git: add config to disable abandoning of unreachable commits

Some users prefer to have commits not get abandoned when importing
refs. This adds a config option for that.

Closes #2504.

CHANGELOG.md

@@ -15,6 +15,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   create a new workspace with its working-copy commit on top of all the parents,
   as if you had run `jj new r1 r2 r3 ...`.
 
+* You can now set `git.abandon-unreachable-commits = false` to disable the
+  usual behavior where commits that became unreachable in the Git repo are
+  abandoned ([#2504](https://github.com/martinvonz/jj/pull/2504)).
+
 ### Fixed bugs
 
 

cli/src/config-schema.json

@@ -225,6 +225,11 @@
                     "description": "Whether jj creates a local branch with the same name when it imports a remote-tracking branch from git. See https://github.com/martinvonz/jj/blob/main/docs/config.md#automatic-local-branch-creation",
                     "default": true
                 },
+                "abandon-unreachable-commits": {
+                    "type": "boolean",
+                    "description": "Whether jj should abandon commits that became unreachable in Git.",
+                    "default": true
+                },
                 "push-branch-prefix": {
                     "type": "string",
                     "description": "Prefix used when pushing a change ID as a new branch",

docs/config.md

@@ -525,6 +525,20 @@ jj branch delete gh-pages
 jj branch untrack gh-pages@upstream
 ```
 
+### Abandon commits that became unreachable in Git
+
+By default, when `jj` imports refs from Git, it will look for commits that used
+to be [reachable][reachable] but no longer are reachable. Those commits will
+then be abandoned, and any descendant commits will be rebased off of them (as
+usual when commits are abandoned). You can disable this behavior and instead
+leave the Git-unreachable commits in your repo by setting:
+
+```toml
+git.abandon-unreachable-commits = false
+```
+
+[reachable]: https://git-scm.com/docs/gitglossary/#Documentation/gitglossary.txt-aiddefreachableareachable
+
 ### Prefix for generated branches on push
 
 `jj git push --change` generates branch names with a prefix of "push-" by

lib/src/git.rs

@@ -313,7 +313,11 @@ pub fn import_some_refs(
         }
     }
 
-    let abandoned_commits = abandon_unreachable_commits(mut_repo, &changed_remote_refs);
+    let abandoned_commits = if git_settings.abandon_unreachable_commits {
+        abandon_unreachable_commits(mut_repo, &changed_remote_refs)
+    } else {
+        vec![]
+    };
     let stats = GitImportStats { abandoned_commits };
     Ok(stats)
 }

lib/src/settings.rs

@@ -40,12 +40,16 @@ pub struct RepoSettings {
 #[derive(Debug, Clone)]
 pub struct GitSettings {
     pub auto_local_branch: bool,
+    pub abandon_unreachable_commits: bool,
 }
 
 impl GitSettings {
     pub fn from_config(config: &config::Config) -> Self {
         GitSettings {
             auto_local_branch: config.get_bool("git.auto-local-branch").unwrap_or(true),
+            abandon_unreachable_commits: config
+                .get_bool("git.abandon-unreachable-commits")
+                .unwrap_or(true),
         }
     }
 }
@@ -54,6 +58,7 @@ impl Default for GitSettings {
     fn default() -> Self {
         GitSettings {
             auto_local_branch: true,
+            abandon_unreachable_commits: true,
         }
     }
 }

lib/tests/test_git.rs

@@ -708,6 +708,7 @@ fn test_import_refs_reimport_with_moved_untracked_remote_ref() {
     let settings = testutils::user_settings();
     let git_settings = GitSettings {
         auto_local_branch: false,
+        ..Default::default()
     };
     let test_workspace = TestRepo::init_with_backend(TestRepoBackend::Git);
     let repo = &test_workspace.repo;
@@ -819,6 +820,37 @@ fn test_import_refs_reimport_all_from_root_removed() {
     assert!(!tx.mut_repo().view().heads().contains(&jj_id(&commit)));
 }
 
+#[test]
+fn test_import_refs_reimport_abandoning_disabled() {
+    // Test that we don't abandoned unreachable commits if configured not to
+    let settings = testutils::user_settings();
+    let git_settings = GitSettings {
+        abandon_unreachable_commits: false,
+        ..Default::default()
+    };
+    let test_repo = TestRepo::init_with_backend(TestRepoBackend::Git);
+    let repo = &test_repo.repo;
+    let git_repo = get_git_repo(repo);
+
+    let commit1 = empty_git_commit(&git_repo, "refs/heads/main", &[]);
+    let commit2 = empty_git_commit(&git_repo, "refs/heads/delete-me", &[&commit1]);
+    let mut tx = repo.start_transaction(&settings, "test");
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    // Test the setup
+    assert!(tx.mut_repo().view().heads().contains(&jj_id(&commit2)));
+
+    // Remove the `delete-me` branch and re-import
+    git_repo
+        .find_reference("refs/heads/delete-me")
+        .unwrap()
+        .delete()
+        .unwrap();
+    git::import_refs(tx.mut_repo(), &git_repo, &git_settings).unwrap();
+    tx.mut_repo().rebase_descendants(&settings).unwrap();
+    assert!(tx.mut_repo().view().heads().contains(&jj_id(&commit2)));
+}
+
 #[test]
 fn test_import_refs_reimport_conflicted_remote_branch() {
     let settings = testutils::user_settings();
@@ -1426,6 +1458,7 @@ fn test_import_export_non_tracking_branch() {
     let test_data = GitRepoData::create();
     let mut git_settings = GitSettings {
         auto_local_branch: false,
+        ..Default::default()
     };
     let git_repo = test_data.git_repo;
     let commit_main_t0 = empty_git_commit(&git_repo, "refs/remotes/origin/main", &[]);