rewrite: add reset-author-timestamp revset

It's common to create empty working-copy commits while using jj, and
currently the author timestamp for a commit is only set when it is first
created. If you create an empty commit, then don't work on a repo for a
few days, and then start working on a new feature without abandoning the
working-copy commit, the author timestamp will remain as the time the
commit was created rather than being updated to the time that work began
or finished.

This commit adds a new revset `reset-author-timestamp` which specifies
which commits should have their author timestamp updated whenever
changes are made to them. This can be configured to fit many different
workflows. By default, empty commits with no description have their
author timestamps reset, meaning that the author timestamp will become
finalized whenever a commit is given a description or becomes non-empty.

Rebasing commits never updates the author timestamp, and commits by a
different author are also never updated.

CHANGELOG.md

@@ -55,6 +55,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * New command `jj branch move` let you update branches by name pattern or source
   revision.
 
+* `revsets.reset-author-timestamp` can be used to configure which author
+  timestamps should be reset when rewriting commits.
+
 ### Fixed bugs
 
 ## [0.18.0] - 2024-06-05

cli/src/cli_util.rs

@@ -1157,8 +1157,47 @@ impl WorkspaceCommandHelper {
         self.check_repo_rewritable(self.repo().as_ref(), commits)
     }
 
+    fn reset_author_timestamp_revset(&self) -> Result<Rc<RevsetExpression>, CommandError> {
+        let reset_author_timestamp = self
+            .settings
+            .config()
+            .get_string("revsets.reset-author-timestamp")
+            .unwrap_or_default();
+        revset::parse(&reset_author_timestamp, &self.revset_parse_context())
+            .map_err(|e| config_error_with_message("Invalid `revsets.reset-author-timestamp`", e))
+    }
+
+    fn evaluate_reset_author_timestamp(
+        mut expression: RevsetExpressionEvaluator,
+        commit_id: &CommitId,
+    ) -> Result<bool, CommandError> {
+        expression.intersect_with(&RevsetExpression::commit(commit_id.clone()));
+        Ok(expression
+            .evaluate_to_commit_ids()
+            .map_err(|e| config_error_with_message("Invalid `revsets.reset-author-timestamp`", e))?
+            .next()
+            .is_some())
+    }
+
+    pub fn check_reset_author_timestamp(
+        &self,
+        repo: &dyn Repo,
+        commit_id: &CommitId,
+    ) -> Result<bool, CommandError> {
+        let expression = RevsetExpressionEvaluator::new(
+            repo,
+            self.revset_extensions.clone(),
+            self.id_prefix_context()?,
+            self.reset_author_timestamp_revset()?,
+        );
+        Self::evaluate_reset_author_timestamp(expression, commit_id)
+    }
+
     #[instrument(skip_all)]
     fn snapshot_working_copy(&mut self, ui: &mut Ui) -> Result<(), CommandError> {
+        let reset_author_timestamp_revset = self.reset_author_timestamp_revset()?;
+        let id_prefix_context = self.id_prefix_context()?.clone();
+
         let workspace_id = self.workspace_id().to_owned();
         let get_wc_commit = |repo: &ReadonlyRepo| -> Result<Option<_>, _> {
             repo.view()
@@ -1229,12 +1268,22 @@ See https://github.com/martinvonz/jj/blob/main/docs/working-copy.md#stale-workin
         })?;
         drop(progress);
         if new_tree_id != *wc_commit.tree_id() {
+            let reset_author_timestamp = Self::evaluate_reset_author_timestamp(
+                RevsetExpressionEvaluator::new(
+                    self.user_repo.repo.as_ref(),
+                    self.revset_extensions.clone(),
+                    &id_prefix_context,
+                    reset_author_timestamp_revset,
+                ),
+                wc_commit.id(),
+            )?;
             let mut tx =
                 start_repo_transaction(&self.user_repo.repo, &self.settings, &self.string_args);
             tx.set_is_snapshot(true);
             let mut_repo = tx.mut_repo();
             let commit = mut_repo
                 .rewrite_commit(&self.settings, &wc_commit)
+                .reset_author_timestamp(reset_author_timestamp)
                 .set_tree_id(new_tree_id)
                 .write()?;
             mut_repo.set_wc_commit(workspace_id, commit.id().clone())?;
@@ -1628,8 +1677,15 @@ impl WorkspaceCommandTransaction<'_> {
     }
 
