git fetch: do a git export of deleted branches before fetch

lib/src/git.rs

@@ -507,6 +507,47 @@ pub fn fetch(
     callbacks: RemoteCallbacks<'_>,
     git_settings: &GitSettings,
 ) -> Result<Option<String>, GitFetchError> {
+    // In non-colocated repositories, it's possible that `jj branch forget` was run
+    // at some point and no `jj git export` happened since.
+    //
+    // This would mean that remote-tracking branches, forgotten in the jj repo,
+    // still exist in the git repo. If the branches didn't move on the remote, and
+    // we fetched them, jj would think that they are unmodified and wouldn't
+    // resurrect them.
+    //
+    // Export will delete the remote-tracking branches in the git repo, so it's
+    // possible to fetch them again.
+    //
+    // For more details, see the `test_branch_forget_fetched_branch` test, and PRs
+    // #1714 and #1771
+    //
+    // Apart from `jj branch forget`, jj doesn't provide commands to manipulate
+    // remote-tracking branches, and local git branches don't affect fetch
+    // behaviors. So, it's unnecessary to export anything else.
+    //
+    // TODO: Create a command the user can use to reset jj's
+    // branch state to the git repo's state. In this case, `jj branch forget`
+    // doesn't work as it tries to delete the latter. One possible name is `jj
+    // git import --reset BRANCH`.
+    // TODO: Once the command described above exists, it should be mentioned in `jj
+    // help branch forget`.
+    let nonempty_branches = mut_repo
+        .base_repo()
+        .view()
+        .branches()
+        .iter()
+        .filter_map(|(branch, target)| target.local_target.as_ref().map(|_| branch.to_owned()))
+        .collect_vec();
+    // TODO: Inform the user if the export failed? In most cases, export is not
+    // essential for fetch to work.
+    let _ = export_some_refs(mut_repo, git_repo, |ref_name| {
+        matches!(
+            ref_name,
+            RefName::RemoteBranch { branch, remote:_ }
+               if !nonempty_branches.contains(branch)
+        )
+    });
+
     let mut remote =
         git_repo
             .find_remote(remote_name)

tests/test_branch_command.rs

@@ -238,9 +238,8 @@ fn test_branch_forget_export() {
     insta::assert_snapshot!(stdout, @"");
     let stdout = test_env.jj_cmd_success(&repo_path, &["branch", "forget", "foo"]);
     insta::assert_snapshot!(stdout, @"");
-    // Forgetting a branch does not delete its local-git tracking branch. This is
-    // the opposite of what happens to remote-tracking branches.
-    // TODO: Consider allowing forgetting local-git tracking branches as an option
+    // Forgetting a branch does not delete its local-git tracking branch. The
+    // git-tracking branch is kept.
     let stdout = test_env.jj_cmd_success(&repo_path, &["branch", "list"]);
     insta::assert_snapshot!(stdout, @r###"
     foo (forgotten)
@@ -346,17 +345,14 @@ fn test_branch_forget_fetched_branch() {
     "###);
 
     // TEST 2: No export/import (otherwise the same as test 1)
-    // Short-term TODO: While it looks like the bug above is fixed, correct behavior
-    // now actually depends on export/import.
     test_env.jj_cmd_success(&repo_path, &["branch", "forget", "feature1"]);
     insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @"");
-    // Short-term TODO: Fix this BUG. It should be possible to fetch `feature1`
-    // again.
+    // Fetch works even without the export-import
     let stdout = test_env.jj_cmd_success(&repo_path, &["git", "fetch", "--remote=origin"]);
-    insta::assert_snapshot!(stdout, @r###"
-    Nothing changed.
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
+    feature1: 9f01a0e04879 message
     "###);
-    insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @"");
 
     // TEST 3: fetch branch that was moved & forgotten
 
@@ -371,18 +367,14 @@ fn test_branch_forget_fetched_branch() {
             &[&git_repo.find_commit(first_git_repo_commit).unwrap()],
         )
         .unwrap();
-    let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "forget", "feature1"]);
-    insta::assert_snapshot!(stderr, @r###"
-    Error: No such branch: feature1
-    "###);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["branch", "forget", "feature1"]);
+    insta::assert_snapshot!(stdout, @"");
 
-    // BUG: fetching a moved branch creates a move-deletion conflict
+    // Fetching a moved branch does not create a conflict
     let stdout = test_env.jj_cmd_success(&repo_path, &["git", "fetch", "--remote=origin"]);
     insta::assert_snapshot!(stdout, @"");
     insta::assert_snapshot!(get_branch_output(&test_env, &repo_path), @r###"
-    feature1 (conflicted):
-      - 9f01a0e04879 message
-      + 38aefb173976 another message
+    feature1: 38aefb173976 another message
     "###);
 }