fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/helpers/callback_once_test.go

@@ -0,0 +1,76 @@
+package helpers
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// Go never frees syscall.NewCallback (hard cap ~2000). Creating one per
+// screenshot frame is a fatal "too many callback functions".
+func TestNewCallbackOnlyAtPackageScope(t *testing.T) {
+	root := filepath.Join("..", "..")
+	fset := token.NewFileSet()
+	err := filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			name := d.Name()
+			if name == ".git" || name == "vendor" || name == "testdata" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !strings.HasSuffix(path, ".go") {
+			return nil
+		}
+		file, err := parser.ParseFile(fset, path, nil, 0)
+		if err != nil {
+			t.Errorf("parse %s: %v", path, err)
+			return nil
+		}
+		for _, decl := range file.Decls {
+			fn, ok := decl.(*ast.FuncDecl)
+			if !ok {
+				continue
+			}
+			ast.Inspect(fn, func(n ast.Node) bool {
+				if !isNewCallbackCall(n) {
+					return true
+				}
+				name := fn.Name.Name
+				if fn.Recv != nil {
+					name = "method " + name
+				}
+				t.Errorf("%s: syscall.NewCallback inside %s; Go never frees these (cap ~2000)", fset.Position(n.Pos()), name)
+				return true
+			})
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func isNewCallbackCall(n ast.Node) bool {
+	call, ok := n.(*ast.CallExpr)
+	if !ok {
+		return false
+	}
+	sel, ok := call.Fun.(*ast.SelectorExpr)
+	if !ok {
+		return false
+	}
+	switch sel.Sel.Name {
+	case "NewCallback", "NewCallbackCDecl":
+		return true
+	default:
+		return false
+	}
+}