, extra: FileConfig) -> FileConfig {
+        let mut profile = self.profile;
+        for (profile_name, profile_options) in extra.profile {
+            let merged = match profile.remove(&profile_name) {
+                Some(existing) => existing.merge_with(profile_options),
+                None => profile_options,
+            };
+            profile.insert(profile_name, merged);
+        }
+        FileConfig {
+            options: self.options.merge_with(extra.options),
+            profile,
+            default_profile: extra.default_profile.or(self.default_profile),
+            extra_config: None,
+            _unknown: HashMap::new(),
+        }
+    }
+
     fn parse(contents: &str) -> Result<Self, SandboxError> {
         let config: Self = toml::from_str(contents).map_err(|e| SandboxError::ConfigParse {
             path: PathBuf::new(),
@@ -363,6 +407,28 @@ pub struct Options {
 }
 
 impl Options {
+    fn merge_with(self, extra: Options) -> Options {
+        let (blacklist, whitelist) = pick_mode_flags(&self, &extra);
+        Options {
+            blacklist,
+            whitelist,
+            hardened: extra.hardened.or(self.hardened),
+            unshare_net: extra.unshare_net.or(self.unshare_net),
+            seccomp: extra.seccomp.or(self.seccomp),
+            env_filter: extra.env_filter.or(self.env_filter),
+            entrypoint: extra.entrypoint.or(self.entrypoint),
+            command: extra.command.or(self.command),
+            dry_run: extra.dry_run.or(self.dry_run),
+            chdir: extra.chdir.or(self.chdir),
+            rw: append(self.rw, extra.rw),
+            ro: append(self.ro, extra.ro),
+            mask: append(self.mask, extra.mask),
+            env: append(self.env, extra.env),
+            unsetenv: append(self.unsetenv, extra.unsetenv),
+            bwrap_args: append(self.bwrap_args, extra.bwrap_args),
+        }
+    }
+
     fn validate_paths(&mut self) -> Result<(), SandboxError> {
         if let Some(ref chdir) = self.chdir {
             self.chdir = Some(expand_and_canonicalize(chdir)?);
@@ -374,6 +440,19 @@ impl Options {
     }
 }
 
+fn append<T>(mut base: Vec<T>, extra: Vec<T>) -> Vec<T> {
+    base.extend(extra);
+    base
+}
+
+fn pick_mode_flags(base: &Options, extra: &Options) -> (Option<bool>, Option<bool>) {
+    if extra.blacklist.is_some() || extra.whitelist.is_some() {
+        (extra.blacklist, extra.whitelist)
+    } else {
+        (base.blacklist, base.whitelist)
+    }
+}
+
 fn expand_and_require_existing(path: &Path) -> Result<PathBuf, SandboxError> {
     let expanded = expand_and_require_absolute(path)?;
     if !expanded.exists() {
@@ -1485,6 +1564,210 @@ mod tests {
         assert!(matches!(result, Err(SandboxError::NoCommand)));
     }
 
