AI codebase review

fix: duplicate search no longer hangs on files swapped for FIFOs

Candidates were reopened by path with os.Open after the scan. A file
replaced by a FIFO (or a symlink to one) blocked open(2) forever, and
context cancellation could not interrupt it, wedging the duplicate view
and --duplicates. Content is now opened with O_NOFOLLOW|O_NONBLOCK on
Unix; the existing regular-file check then skips it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/duplicate/fifo_unix_test.go

@@ -0,0 +1,51 @@
+//go:build unix
+
+package duplicate
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/andydixon/mapsize/internal/scan"
+)
+
+// A candidate replaced by a FIFO (or a symlink to one) after the scan must
+// be skipped, not block the search forever in open(2).
+func TestFindSkipsSwappedFIFO(t *testing.T) {
+	root := t.TempDir()
+	for _, n := range []string{"a", "b", "c"} {
+		os.WriteFile(filepath.Join(root, n), []byte("same"), 0o644)
+	}
+	s, err := scan.Start(context.Background(), root, scan.Options{Workers: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	<-s.Done()
+	os.Remove(filepath.Join(root, "b"))
+	if err := syscall.Mkfifo(filepath.Join(root, "b"), 0o644); err != nil {
+		t.Skip("mkfifo:", err)
+	}
+	fifo2 := filepath.Join(t.TempDir(), "p")
+	syscall.Mkfifo(fifo2, 0o644)
+	os.Remove(filepath.Join(root, "c"))
+	os.Symlink(fifo2, filepath.Join(root, "c"))
+
+	done := make(chan error, 1)
+	var f Finder
+	go func() { _, err := f.Find(context.Background(), s.Tree, Options{}); done <- err }()
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatal(err)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("duplicate search blocked on a FIFO")
+	}
+	if p := f.Progress(); p.Skipped != 2 {
+		t.Fatalf("skipped=%d want 2", p.Skipped)
+	}
+}