Add failed: "/tmp/gogit-worktree-test/main-repo/.git/objects/pack/tmp_obj_847737165": path outside base dir "/tmp/gogit-worktree-test/main-repo/.git/worktrees/worktree": file does not exist

The current implementation assumes path are relativ but for tmp files (in combination with worktree!?) they are absolute. So the current logic that decides which filesystem should be returned isn't working at all and always returnsdotGitFs but it should returncommonDotGitFs instead.

This came out of a debugging session with Claude Code first on our code base and then on go-git main branch. I reviewed the code and I'm pretty confident it's a good approach but still wanted to flag it for transparency. Especially the amount of comments is a bit on the verbose side, so happy to remove/reduce if that's too much.

Here is a script to replicate this:

open for source

package main  import (      "fmt"      "os"      "os/exec"      "path/filepath"      "time"      "github.com/go-git/go-git/v6"      "github.com/go-git/go-git/v6/plumbing/object"  )  func main() {      // Create test structure      tmpDir := "/tmp/gogit-worktree-test"      os.RemoveAll(tmpDir)      os.MkdirAll(tmpDir, 0755)      mainRepoDir := filepath.Join(tmpDir, "main-repo")      worktreeDir := filepath.Join(tmpDir, "worktree")      // Init main repo      exec.Command("git", "init", mainRepoDir).Run()      // Create initial commit      os.WriteFile(filepath.Join(mainRepoDir, "file.txt"), []byte("test"), 0644)      exec.Command("git", "-C", mainRepoDir, "add", ".").Run()      exec.Command("git", "-C", mainRepoDir, "commit", "-m", "initial").Run()      // Create worktree      exec.Command("git", "-C", mainRepoDir, "worktree", "add", worktreeDir, "-b", "test-branch").Run()      // Change to worktree directory      os.Chdir(worktreeDir)      // Open with go-git v6      repo, err := git.PlainOpenWithOptions(".", &git.PlainOpenOptions{          EnableDotGitCommonDir: true,      })      if err != nil {          fmt.Printf("Open failed: %v\n", err)          return      }      wt, _ := repo.Worktree()      // Create a new file      os.WriteFile(filepath.Join(worktreeDir, "newfile.txt"), []byte("content"), 0644)      // Try to add it - THIS FAILS      _, err = wt.Add("newfile.txt")      if err != nil {          fmt.Printf("Add failed: %v\n", err)          return      }      // Try to commit      _, err = wt.Commit("test commit", &git.CommitOptions{          Author: &object.Signature{Name: "Test", Email: "test@test.com", When: time.Now()},      })      if err != nil {          fmt.Printf("Commit failed: %v\n", err)          return      }      fmt.Println("Success!")  }