+    #[test]
+    fn merge_options_scalar_extra_overrides_base() {
+        let base = Options {
+            hardened: Some(false),
+            ..Options::default()
+        };
+        let extra = Options {
+            hardened: Some(true),
+            ..Options::default()
+        };
+        assert_eq!(base.merge_with(extra).hardened, Some(true));
+    }
+
+    #[test]
+    fn merge_options_scalar_extra_unset_inherits_base() {
+        let base = Options {
+            hardened: Some(true),
+            ..Options::default()
+        };
+        assert_eq!(base.merge_with(Options::default()).hardened, Some(true));
+    }
+
+    #[test]
+    fn merge_options_extra_mode_replaces_base_mode() {
+        let base = Options {
+            whitelist: Some(true),
+            ..Options::default()
+        };
+        let extra = Options {
+            blacklist: Some(true),
+            ..Options::default()
+        };
+        let merged = base.merge_with(extra);
+        assert_eq!(merged.blacklist, Some(true));
+        assert_eq!(merged.whitelist, None);
+    }
+
+    #[test]
+    fn merge_options_base_mode_inherited_when_extra_silent() {
+        let base = Options {
+            whitelist: Some(true),
+            ..Options::default()
+        };
+        let merged = base.merge_with(Options::default());
+        assert_eq!(merged.whitelist, Some(true));
+        assert_eq!(merged.blacklist, None);
+    }
+
+    #[test]
+    fn merge_options_lists_append_base_then_extra() {
+        let base = Options {
+            rw: vec!["/a".into()],
+            env: vec!["A=1".into()],
+            ..Options::default()
+        };
+        let extra = Options {
+            rw: vec!["/b".into()],
+            env: vec!["B=2".into()],
+            ..Options::default()
+        };
+        let merged = base.merge_with(extra);
+        assert_eq!(merged.rw, vec!["/a", "/b"]);
+        assert_eq!(merged.env, vec!["A=1", "B=2"]);
+    }
+
+    #[test]
+    fn merge_file_config_profile_merged_by_name() {
+        let base = FileConfig {
+            profile: HashMap::from([(
+                "claude".into(),
+                Options {
+                    rw: vec!["/a".into()],
+                    hardened: Some(false),
+                    ..Options::default()
+                },
+            )]),
+            ..FileConfig::default()
+        };
+        let extra = FileConfig {
+            profile: HashMap::from([
+                (
+                    "claude".into(),
+                    Options {
+                        rw: vec!["/b".into()],
+                        hardened: Some(true),
+                        ..Options::default()
+                    },
+                ),
+                (
+                    "codex".into(),
+                    Options {
+                        unshare_net: Some(true),
+                        ..Options::default()
+                    },
+                ),
+            ]),
+            ..FileConfig::default()
+        };
+        let merged = base.merge_with(extra);
+        assert_eq!(merged.profile["claude"].hardened, Some(true));
+        assert_eq!(merged.profile["claude"].rw, vec!["/a", "/b"]);
+        assert_eq!(merged.profile["codex"].unshare_net, Some(true));
+    }
+
+    #[test]
+    fn merge_file_config_default_profile_extra_overrides() {
+        let base = FileConfig {
+            default_profile: Some("claude".into()),
+            ..FileConfig::default()
+        };
+        let extra = FileConfig {
+            default_profile: Some("codex".into()),
+            ..FileConfig::default()
+        };
+        assert_eq!(base.merge_with(extra).default_profile, Some("codex".into()));
+    }
+
+    #[test]
+    fn merge_file_config_default_profile_extra_unset_inherits() {
+        let base = FileConfig {
+            default_profile: Some("claude".into()),
+            ..FileConfig::default()
+        };
+        assert_eq!(
+            base.merge_with(FileConfig::default()).default_profile,
+            Some("claude".into())
+        );
+    }
+
+    #[test]
+    fn load_extra_applies_overlay() {
+        let dir = TempDir::new().unwrap();
+        let base_path = dir.path().join("base.toml");
+        let extra_path = dir.path().join("extra.toml");
+        std::fs::write(&extra_path, "hardened = true\nrw = [\"/b\"]\n").unwrap();
+        std::fs::write(
+            &base_path,
+            format!(
+                "hardened = false\nrw = [\"/a\"]\nextra-config = \"{}\"\n",
+                extra_path.display()
+            ),
+        )
+        .unwrap();
+
+        let config = FileConfig::load(&base_path).unwrap();
+        assert_eq!(config.options.hardened, Some(true));
+        assert_eq!(config.options.rw, vec!["/a", "/b"]);
+        assert_eq!(config.extra_config, None);
+    }
+
+    #[test]
+    fn load_extra_missing_file_is_skipped() {
+        let dir = TempDir::new().unwrap();
+        let base_path = dir.path().join("base.toml");
+        let extra_path = dir.path().join("does-not-exist.toml");
+        std::fs::write(
+            &base_path,
+            format!(
+                "hardened = true\nextra-config = \"{}\"\n",
+                extra_path.display()
+            ),
+        )
+        .unwrap();
+
+        let config = FileConfig::load(&base_path).unwrap();
+        assert_eq!(config.options.hardened, Some(true));
+    }
+
+    #[test]
+    fn load_extra_nested_rejected() {
+        let dir = TempDir::new().unwrap();
+        let base_path = dir.path().join("base.toml");
+        let extra_path = dir.path().join("extra.toml");
+        let deeper_path = dir.path().join("deeper.toml");
+        std::fs::write(&deeper_path, "hardened = true\n").unwrap();
+        std::fs::write(
+            &extra_path,
+            format!("extra-config = \"{}\"\n", deeper_path.display()),
+        )
+        .unwrap();
+        std::fs::write(
+            &base_path,
+            format!("extra-config = \"{}\"\n", extra_path.display()),
+        )
+        .unwrap();
+
+        assert!(matches!(
+            FileConfig::load(&base_path),
+            Err(SandboxError::NestedExtraConfig(_))
+        ));
+    }
+
+    #[test]
+    fn load_extra_relative_path_rejected() {
+        let dir = TempDir::new().unwrap();
+        let base_path = dir.path().join("base.toml");
+        std::fs::write(&base_path, "extra-config = \"extra.toml\"\n").unwrap();
+
+        assert!(matches!(
+            FileConfig::load(&base_path),
+            Err(SandboxError::ConfigPathNotAbsolute(_))
+        ));
+    }
+
     fn assert_paths(actual: &[String], expected: &[&str]) {
         let expected: Vec<String> = expected.iter().map(|s| s.to_string()).collect();
         assert_eq!(actual, &expected);

src/errors.rs

@@ -25,6 +25,7 @@ pub enum SandboxError {
     ConflictingEnvKey(String),
     InvalidEnvEntry(String),
     UnknownConfigKey(String),
+    NestedExtraConfig(PathBuf),
     ConfigPathNotAbsolute(PathBuf),
     InvalidBwrapArg(String),
     InvalidBindSpec(String),
@@ -75,6 +76,11 @@ impl std::fmt::Display for SandboxError {
                 write!(f, "invalid env entry (expected KEY or KEY=VALUE): {raw:?}")
             }
             Self::UnknownConfigKey(key) => write!(f, "unknown config key: {key}"),
+            Self::NestedExtraConfig(p) => write!(
+                f,
+                "extra-config file '{}' sets its own extra-config (nesting not supported)",
+                p.display()
+            ),
             Self::ConfigPathNotAbsolute(p) => {
                 write!(f, "config path is not absolute: {}", p.display())
             }