cli: let the user know if there's been a concurrent modification (#111)

src/commands.rs

@@ -47,7 +47,7 @@ use jujutsu_lib::op_heads_store::OpHeadsStore;
 use jujutsu_lib::op_store::{OpStore, OpStoreError, OperationId, RefTarget, WorkspaceId};
 use jujutsu_lib::operation::Operation;
 use jujutsu_lib::refs::{classify_branch_push_action, BranchPushAction};
-use jujutsu_lib::repo::{MutableRepo, ReadonlyRepo, RepoRef};
+use jujutsu_lib::repo::{MutableRepo, ReadonlyRepo, RepoAtHead, RepoRef};
 use jujutsu_lib::repo_path::RepoPath;
 use jujutsu_lib::revset::{RevsetError, RevsetExpression, RevsetParseError};
 use jujutsu_lib::revset_graph_iterator::RevsetGraphEdgeType;
@@ -184,7 +184,7 @@ impl<'help> CommandHelper<'help> {
         &self.args
     }
 
-    fn workspace_helper(&self, ui: &Ui) -> Result<WorkspaceCommandHelper, CommandError> {
+    fn workspace_helper(&self, ui: &mut Ui) -> Result<WorkspaceCommandHelper, CommandError> {
         let wc_path_str = self.args.repository.as_deref().unwrap_or(".");
         let wc_path = ui.cwd().join(wc_path_str);
         let workspace = match Workspace::load(ui.settings(), wc_path) {
@@ -211,7 +211,16 @@ jj init --git-repo=.";
         let repo_loader = workspace.repo_loader();
         let op_str = &self.args.at_operation;
         let repo = if op_str == "@" {
-            repo_loader.load_at_head().resolve()
+            match repo_loader.load_at_head() {
+                RepoAtHead::Single(repo) => repo,
+                RepoAtHead::Unresolved(unresolved) => {
+                    writeln!(
+                        ui,
+                        "Concurrent modification detected, resolving automatically.",
+                    )?;
+                    unresolved.resolve()
+                }
+            }
         } else {
             let op = resolve_single_op_from_store(
                 repo_loader.op_store(),

tests/test_concurrent_operations.rs

@@ -0,0 +1,56 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use jujutsu::testutils::{get_stdout_string, TestEnvironment};
+
+#[test]
+fn test_concurrent_operations() {
+    let test_env = TestEnvironment::default();
+    test_env
+        .jj_cmd(test_env.env_root(), &["init", "repo", "--git"])
+        .assert()
+        .success();
+    let repo_path = test_env.env_root().join("repo");
+
+    let assert = test_env
+        .jj_cmd(&repo_path, &["op", "log"])
+        .assert()
+        .success();
+    let op_id_hex = get_stdout_string(&assert)[2..14].to_string();
+
+    test_env
+        .jj_cmd(&repo_path, &["describe", "-m", "message 1"])
+        .assert()
+        .success();
+    test_env
+        .jj_cmd(
+            &repo_path,
+            &["describe", "-m", "message 2", "--at-op", &op_id_hex],
+        )
+        .assert()
+        .success();
+
+    // We should be informed about the concurrent modification
+    let assert = test_env
+        .jj_cmd(&repo_path, &["log", "-T", "description"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    Concurrent modification detected, resolving automatically.
+    o message 2
+    | @ message 1
+    |/  
+    o 
+    "###);
+}