workspace: don't lose sparsed-away paths when recovering workspace

When an operation is missing and we recover the workspace, we create a
new working-copy commit on top of the desired working-copy commit (per
the available head operation). We then reset the working copy to an
empty tree because it shouldn't really matter much which commit we
reset to. However, when the workspace is sparse, it does matter, as
the test case from the previous patch shows. This patch fixes it by
replacing the `reset_to_empty()` method by a new `recover(&Commit)`,
which effectively resets to the empty tree and then resets to the
commit. That way, any subsequent snapshotting will result keep the
paths from that tree for paths outside the sparse patterns.

cli/examples/custom-working-copy/main.rs

@@ -241,8 +241,8 @@ impl LockedWorkingCopy for LockedConflictsWorkingCopy {
         self.inner.reset(commit)
     }
 
-    fn reset_to_empty(&mut self) -> Result<(), ResetError> {
-        self.inner.reset_to_empty()
+    fn recover(&mut self, commit: &Commit) -> Result<(), ResetError> {
+        self.inner.recover(commit)
     }
 
     fn sparse_patterns(&self) -> Result<&[RepoPathBuf], WorkingCopyStateError> {

cli/src/commands/workspace.rs

@@ -324,7 +324,7 @@ fn create_and_check_out_recovery_commit(
     mut_repo.set_wc_commit(workspace_id, new_commit.id().clone())?;
     let repo = tx.commit("recovery commit");
 
-    locked_workspace.locked_wc().reset_to_empty()?;
+    locked_workspace.locked_wc().recover(&new_commit)?;
     locked_workspace.finish(repo.op_id().clone())?;
 
     writeln!(

cli/tests/test_workspaces.rs

@@ -494,7 +494,7 @@ fn test_workspaces_current_op_discarded_by_other() {
     insta::assert_snapshot!(stdout, @"");
 
     insta::assert_snapshot!(get_log_output(&test_env, &main_path), @r###"
-    ◉  8fd911b4e595 secondary@
+    ◉  b93a924213f3 secondary@
     ◉  ec4904a30161
     │ @  74769415363f default@
     ├─╯
@@ -511,14 +511,12 @@ fn test_workspaces_current_op_discarded_by_other() {
     "###);
     let (stdout, stderr) = test_env.jj_cmd_ok(&secondary_path, &["st"]);
     insta::assert_snapshot!(stderr, @"");
-    // TODO: The file outside the sparse patterns should still be there
     insta::assert_snapshot!(stdout, @r###"
     Working copy changes:
     A added
     D deleted
     M modified
-    D sparse
-    Working copy : kmkuslsw 8fd911b4 (no description set)
+    Working copy : kmkuslsw b93a9242 (no description set)
     Parent commit: rzvqmyuk ec4904a3 (empty) (no description set)
     "###);
     // The modified file should have the same contents it had before (not reset to
@@ -530,7 +528,7 @@ fn test_workspaces_current_op_discarded_by_other() {
     let (stdout, stderr) = test_env.jj_cmd_ok(&secondary_path, &["obslog"]);
     insta::assert_snapshot!(stderr, @"");
     insta::assert_snapshot!(stdout, @r###"
-    @  kmkuslsw test.user@example.com 2001-02-03 04:05:18.000 +07:00 secondary@ 8fd911b4
+    @  kmkuslsw test.user@example.com 2001-02-03 04:05:18.000 +07:00 secondary@ b93a9242
     │  (no description set)
     ◉  kmkuslsw hidden test.user@example.com 2001-02-03 04:05:18.000 +07:00 30ee0d1f
        (empty) (no description set)

lib/src/local_working_copy.rs

@@ -1457,9 +1457,10 @@ impl TreeState {
         Ok(())
     }
 
-    pub fn reset_to_empty(&mut self) {
+    pub async fn recover(&mut self, new_tree: &MergedTree) -> Result<(), ResetError> {
         self.file_states.clear();
         self.tree_id = self.store.empty_merged_tree_id();
+        self.reset(new_tree).await
     }
 }
 
@@ -1778,14 +1779,16 @@ impl LockedWorkingCopy for LockedLocalWorkingCopy {
         Ok(())
     }
 
-    fn reset_to_empty(&mut self) -> Result<(), ResetError> {
+    fn recover(&mut self, commit: &Commit) -> Result<(), ResetError> {
+        let new_tree = commit.tree()?;
         self.wc
             .tree_state_mut()
             .map_err(|err| ResetError::Other {
                 message: "Failed to read the working copy state".to_string(),
                 err: err.into(),
             })?
-            .reset_to_empty();
+            .recover(&new_tree)
+            .block_on()?;
         self.tree_state_dirty = true;
         Ok(())
     }

lib/src/working_copy.rs

@@ -107,8 +107,9 @@ pub trait LockedWorkingCopy {
     /// Update to another commit without touching the files in the working copy.
     fn reset(&mut self, commit: &Commit) -> Result<(), ResetError>;
 
-    /// Update to the empty tree without touching the files in the working copy.
-    fn reset_to_empty(&mut self) -> Result<(), ResetError>;
+    /// Update to another commit without touching the files in the working copy,
+    /// without assuming that the previous tree exists.
+    fn recover(&mut self, commit: &Commit) -> Result<(), ResetError>;
 
     /// See `WorkingCopy::sparse_patterns()`
     fn sparse_patterns(&self) -> Result<&[RepoPathBuf], WorkingCopyStateError>;