     pub fn rewrite_commit(&mut self, commit: &Commit) -> Result<CommitBuilder, CommandError> {
+        let reset_author_timestamp = self
+            .helper
+            .check_reset_author_timestamp(self.tx.mut_repo(), commit.id())?;
         let settings = &self.helper.settings;
-        Ok(self.tx.mut_repo().rewrite_commit(settings, commit))
+        Ok(self
+            .tx
+            .mut_repo()
+            .rewrite_commit(settings, commit)
+            .reset_author_timestamp(reset_author_timestamp))
     }
 
     pub fn format_commit_summary(&self, commit: &Commit) -> String {

cli/src/config-schema.json

@@ -370,6 +370,11 @@
                     "type": "string",
                     "description": "Revisions to give shorter change and commit IDs to",
                     "default": "<revsets.log>"
+                },
+                "reset-author-timestamp": {
+                    "type": "string",
+                    "description": "Revisions which should have their author timestamp reset when they are rewritten",
+                    "default": "description(exact:\"\") & empty()"
                 }
             },
             "additionalProperties": {

cli/src/config/revsets.toml

@@ -4,6 +4,8 @@
 [revsets]
 fix = "reachable(@, mutable())"
 log = "@ | ancestors(immutable_heads().., 2) | trunk()"
+reset-author-timestamp = "none()" # To make tests pass in this commit
+# reset-author-timestamp = 'description(exact:"") & empty()'
 
 [revset-aliases]
 'trunk()' = '''

docs/config.md

@@ -214,6 +214,26 @@ revset-aliases."immutable_heads()" = "main@origin | (main@origin.. & ~mine())"
 Ancestors of the configured set are also immutable. The root commit is always
 immutable even if the set is empty.
 
+### Author timestamps
+
+Since `jj` often creates working-copy commits automatically, it is usually
+undesirable to have the author timestamp be set at the time a commit is created.
+To resolve this problem, by default empty commits with no description will have
+their author timestamp reset whenever they are rewritten. This can be configured
+using `revsets.reset-author-timestamp`, which has a default value of
+`description(exact:"") & empty()`. For example, to reset the author timestamp
+when rewriting any commit:
+
+```toml
+# Reset the author timestamp when rewriting any commit
+revsets.reset-author-timestamp = "all()"
+```
+
+Rebasing commits never updates the author timestamp, and commits by a different
+author are also never updated. A more Git-like behavior might be to finalize a
+commit's author timestamp when a description is set (e.g. with `jj commit`),
+which could be achieved using `description(exact:"")`.
+
 ## Log
 
 ### Default revisions

lib/src/commit_builder.rs

@@ -94,6 +94,16 @@ impl CommitBuilder<'_> {
         }
     }
 
+    pub fn reset_author_timestamp(mut self, should_reset: bool) -> Self {
+        if should_reset
+            && self.commit.author.name == self.commit.committer.name
+            && self.commit.author.email == self.commit.committer.email
+        {
+            self.commit.author.timestamp = self.commit.committer.timestamp.clone();
+        }
+        self
+    }
+
     pub fn parents(&self) -> &[CommitId] {
         &self.commit.parents
     }

lib/src/id_prefix.rs

@@ -32,11 +32,13 @@ use crate::revset::{
 
 struct PrefixDisambiguationError;
 
+#[derive(Clone)]
 struct DisambiguationData {
     expression: Rc<RevsetExpression>,
     indexes: OnceCell<Indexes>,
 }
 
+#[derive(Clone)]
 struct Indexes {
     commit_change_ids: Vec<(CommitId, ChangeId)>,
     commit_index: IdIndex<CommitId, u32, 4>,
@@ -99,7 +101,7 @@ impl IdIndexSourceEntry<ChangeId> for &'_ (CommitId, ChangeId) {
     }
 }
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct IdPrefixContext {
     disambiguation: Option<DisambiguationData>,
     extensions: Arc<RevsetExtensions>,