Allow simultaneous rebasing of duplicate commits

Fixes #694

CHANGELOG.md

@@ -67,6 +67,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   the jj repo from a Git repo. They will now be considered as non-existent if
   referenced explicitly instead of crashing.
 
+* `jj duplicate` followed by `jj rebase` of a tree containing both the original
+  and duplicate commit no longer crashes.
+
 ### Contributors
 
 Thanks to the people who made this release happen!

lib/src/store.rs

@@ -16,9 +16,9 @@ use std::collections::HashMap;
 use std::io::Read;
 use std::sync::{Arc, RwLock};
 
-use crate::backend;
 use crate::backend::{
-    Backend, BackendResult, CommitId, Conflict, ConflictId, FileId, SymlinkId, TreeId,
+    self, Backend, BackendError, BackendResult, CommitId, Conflict, ConflictId, FileId, SymlinkId,
+    TreeId,
 };
 use crate::commit::Commit;
 use crate::repo_path::RepoPath;
@@ -81,11 +81,24 @@ impl Store {
         write_locked_cache.insert(id.clone(), data.clone());
         Ok(data)
     }
-
     pub fn write_commit(self: &Arc<Self>, commit: backend::Commit) -> BackendResult<Commit> {
         assert!(!commit.parents.is_empty());
-        let commit_id = self.backend.write_commit(&commit)?;
-        let data = Arc::new(commit);
+        let mut mut_commit = commit;
+        let commit_id = loop {
+            // It's possible that another commit with the same commit id (but different
+            // change id) already exists. Fundge the timestamp until this is no
+            // longer the case.
+            let maybe_commit_id = self.backend.write_commit(&mut_commit);
+            match maybe_commit_id {
+                Err(BackendError::AlreadyHaveCommitWithId { .. }) => {
+                    // This is measured in milliseconds, and Git can't handle durations
+                    // less than 1 second.
+                    mut_commit.committer.timestamp.timestamp.0 -= 1000
+                }
+                _ => break maybe_commit_id?,
+            }
+        };
+        let data = Arc::new(mut_commit);
         {
             let mut write_locked_cache = self.commit_cache.write().unwrap();
             write_locked_cache.insert(commit_id.clone(), data.clone());

tests/test_duplicate_command.rs

@@ -127,10 +127,21 @@ fn test_rebase_duplicates() {
     o 000000000000   (no description set) @ 1970-01-01 00:00:00.000 +00:00
     "###);
 
-    // This is the bug: this should succeed
-    let stderr = test_env.jj_cmd_failure(&repo_path, &["rebase", "-s", "a", "-d", "a-"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Error: Unexpected error from backend: Git commit '29bd36b60e6002f04e03c5077f989c93e3c910e1' already exists with different associated non-Git meta-data
+    let stdout = test_env.jj_cmd_success(&repo_path, &["rebase", "-s", "a", "-d", "a-"]);
+    insta::assert_snapshot!(stdout, @r###"
+    Rebased 4 commits
+    Working copy now at: 29bd36b60e60 b
+    "###);
+    // One of the duplicate commit's timestamps was changed a little to make it have
+    // a different commit id from the other.
+    insta::assert_snapshot!(get_log_output_with_ts(&test_env, &repo_path), @r###"
+    o b43fe7354758   b @ 2001-02-03 04:05:14.000 +07:00
+    | o 08beb14c3ead   b @ 2001-02-03 04:05:15.000 +07:00
+    |/  
+    | @ 29bd36b60e60   b @ 2001-02-03 04:05:16.000 +07:00
+    |/  
+    o 2f6dc5a1ffc2   a @ 2001-02-03 04:05:16.000 +07:00
+    o 000000000000   (no description set) @ 1970-01-01 00:00:00.000 +00:00
     "###);
 }
 fn get_log_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
@@ -144,7 +155,6 @@ fn get_log_output(test_env: &TestEnvironment, repo_path: &Path) -> String {
     )
 }
 
-// The timestamp is relevant for the bugfix
 fn get_log_output_with_ts(test_env: &TestEnvironment, repo_path: &Path) -> String {
     test_env.jj_cmd_success(
         repo_path,