handle errors when calling NewContextFromSnapshot at V8 level

context.go

@@ -8,7 +8,6 @@ package v8go
 // #include "v8go.h"
 import "C"
 import (
-	"errors"
 	"runtime"
 	"sync"
 	"unsafe"
@@ -78,21 +77,22 @@ func NewContext(opt ...ContextOption) *Context {
 }
 
 // NewContextFromSnapshot creates a new JavaScript context from the Isolate startup data;
-// If the Isolate has no startup data associated returns an error.
+// error will be of type `JSError` if not nil.
 func NewContextFromSnapshot(iso *Isolate, snapshot_index int) (*Context, error) {
 	ctxMutex.Lock()
 	ctxSeq++
 	ref := ctxSeq
 	ctxMutex.Unlock()
 
-	createParams := iso.createParams
-	if createParams == nil || createParams.startupData == nil {
-		return nil, errors.New("v8go: The isolate must have startupData associated with it")
+	rtn := C.NewContextFromSnapshot(iso.ptr, C.size_t(snapshot_index), C.int(ref))
+
+	if rtn.context == nil {
+		return nil, newJSError(rtn.error)
 	}
 
 	ctx := &Context{
 		ref: ref,
-		ptr: C.NewContextFromSnapshot(iso.ptr, C.size_t(snapshot_index), C.int(ref)),
+		ptr: rtn.context,
 		iso: iso,
 	}
 

context_test.go

@@ -55,6 +55,45 @@ func TestNewContextFromSnapshotErrorWhenIsolateHasNoStartupData(t *testing.T) {
 	}
 }
 
+func TestNewContextFromSnapshotErrorWhenIndexOutOfRange(t *testing.T) {
+	t.Parallel()
+
+	snapshotCreator := v8.NewSnapshotCreator()
+	snapshotCreatorIso, err := snapshotCreator.GetIsolate()
+	fatalIf(t, err)
+
+	snapshotCreatorCtx := v8.NewContext(snapshotCreatorIso)
+	defer snapshotCreatorCtx.Close()
+
+	snapshotCreatorCtx.RunScript(`const add = (a, b) => a + b`, "add.js")
+	snapshotCreatorCtx.RunScript(`function run() { return add(3, 4); }`, "main.js")
+	err = snapshotCreator.SetDefaultContext(snapshotCreatorCtx)
+	fatalIf(t, err)
+
+	snapshotCreatorCtx2 := v8.NewContext(snapshotCreatorIso)
+	defer snapshotCreatorCtx2.Close()
+
+	snapshotCreatorCtx2.RunScript(`const multiply = (a, b) => a * b`, "add.js")
+	snapshotCreatorCtx2.RunScript(`function run() { return multiply(3, 4); }`, "main.js")
+	index, err := snapshotCreator.AddContext(snapshotCreatorCtx2)
+	fatalIf(t, err)
+
+	data, err := snapshotCreator.Create(v8.FunctionCodeHandlingClear)
+	fatalIf(t, err)
+
+	iso := v8.NewIsolate(v8.WithStartupData(data))
+	defer iso.Dispose()
+
+	ctx, err := v8.NewContextFromSnapshot(iso, index+1)
+
+	if ctx != nil {
+		t.Errorf("expected nil context got: %+v", ctx)
+	}
+	if err == nil {
+		t.Error("error expected but was <nil>")
+	}
+}
+
 func TestJSExceptions(t *testing.T) {
 	t.Parallel()
 

v8go.cc

@@ -668,28 +668,40 @@ ContextPtr NewContext(IsolatePtr iso,
   return ctx;
 }
 
-ContextPtr NewContextFromSnapshot(IsolatePtr iso,
+RtnContext NewContextFromSnapshot(IsolatePtr iso,
                                   size_t snapshot_blob_index,
                                   int ref) {
   Locker locker(iso);
   Isolate::Scope isolate_scope(iso);
   HandleScope handle_scope(iso);
 
+  RtnContext rtn = {};
+
   // For function callbacks we need a reference to the context, but because of
   // the complexities of C -> Go function pointers, we store a reference to the
   // context as a simple integer identifier; this can then be used on the Go
   // side to lookup the context in the context registry. We use slot 1 as slot 0
   // has special meaning for the Chrome debugger.
 
-  Local<Context> local_ctx =
-      Context::FromSnapshot(iso, snapshot_blob_index).ToLocalChecked();
+  Local<Context> local_ctx;
+  MaybeLocal<Context> maybe_local_ctx =
+      Context::FromSnapshot(iso, snapshot_blob_index);
+
+  if (!maybe_local_ctx.ToLocal(&local_ctx)) {
+    RtnError error = {nullptr, nullptr, nullptr};
+    error.msg = CopyString("Failed to create context from snapshot index: " + std::to_string(snapshot_blob_index));
+    rtn.error = error;
+    return rtn;
+  }
+
   local_ctx->SetEmbedderData(1, Integer::New(iso, ref));
 
   m_ctx* ctx = new m_ctx;
   ctx->ptr.Reset(iso, local_ctx);
   ctx->iso = iso;
   ctx->startup_data = nullptr;
-  return ctx;
+  rtn.context = ctx;
+  return rtn;
 }
 
 void ContextFree(ContextPtr ctx) {

v8go.h

@@ -124,6 +124,11 @@ typedef struct {
   RtnError error;
 } RtnValue;
 
+typedef struct {
+  ContextPtr context;
+  RtnError error;
+} RtnContext;
+
 typedef struct {
   const char* string;
   RtnError error;
@@ -189,7 +194,7 @@ extern void CPUProfileDelete(CPUProfile* ptr);
 extern ContextPtr NewContext(IsolatePtr iso_ptr,
                              TemplatePtr global_template_ptr,
                              int ref);
-extern ContextPtr NewContextFromSnapshot(IsolatePtr iso,
+extern RtnContext NewContextFromSnapshot(IsolatePtr iso,
                                          size_t snapshot_blob_index,
                                          int ref);
 extern void ContextFree(ContextPtr ptr);