fix: prevent squash on out-of-sync branches

cmd/av/helpers.go

@@ -165,3 +165,47 @@ func branchNameArgs(
 	branches, _ := allBranches(cmd.Context())
 	return branches, cobra.ShellCompDirectiveNoSpace
 }
+
+// validateBranchSync checks if the current branch is in sync with its parent branch.
+// Only enforces this check for non-trunk branches when we have a stored parent head.
+// This validation prevents commands from operating on branches that are out of sync
+// with their parent, which could lead to unexpected behavior or loss of commits.
+func validateBranchSync(ctx context.Context, repo *git.Repo, branch meta.Branch) error {
+	if branch.Parent.Trunk {
+		// If there's no parent or the branch is a trunk branch, we don't need to check sync
+		return nil
+	}
+
+	// Get the current head of the parent branch
+	currentParentHead, err := repo.RevParse(ctx, &git.RevParse{Rev: branch.Parent.Name})
+	if err != nil {
+		return errors.Errorf(
+			"cannot get current head of parent branch %s: %v",
+			branch.Parent.Name,
+			err,
+		)
+	}
+
+	// If the current parent head differs from the stored parent head,
+	// then the parent has moved and we need to sync before proceeding
+	if currentParentHead != branch.Parent.Head {
+		return errors.Errorf(
+			"branch is not in sync with parent branch %s, please run 'av sync' first",
+			branch.Parent.Name,
+		)
+	}
+
+	// Additional check: ensure the child branch is actually based on the stored parent head
+	mergeBase, err := repo.MergeBase(ctx, branch.Parent.Head, branch.Name)
+	if err != nil {
+		return errors.Errorf("cannot find merge base with parent branch: %v", err)
+	}
+
+	if mergeBase != branch.Parent.Head {
+		return errors.Errorf(
+			"branch is not in sync with parent branch %s, please run 'av sync' first",
+			branch.Parent.Name,
+		)
+	}
+	return nil
+}

cmd/av/squash.go

@@ -70,8 +70,23 @@ func runSquash(ctx context.Context, repo *git.Repo, db meta.DB) error {
 		return errors.New("this branch has already been merged, squashing is not allowed")
 	}
 
+	// Check if branch is in sync with parent before squashing
+	if err := validateBranchSync(ctx, repo, branch); err != nil {
+		return err
+	}
+
+	// Use the parent's head commit hash if available, otherwise fall back to branch name
+	// Since we've already validated sync, we can safely use the stored parent head
+	var parentRef string
+	if branch.Parent.Head != "" {
+		parentRef = branch.Parent.Head
+	} else {
+		// Fallback to branch name when no head commit is stored
+		parentRef = branch.Parent.Name
+	}
+
 	commitIDs, err := repo.RevList(ctx, git.RevListOpts{
-		Specifiers: []string{currentBranchName, "^" + branch.Parent.Name},
+		Specifiers: []string{currentBranchName, "^" + parentRef},
 		Reverse:    true,
 	})
 	if err != nil {

