Fix move-branch failing when base segment has no ref name

crates/but-workspace/src/branch/move_branch.rs

@@ -375,20 +375,10 @@ fn get_disconnect_parameters<'ws, 'meta, M: RefMetadata>(
 
     let parents_to_disconnect = if let Some(stack_base_segment) = stack_base_segment {
         // Base segment is part of the source stack.
-        let base_segment_ref_name = stack_base_segment
-            .ref_name()
-            .context("Base segment doesn't have a ref name.")?;
-        let reference_selector = editor.select_reference(base_segment_ref_name)?;
-        let selectors = SomeSelectors::new(vec![reference_selector])?;
-        SelectorSet::Some(selectors)
+        select_segment(editor, stack_base_segment)?
     } else if let Some(graph_base_segment) = graph_base_segment {
-        // Base segment is outside of workspace (probably target branch).
-        let ref_name = graph_base_segment
-            .ref_name()
-            .context("Graph base segment doesn't have a ref name.")?;
-        let reference_selector = editor.select_reference(ref_name)?;
-        let selectors = SomeSelectors::new(vec![reference_selector])?;
-        SelectorSet::Some(selectors)
+        // Base segment is outside the stack (e.g. the target branch, or an unnamed fork-point segment).
+        select_segment(editor, graph_base_segment)?
     } else if subject_segment.base_segment_id.is_some() {
         // Base segment could not be found, but there is an ID defined. Error out.
         bail!(
@@ -437,3 +427,47 @@ fn get_disconnect_parameters<'ws, 'meta, M: RefMetadata>(
 
     Ok((delimiter, children_to_disconnect, parents_to_disconnect))
 }
+
+/// Select a segment for use as a disconnect point.
+///
+/// Prefers the ref name when available, otherwise falls back to the tip commit.
+/// Fails if the segment has neither (empty and unnamed).
+fn select_segment<M: RefMetadata>(
+    editor: &Editor<'_, '_, M>,
+    segment: &impl SegmentLike,
+) -> anyhow::Result<SelectorSet> {
+    let selector = if let Some(ref_name) = segment.ref_name() {
+        editor.select_reference(ref_name)?
+    } else if let Some(tip) = segment.tip() {
+        editor.select_commit(tip)?
+    } else {
+        bail!("Base segment has neither a ref name nor any commits.");
+    };
+    let selectors = SomeSelectors::new(vec![selector])?;
+    Ok(SelectorSet::Some(selectors))
+}
+
+/// Common interface for selecting a graph segment, abstracting over
+/// [`StackSegment`] (workspace projection) and [`but_graph::Segment`] (raw graph).
+trait SegmentLike {
+    fn ref_name(&self) -> Option<&gix::refs::FullNameRef>;
+    fn tip(&self) -> Option<gix::ObjectId>;
+}
+
+impl SegmentLike for StackSegment {
+    fn ref_name(&self) -> Option<&gix::refs::FullNameRef> {
+        self.ref_name()
+    }
+    fn tip(&self) -> Option<gix::ObjectId> {
+        self.tip()
+    }
+}
+
+impl SegmentLike for but_graph::Segment {
+    fn ref_name(&self) -> Option<&gix::refs::FullNameRef> {
+        self.ref_name()
+    }
+    fn tip(&self) -> Option<gix::ObjectId> {
+        self.tip()
+    }
+}

e2e/playwright/scripts/fetch-in-clone.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+echo "GIT CONFIG $GIT_CONFIG_GLOBAL"
+echo "DATA DIR $E2E_TEST_APP_DATA_DIR"
+echo "DIRECTORY: $1"
+
+# Fetch updates from the remote.
+pushd "$1"
+  git fetch origin
+popd

e2e/playwright/tests/branches.spec.ts

@@ -643,3 +643,41 @@ test("should be able to unapply a stack", async ({ page, context }, testInfo) =>
 	branchHeaders = getByTestId(page, "branch-header").filter({ hasText: "branch1" });
 	await expect(branchHeaders).toHaveCount(0);
 });
+
+test("should be able to move a branch when origin/master has advanced past the fork point", async ({
+	page,
+	context,
+}, testInfo) => {
+	const workdir = testInfo.outputPath("workdir");
+	const configdir = testInfo.outputPath("config");
+	gitbutler = await startGitButler(workdir, configdir, context);
+
+	// Set up project with branches (branch1 and branch3 fork from initial master).
+	await gitbutler.runScript("project-with-remote-branches.sh");
+	// Apply branch1 and branch3 as two separate stacks.
+	await gitbutler.runScript("apply-upstream-branch.sh", ["branch1", "local-clone"]);
+	await gitbutler.runScript("apply-upstream-branch.sh", ["branch3", "local-clone"]);
+
+	// Advance remote master past the fork point. This creates a commit on
+	// origin/master that is ahead of where branch1 and branch3 diverge.
+	await gitbutler.runScript("project-with-remote-branches__add-commit-to-base.sh");
+	// Fetch so that origin/master advances in the local clone, making the
+	// old fork point an unnamed segment in the graph.
+	await gitbutler.runScript("fetch-in-clone.sh", ["local-clone"]);
+
+	// Move branch3 on top of branch1. This should succeed even though the
+	// base segment at the old fork point has no ref name.
+	await gitbutler.runScript("move-branch.sh", ["branch3", "branch1", "local-clone"]);
+
+	await page.goto("/");
+
+	// Should load the workspace
+	await waitForTestId(page, "workspace-view");
+
+	// There should be one stack with both branches
+	const stacks = getByTestId(page, "stack");
+	await expect(stacks).toHaveCount(1);
+
+	const branchHeaders = getByTestId(page, "branch-header");
+	await expect(branchHeaders).toHaveCount(2);
+});