Fix nil-pointer panic in checkpoint parent-load error path

internal/planner/checkpoint.go

@@ -51,7 +51,7 @@ func FirstParentChainStoppingAt(store storer.EncodedObjectStorer, tip plumbing.H
 		}
 		commit, err = object.GetCommit(store, parent)
 		if err != nil {
-			return nil, fmt.Errorf("load parent commit %s: %w", commit.ParentHashes[0], err)
+			return nil, fmt.Errorf("load parent commit %s: %w", parent, err)
 		}
 	}
 	// Reverse in-place to get root-to-tip order.

internal/planner/checkpoint_test.go

@@ -0,0 +1,30 @@
+package planner
+
+import (
+	"strings"
+	"testing"
+
+	git "github.com/go-git/go-git/v6"
+	"github.com/go-git/go-git/v6/plumbing"
+	"github.com/go-git/go-git/v6/storage/memory"
+)
+
+// When a first-parent is absent from the store, the error path must not
+// dereference the (nil) commit it just failed to load, and must name the
+// parent hash it could not find.
+func TestFirstParentChainStoppingAtMissingParentErrors(t *testing.T) {
+	repo, err := git.Init(memory.NewStorage(), nil)
+	if err != nil {
+		t.Fatalf("init repo: %v", err)
+	}
+	missingParent := plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	tip := seedCommit(t, repo, []plumbing.Hash{missingParent})
+
+	_, err = FirstParentChainStoppingAt(repo.Storer, tip, map[plumbing.Hash]struct{}{})
+	if err == nil {
+		t.Fatal("expected error when first parent is missing from the store")
+	}
+	if !strings.Contains(err.Error(), missingParent.String()) {
+		t.Fatalf("error should name the missing parent %s, got %q", missingParent, err.Error())
+	}
+}