From af145e8ea516c35c82db4bc6f478532bc7112b4a Mon Sep 17 00:00:00 2001
From: Martin von Zweigbergk <martinvonz@google.com>
Date: Sun, 13 Aug 2023 15:41:53 -0700
Subject: [PATCH] cli: include hint when push is not fast-forward

---
 cli/src/commands/git.rs    |  5 +++++
 cli/tests/test_git_push.rs | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/cli/src/commands/git.rs b/cli/src/commands/git.rs
index c336ce4d5..7d443aaaa 100644
--- a/cli/src/commands/git.rs
+++ b/cli/src/commands/git.rs
@@ -975,6 +975,11 @@ fn cmd_git_push(
     })
     .map_err(|err| match err {
         GitPushError::InternalGitError(err) => map_git_error(err),
+        GitPushError::NotFastForward => user_error_with_hint(
+            "The push conflicts with changes made on the remote (it is not fast-forwardable).",
+            "Try fetching from the remote, then make the branch point to where you want it to be, \
+             and push again.",
+        ),
         _ => user_error(err.to_string()),
     })?;
     git::import_refs(tx.mut_repo(), &git_repo, &command.settings().git_settings())?;
diff --git a/cli/tests/test_git_push.rs b/cli/tests/test_git_push.rs
index 57dc5e00a..c3ab6ccbf 100644
--- a/cli/tests/test_git_push.rs
+++ b/cli/tests/test_git_push.rs
@@ -14,7 +14,7 @@
 
 use std::path::PathBuf;
 
-use crate::common::TestEnvironment;
+use crate::common::{get_stderr_string, get_stdout_string, TestEnvironment};
 
 pub mod common;
 
@@ -169,6 +169,37 @@ fn test_git_push_current_branch_unchanged() {
     "###);
 }
 
+#[test]
+fn test_git_push_not_fast_forward() {
+    let (test_env, workspace_root) = set_up();
+
+    // Move branch1 forward on the remote
+    let origin_path = test_env.env_root().join("origin");
+    test_env.jj_cmd_success(&origin_path, &["new", "branch1", "-m=remote"]);
+    std::fs::write(origin_path.join("remote"), "remote").unwrap();
+    test_env.jj_cmd_success(&origin_path, &["branch", "set", "branch1"]);
+    test_env.jj_cmd_success(&origin_path, &["git", "export"]);
+
+    // Move branch1 forward to another commit locally
+    test_env.jj_cmd_success(&workspace_root, &["new", "branch1", "-m=local"]);
+    std::fs::write(workspace_root.join("local"), "local").unwrap();
+    test_env.jj_cmd_success(&workspace_root, &["branch", "set", "branch1"]);
+
+    // Pushing should fail
+    let assert = test_env
+        .jj_cmd(&workspace_root, &["git", "push"])
+        .assert()
+        .code(1);
+    insta::assert_snapshot!(get_stdout_string(&assert), @r###"
+    Branch changes to push to origin:
+      Move branch branch1 from 45a3aa29e907 to c35839cb8e8c
+    "###);
+    insta::assert_snapshot!(get_stderr_string(&assert), @r###"
+    Error: The push conflicts with changes made on the remote (it is not fast-forwardable).
+    Hint: Try fetching from the remote, then make the branch point to where you want it to be, and push again.
+    "###);
+}
+
 #[test]
 fn test_git_push_multiple() {
     let (test_env, workspace_root) = set_up();