Make change ids in tests repeatable

This will be needed to test functionality for showing shortest
unique prefix for commit and change ids. As a bonus, this also
allows us to test log output with change ids.

As another bonus, this will prevent occasional CI failures like
https://github.com/martinvonz/jj/actions/runs/3817554687/jobs/6493881468.

lib/src/commit_builder.rs

@@ -12,25 +12,21 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use uuid::Uuid;
+use std::sync::Arc;
 
-use crate::backend::{self, BackendResult, ChangeId, CommitId, ObjectId, Signature, TreeId};
+use crate::backend::{self, BackendResult, ChangeId, CommitId, Signature, TreeId};
 use crate::commit::Commit;
 use crate::repo::MutableRepo;
-use crate::settings::UserSettings;
+use crate::settings::{JJRng, UserSettings};
 
 #[must_use]
 pub struct CommitBuilder<'repo> {
     mut_repo: &'repo mut MutableRepo,
+    rng: Arc<JJRng>,
     commit: backend::Commit,
     rewrite_source: Option<Commit>,
 }
 
-// TODO: This function will be replaced in the next commit.
-pub fn new_change_id() -> ChangeId {
-    ChangeId::from_bytes(Uuid::new_v4().as_bytes())
-}
-
 impl CommitBuilder<'_> {
     pub fn for_new_commit<'repo>(
         mut_repo: &'repo mut MutableRepo,
@@ -40,17 +36,19 @@ impl CommitBuilder<'_> {
     ) -> CommitBuilder<'repo> {
         let signature = settings.signature();
         assert!(!parents.is_empty());
+        let rng = settings.get_rng();
         let commit = backend::Commit {
             parents,
             predecessors: vec![],
             root_tree: tree_id,
-            change_id: new_change_id(),
+            change_id: rng.new_change_id(),
             description: String::new(),
             author: signature.clone(),
             committer: signature,
         };
         CommitBuilder {
             mut_repo,
+            rng,
             commit,
             rewrite_source: None,
         }
@@ -75,6 +73,7 @@ impl CommitBuilder<'_> {
         CommitBuilder {
             mut_repo,
             commit,
+            rng: settings.get_rng(),
             rewrite_source: Some(predecessor.clone()),
         }
     }
@@ -101,7 +100,7 @@ impl CommitBuilder<'_> {
     }
 
     pub fn generate_new_change_id(mut self) -> Self {
-        self.commit.change_id = new_change_id();
+        self.commit.change_id = self.rng.new_change_id();
         self
     }
 

lib/src/settings.rs

@@ -19,7 +19,7 @@ use chrono::DateTime;
 use rand::prelude::*;
 use rand_chacha::ChaCha20Rng;
 
-use crate::backend::{Signature, Timestamp};
+use crate::backend::{ChangeId, ObjectId, Signature, Timestamp};
 
 #[derive(Debug, Clone)]
 pub struct UserSettings {
@@ -178,8 +178,19 @@ impl UserSettings {
 #[derive(Debug)]
 pub struct JJRng(Mutex<ChaCha20Rng>);
 impl JJRng {
-    /// Wraps Rng::gen but only requires an immutable reference.
+    pub fn new_change_id(&self) -> ChangeId {
-    pub fn gen<T>(&self) -> T
+        let random_bytes = self.gen();
+        // The `uuid` crate is used merely to specify the number of random bytes (16)
+        ChangeId::from_bytes(
+            uuid::Builder::from_random_bytes(random_bytes)
+                .into_uuid()
+                .as_bytes(),
+        )
+    }
+
+    /// Wraps Rng::gen but only requires an immutable reference. Can be made
+    /// public if there's a use for it.
+    fn gen<T>(&self) -> T
     where
         rand::distributions::Standard: rand::distributions::Distribution<T>,
     {

tests/test_log_command.rs

@@ -61,6 +61,16 @@ fn test_log_with_or_without_diff() {
     o (no description set)
     "###);
 
+    // Test default log output format
+    insta::assert_snapshot!(test_env.jj_cmd_success(&repo_path, &["log"]), @r###"
+    @ ffdaa62087a2 test.user@example.com 2001-02-03 04:05:10.000 +07:00 789e536fd2e0
+    | a new commit
+    o 9a45c67d3e96 test.user@example.com 2001-02-03 04:05:08.000 +07:00 4291e264ae97
+    | add a file
+    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+      (no description set)
+    "###);
+
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "description", "--no-graph"]);
     insta::assert_snapshot!(stdout, @r###"
     a new commit