e2e_tests/squash_test.go

@@ -1,6 +1,7 @@
 package e2e_tests
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/aviator-co/av/internal/git/gittest"
@@ -321,3 +322,108 @@ func TestSquashStackedBranchBDoesNotAffectBranchA(t *testing.T) {
 	require.FileExists(t, repo.RepoDir+"/b2.txt")
 	require.FileExists(t, repo.RepoDir+"/b3.txt")
 }
+
+func TestSquashWithModifiedParentBranch(t *testing.T) {
+	repo := gittest.NewTempRepo(t)
+	Chdir(t, repo.RepoDir)
+	repo.Git(t, "fetch")
+
+	// Create parent branch
+	RequireAv(t, "branch", "parent-branch")
+	repo.CreateFile(t, "parent1.txt", "parent content 1")
+	repo.AddFile(t, "parent1.txt")
+	RequireAv(t, "commit", "-m", "parent commit 1")
+
+	repo.CreateFile(t, "parent2.txt", "parent content 2")
+	repo.AddFile(t, "parent2.txt")
+	RequireAv(t, "commit", "-m", "parent commit 2")
+
+	// Create child branch from parent
+	RequireAv(t, "branch", "child-branch")
+	repo.CreateFile(t, "child1.txt", "child content 1")
+	repo.AddFile(t, "child1.txt")
+	RequireAv(t, "commit", "-m", "child commit 1")
+
+	repo.CreateFile(t, "child2.txt", "child content 2")
+	repo.AddFile(t, "child2.txt")
+	RequireAv(t, "commit", "-m", "child commit 2")
+
+	// Simulate external changes to parent branch (force push scenario)
+	repo.Git(t, "checkout", "parent-branch")
+	repo.Git(t, "reset", "--hard", "HEAD~1") // Reset to parent commit 1
+	repo.CreateFile(t, "parent_new.txt", "new parent content")
+	repo.AddFile(t, "parent_new.txt")
+	repo.Git(t, "commit", "-m", "parent new commit")
+
+	// Switch back to child branch and attempt squash
+	RequireAv(t, "switch", "child-branch")
+
+	// Squash should now fail because branch is not in sync with parent
+	output := Av(t, "squash")
+	require.NotEqual(t, 0, output.ExitCode)
+	require.Contains(t, output.Stderr, "branch is not in sync with parent branch")
+	require.Contains(t, output.Stderr, "please run 'av sync' first")
+
+	// Verify child files are still present (no partial squash happened)
+	require.FileExists(t, repo.RepoDir+"/child1.txt")
+	require.FileExists(t, repo.RepoDir+"/child2.txt")
+
+	// Verify we still have the expected commits (no squashing occurred)
+	commitCount := strings.TrimSpace(
+		repo.Git(t, "rev-list", "--count", "child-branch", "^parent-branch"),
+	)
+	// After the parent branch was force-pushed (reset and new commits), the child branch
+	// now has more commits relative to the new parent head, which is expected behavior
+	require.NotEqual(
+		t,
+		"1",
+		commitCount,
+		"Should have more than 1 commit, confirming squash was prevented",
+	)
+
+	// The key achievement: squash was prevented, avoiding the bug where we would
+	// squash into a parent commit due to the force-pushed parent branch
+}
+
+func TestSquashFailsWhenBranchOutOfSync(t *testing.T) {
+	repo := gittest.NewTempRepo(t)
+	Chdir(t, repo.RepoDir)
+	repo.Git(t, "fetch")
+
+	// Create parent branch
+	RequireAv(t, "branch", "parent-branch")
+	repo.CreateFile(t, "parent1.txt", "parent content 1")
+	repo.AddFile(t, "parent1.txt")
+	RequireAv(t, "commit", "-m", "parent commit 1")
+
+	// Create child branch from parent
+	RequireAv(t, "branch", "child-branch")
+	repo.CreateFile(t, "child1.txt", "child content 1")
+	repo.AddFile(t, "child1.txt")
+	RequireAv(t, "commit", "-m", "child commit 1")
+
+	repo.CreateFile(t, "child2.txt", "child content 2")
+	repo.AddFile(t, "child2.txt")
+	RequireAv(t, "commit", "-m", "child commit 2")
+
+	// Modify parent branch externally to create out-of-sync condition
+	repo.Git(t, "checkout", "parent-branch")
+	repo.CreateFile(t, "parent2.txt", "parent content 2")
+	repo.AddFile(t, "parent2.txt")
+	repo.Git(t, "commit", "-m", "parent commit 2")
+
+	// Switch back to child branch
+	RequireAv(t, "switch", "child-branch")
+
+	// Squash should fail because branch is not in sync with parent
+	output := Av(t, "squash")
+	require.NotEqual(t, 0, output.ExitCode)
+	require.Contains(t, output.Stderr, "branch is not in sync with parent branch parent-branch")
+	require.Contains(t, output.Stderr, "please run 'av sync' first")
+
+	// Verify no changes were made (both child commits should still exist)
+	commitCount := strings.TrimSpace(
+		repo.Git(t, "rev-list", "--count", "child-branch", "^parent-branch"),
+	)
+	require.Equal(t, "2", commitCount)
+}