config: add hostname-based conditional config scopes

This enables configuration to be conditionally applied based on the
hostname set in `operation.hostname`. Users can now use
`--when.hostnames = ["host-a", "host-b"]` in their config files to apply
settings only on specific machines.

Fixes: #6441

Author: kaffarell

CHANGELOG.md

@@ -56,6 +56,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 * Add support for `--when.workspaces` config scopes.
 
+* Add support for `--when.hostnames` config scopes. This allows configuration to
+  be conditionally applied based on the hostname set in `operation.hostname`.
+
 * `jj bisect run` accepts the command and arguments to pass to the command
   directly as positional arguments, such as
   `jj bisect --range=..main -- cargo check --all-targets`.

cli/src/cli_util.rs

@@ -3916,6 +3916,10 @@ impl<'a> CliRunner<'a> {
             process_global_args_fn(ui, &matches)?;
         }
         config_env.set_command_name(command_name(&matches));
+        // Set hostname for --when.hostnames conditional config scopes.
+        if let Ok(hostname) = whoami::fallible::hostname() {
+            config_env.set_hostname(hostname);
+        }
 
         let maybe_workspace_loader = if let Some(path) = &args.global_args.repository {
             // TODO: maybe path should be canonicalized by WorkspaceLoader?

cli/src/config-schema.json

@@ -950,6 +950,13 @@
                         "type": "string"
                     }
                 },
+                "hostnames": {
+                    "type": "array",
+                    "description": "List of hostnames to match the hostname",
+                    "items": {
+                        "type": "string"
+                    }
+                },
                 "workspaces": {
                     "type": "array",
                     "description": "List of paths to match the workspace path prefix",

cli/src/config.rs

@@ -376,6 +376,7 @@ pub struct ConfigEnv {
     repo_config_path: Option<ConfigPath>,
     workspace_config_path: Option<ConfigPath>,
     command: Option<String>,
+    hostname: Option<String>,
 }
 
 impl ConfigEnv {
@@ -423,13 +424,18 @@ impl ConfigEnv {
             repo_config_path: None,
             workspace_config_path: None,
             command: None,
+            hostname: None,
         }
     }
 
     pub fn set_command_name(&mut self, command: String) {
         self.command = Some(command);
     }
 
+    pub fn set_hostname(&mut self, hostname: String) {
+        self.hostname = Some(hostname);
+    }
+
     /// Returns the paths to the user-specific config files or directories.
     pub fn user_config_paths(&self) -> impl Iterator<Item = &Path> {
         self.user_config_paths.iter().map(ConfigPath::as_path)
@@ -602,6 +608,7 @@ impl ConfigEnv {
             repo_path: self.repo_path.as_deref(),
             workspace_path: self.workspace_path.as_deref(),
             command: self.command.as_deref(),
+            hostname: self.hostname.as_deref().unwrap_or(""),
         };
         jj_lib::config::resolve(config.as_ref(), &context)
     }
@@ -1852,6 +1859,7 @@ mod tests {
             repo_config_path: None,
             workspace_config_path: None,
             command: None,
+            hostname: None,
         }
     }
 }

docs/config.md

@@ -1883,6 +1883,12 @@ email = "[email protected]"
 [--scope.user]
 email = "[email protected]"
 
+# override ui.pager on specific machines
+[[--scope]]
+--when.hostnames = ["work-laptop", "work-desktop"]
+[--scope.ui]
+pager = "delta"
+
 # disable pagination for `jj status`, use `delta` for `jj diff` and `jj show`
 [[--scope]]
 --when.commands = ["status"]
@@ -1940,6 +1946,16 @@ wip = ["log", "-r", "work"]
 
   Use `jj root` to see the workspace root directory.
 
+* `--when.hostnames`: List of hostnames to match against the `operation.hostname`
+  config setting.
+
+  Hostnames are compared case-sensitively and must match exactly.
+
+  ```toml
+  --when.hostnames = ["work-laptop"]               # matches only "work-laptop"
+  --when.hostnames = ["home-desktop", "laptop"]    # matches "home-desktop" OR "laptop"
+  ```
+
 * `--when.commands`: List of subcommands to match.
 
   Subcommands are space-separated and matched by prefix.