Fix the data race with injectedScript

A data race can occur when two tests perform an action on the same page
but under two different threads. It occurs in ExecutionContext with its
injectedScript parameter. Since we call
ExecutionContext.getInjectedScript() in ElementHandle.evalWithScript(),
it can be called in parallel from different goroutines, so a mutex is
a good fix for this.

Closes: #426

Author: ankur22

common/execution_context.go

@@ -26,6 +26,7 @@ import (
 	"errors"
 	"fmt"
 	"regexp"
+	"sync"
 
 	"github.com/grafana/xk6-browser/api"
 	"github.com/grafana/xk6-browser/k6ext"
@@ -71,6 +72,7 @@ type ExecutionContext struct {
 	session        session
 	frame          *Frame
 	id             runtime.ExecutionContextID
+	isMutex        sync.RWMutex
 	injectedScript api.JSHandle
 	vu             k6modules.VU
 
@@ -267,9 +269,13 @@ func (e *ExecutionContext) getInjectedScript(apiCtx context.Context) (api.JSHand
 		"sid:%s stid:%s fid:%s ectxid:%d efurl:%s",
 		e.sid, e.stid, e.fid, e.id, e.furl)
 
+	e.isMutex.RLock()
 	if e.injectedScript != nil {
-		return e.injectedScript, nil
+		injectedScript := e.injectedScript
+		e.isMutex.RUnlock()
+		return injectedScript, nil
 	}
+	e.isMutex.RUnlock()
 
 	var (
 		suffix                  = `//# sourceURL=` + evaluationScriptURL
@@ -295,9 +301,11 @@ func (e *ExecutionContext) getInjectedScript(apiCtx context.Context) (api.JSHand
 	if !ok {
 		return nil, ErrJSHandleInvalid
 	}
+	e.isMutex.Lock()
 	e.injectedScript = injectedScript
+	e.isMutex.Unlock()
 
-	return e.injectedScript, nil
+	return injectedScript, nil
 }
 
 // Eval evaluates the provided JavaScript within this execution context and