Protect regular files from FIFO setup

main.go

@@ -87,7 +87,16 @@ func setup_fifo(fn string) (chan string, error) {
 	if fn == "" {
 		return t, nil
 	}
-	os.Remove(fn)
+	if info, err := os.Lstat(fn); err == nil {
+		if info.Mode()&os.ModeNamedPipe == 0 {
+			return t, fmt.Errorf("FIFO path %q already exists and is not a FIFO", fn)
+		}
+		if err := os.Remove(fn); err != nil {
+			return t, err
+		}
+	} else if !os.IsNotExist(err) {
+		return t, err
+	}
 	err := syscall.Mkfifo(fn, 0600)
 	if err != nil {
 		return t, err

main_fifo_test.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSetupFIFORefusesToReplaceRegularFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "not-a-fifo")
+	if err := os.WriteFile(path, []byte("keep"), 0600); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := setup_fifo(path); err == nil {
+		t.Fatal("setup_fifo replaced a regular file")
+	}
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(contents) != "keep" {
+		t.Fatalf("regular file was modified: %q", contents)
+	}
+}