Fix read/write race condition for edge case

This mutex is only needed in an edge case where we have multiple
instances of k6 connecting to the same chrome instance. In this
case when a page is created by the first k6 instance, the second
instance of k6 will also receive an onAttachedToTarget event. When
this occurs there's a small chance that at the same time a new
context is being created by the second k6 instance. So the read
occurs in getDefaultBrowserContextOrMatchedID which is called by
onAttachedToTarget, and the write in NewContext. This mutex protects
the read/write race condition for this one case.

common/browser.go

@@ -47,6 +47,16 @@ type Browser struct {
 	// A *Connection is saved to this field, see: connect().
 	conn connection
 
+	// This mutex is only needed in an edge case where we have multiple
+	// instances of k6 connecting to the same chrome instance. In this
+	// case when a page is created by the first k6 instance, the second
+	// instance of k6 will also receive an onAttachedToTarget event. When
+	// this occurs there's a small chance that at the same time a new
+	// context is being created by the second k6 instance. So the read
+	// occurs in getDefaultBrowserContextOrMatchedID which is called by
+	// onAttachedToTarget, and the write in NewContext. This mutex protects
+	// the read/write race condition for this one case.
+	contextMu      sync.RWMutex
 	context        *BrowserContext
 	defaultContext *BrowserContext
 
@@ -139,6 +149,9 @@ func (b *Browser) disposeContext(id cdp.BrowserContextID) error {
 // getDefaultBrowserContextOrMatchedID returns the BrowserContext for the given browser context ID.
 // If the browser context is not found, the default BrowserContext is returned.
 func (b *Browser) getDefaultBrowserContextOrMatchedID(id cdp.BrowserContextID) *BrowserContext {
+	b.contextMu.RLock()
+	defer b.contextMu.RUnlock()
+
 	if b.context == nil || b.context.id != id {
 		return b.defaultContext
 	}
@@ -512,6 +525,9 @@ func (b *Browser) NewContext(opts goja.Value) (api.BrowserContext, error) {
 	if err != nil {
 		return nil, fmt.Errorf("new context: %w", err)
 	}
+
+	b.contextMu.Lock()
+	defer b.contextMu.Unlock()
 	b.context = browserCtx
 
 	return browserCtx, nil