cli: prevent pushing commits with conflicts

It rarely makes sense to push commits with conflicts to a remote Git
repo (which is the only kind of remote we support so far), so let's
just error out instead of pushing a commit that others pulling from
the remote probably can't make sense of.

I've only added a simple test for the error case for now. `libgit2`
doesn't support pushing to a local repo, so it's harder to test the
success case. I suppose we'll have to have the regular `git` binary
running local servers in test eventually.

Closes #60.

src/commands.rs

@@ -4365,9 +4365,11 @@ fn cmd_git_push(
     }
 
     let mut ref_updates = vec![];
+    let mut new_heads = vec![];
     for (branch_name, update) in branch_updates {
         let qualified_name = format!("refs/heads/{}", branch_name);
         if let Some(new_target) = update.new_target {
+            new_heads.push(new_target.clone());
             let force = match update.old_target {
                 None => false,
                 Some(old_target) => !repo.index().is_ancestor(&old_target, &new_target),
@@ -4386,6 +4388,24 @@ fn cmd_git_push(
         }
     }
 
+    // Check if there are conflicts in any commits we're about to push that haven't
+    // already been pushed.
+    let mut old_heads = vec![];
+    for branch_target in repo.view().branches().values() {
+        if let Some(old_head) = branch_target.remote_targets.get(remote_name) {
+            old_heads.extend(old_head.adds());
+        }
+    }
+    for index_entry in repo.index().walk_revs(&new_heads, &old_heads) {
+        let commit = repo.store().get_commit(&index_entry.commit_id())?;
+        if commit.tree().has_conflict() {
+            return Err(UserError(format!(
+                "Won't push commit {} since it has conflicts",
+                short_commit_hash(commit.id())
+            )));
+        }
+    }
+
     let git_repo = get_git_repo(repo.store())?;
     git::push_updates(&git_repo, remote_name, &ref_updates)
         .map_err(|err| CommandError::UserError(err.to_string()))?;

tests/test_git_push.rs

@@ -0,0 +1,70 @@
+// 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_git_push() {
+    let test_env = TestEnvironment::default();
+    let git_repo_path = test_env.env_root().join("git-repo");
+    git2::Repository::init(&git_repo_path).unwrap();
+
+    test_env
+        .jj_cmd(
+            test_env.env_root(),
+            &["git", "clone", "git-repo", "jj-repo"],
+        )
+        .assert()
+        .success();
+    let workspace_root = test_env.env_root().join("jj-repo");
+
+    // No branches to push yet
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["git", "push"])
+        .assert()
+        .success();
+    insta::assert_snapshot!(get_stdout_string(&assert), @"Nothing changed.
+");
+
+    // Try pushing a conflict
+    std::fs::write(workspace_root.join("file"), "first").unwrap();
+    test_env
+        .jj_cmd(&workspace_root, &["close", "-m", "first"])
+        .assert()
+        .success();
+    std::fs::write(workspace_root.join("file"), "second").unwrap();
+    test_env
+        .jj_cmd(&workspace_root, &["close", "-m", "second"])
+        .assert()
+        .success();
+    std::fs::write(workspace_root.join("file"), "third").unwrap();
+    test_env
+        .jj_cmd(&workspace_root, &["rebase", "-d", "@--"])
+        .assert()
+        .success();
+    test_env
+        .jj_cmd(&workspace_root, &["branch", "my-branch"])
+        .assert()
+        .success();
+    test_env
+        .jj_cmd(&workspace_root, &["close", "-m", "third"])
+        .assert()
+        .success();
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["git", "push"])
+        .assert()
+        .failure();
+    insta::assert_snapshot!(get_stdout_string(&assert), @"Error: Won't push commit 56e09a8ca383 since it has conflicts
+");
+}