config: add support for a $JJ_CONFIG environment variable

It's useful for tests, scripts, and debugging to be able to use
specific config instead of the user's config. That's especially true
for our automated tests because they didn't have a place to read
config from on Windows before this patch (they read their config from
`{FOLDERID_RoamingAppData}`, which I don't think we can override in
tests).

CHANGELOG.md

@@ -15,6 +15,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `~/Library/Application Support/` on macOS, and `~\AppData\Roaming\` on
   Windows.
 
+### New features
+
+* You can now set an environment variable called `$JJ_CONFIG` to a path to a
+  config file. That will then be read instead of your regular config file. This
+  is mostly intended for testing and scripts.
+
 ## [0.3.3] - 2022-03-16
 
 No changes, only trying to get the automated build to work.

src/main.rs

@@ -13,17 +13,26 @@
 // limitations under the License.
 
 use std::env;
+use std::path::PathBuf;
 
 use jujutsu::commands::dispatch;
 use jujutsu::ui::Ui;
 use jujutsu_lib::settings::UserSettings;
 
+fn config_path() -> Option<PathBuf> {
+    if let Ok(config_path) = env::var("JJ_CONFIG") {
+        Some(PathBuf::from(config_path))
+    } else {
+        dirs::config_dir().map(|config_dir| config_dir.join("jj").join("config.toml"))
+    }
+}
+
 fn read_config() -> Result<UserSettings, config::ConfigError> {
     let mut config_builder = config::Config::builder();
 
-    if let Some(config_dir) = dirs::config_dir() {
+    if let Some(config_path) = config_path() {
         config_builder = config_builder.add_source(
-            config::File::from(config_dir.join("jj").join("config.toml"))
+            config::File::from(config_path)
                 .required(false)
                 .format(config::FileFormat::Toml),
         );

src/testutils.rs

@@ -23,6 +23,7 @@ pub struct TestEnvironment {
     _temp_dir: TempDir,
     env_root: PathBuf,
     home_dir: PathBuf,
+    config_path: PathBuf,
     command_number: RefCell<i64>,
 }
 
@@ -32,10 +33,13 @@ impl Default for TestEnvironment {
         let env_root = tmp_dir.path().canonicalize().unwrap();
         let home_dir = env_root.join("home");
         std::fs::create_dir(&home_dir).unwrap();
+        let config_path = env_root.join("config.toml");
+        std::fs::write(&config_path, b"").unwrap();
         Self {
             _temp_dir: tmp_dir,
             env_root,
             home_dir,
+            config_path,
             command_number: RefCell::new(0),
         }
     }
@@ -52,6 +56,7 @@ impl TestEnvironment {
         let timestamp = chrono::DateTime::parse_from_rfc3339("2001-02-03T04:05:06+07:00").unwrap();
         let mut command_number = self.command_number.borrow_mut();
         *command_number += 1;
+        cmd.env("JJ_CONFIG", self.config_path.to_str().unwrap());
         let timestamp = timestamp + chrono::Duration::seconds(*command_number);
         cmd.env("JJ_TIMESTAMP", timestamp.to_rfc3339());
         cmd.env("JJ_USER", "Test User");
@@ -66,6 +71,10 @@ impl TestEnvironment {
     pub fn home_dir(&self) -> &Path {
         &self.home_dir
     }
+
+    pub fn config_path(&self) -> &Path {
+        &self.config_path
+    }
 }
 
 pub fn get_stdout_string(assert: &assert_cmd::assert::Assert) -> String {

tests/test_global_opts.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::io::Write;
+
 use jujutsu::testutils::{get_stdout_string, TestEnvironment};
 
 #[test]
@@ -79,3 +81,33 @@ fn test_repo_arg_with_git_clone() {
     insta::assert_snapshot!(get_stdout_string(&assert), @"Error: '--repository' cannot be used with 'git clone'
 ");
 }
+
+#[test]
+fn test_color_config() {
+    let test_env = TestEnvironment::default();
+    let mut config_file = std::fs::File::options()
+        .append(true)
+        .open(test_env.config_path())
+        .unwrap();
+    config_file
+        .write_all(
+            br#"[ui]
+color="always""#,
+        )
+        .unwrap();
+    config_file.flush().unwrap();
+    test_env
+        .jj_cmd(test_env.env_root(), &["init", "repo", "--git"])
+        .assert()
+        .success();
+
+    let repo_path = test_env.env_root().join("repo");
+    let assert = test_env
+        .jj_cmd(&repo_path, &["log", "-T", "commit_id"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    @ 230dd059e1b059aefc0da06a2e5a7dbf22362f22
+    o 0000000000000000000000000000000000000000
+    "###);
+}