Keep v8 context alive for future reference

[Austin Einter]

Dear All

I am quite new to v8. I am a c developer, with limited c++ knowledge. Sorry if I ask very basic questions.

I have a C application, that interacts with web servers. Gets javascript. Need to execute javascript.

Imagine a situation, where I get total 2 javascript files from server.

js1.js

function js_add_elements(var1, var2) 
{ 
    var var3 = parseInt(var1) + parseInt(var2); 
    var result = 'Addition of ' + var1 + ' and ' + var2 + ' results ' + var3; 
    return result; 
}

js2.js

js_add_elements(2, 3);

I have my c code as below.

Header file

typedef struct

{

    std::unique_ptr<v8::Platform> platform;

    v8::Isolate::CreateParams create_params;

    v8::Isolate *isolate;

    v8::Isolate::Scope *isolate_scope;

}MJSEInstance;

#ifdef __cplusplus

extern "C" {

#endif

/**

* @brief Create MJSE instance

*/

void *mjse_create_instance(char **argv);

/**

* @brief Destroy MJSE instance

* @param instance -> MJSE instance

*/

void mjse_destroy_instance(void *instance);

/**

* @brief Executes a script

* @param minstance - MJSE instance

* @param jscript - the javascript

* @param scriptlen - Length of script

*/

void mjse_execute_script(void *instance, char *jscript);

#ifdef __cplusplus

}

#endif

CPP File

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include "include/libplatform/libplatform.h"

#include "include/v8.h"

#include "mjse.h"

#include "mjse_int.h"

#include <new>

extern "C"  {

using namespace v8;

void *mjse_create_instance(char **argv)

{

    MJSEInstance *instance = (MJSEInstance *)calloc(1, sizeof(MJSEInstance));

    

    v8::V8::InitializeICUDefaultLocation(argv[0]);

    v8::V8::InitializeExternalStartupData(argv[0]);

    instance->platform = v8::platform::NewDefaultPlatform();

    v8::V8::InitializePlatform(instance->platform.get());

    v8::V8::Initialize();

    

    instance->create_params.array_buffer_allocator = v8::ArrayBuffer::Allocator::NewDefaultAllocator();

    instance->isolate = v8::Isolate::New(instance->create_params);

    instance->isolate_scope  = new v8::Isolate::Scope(instance->isolate);

    

    return instance;

}

void mjse_destroy_instance(void *minstance)

{

    MJSEInstance *instance = (MJSEInstance *)minstance;

    

    if(instance == NULL)

        return;

    

    if(instance->isolate_scope)

    {

        delete instance->isolate_scope;

    }

    

    if(instance->isolate)

    {

        instance->isolate->Dispose();

    }

    v8::V8::Dispose();

    v8::V8::ShutdownPlatform();

    if(instance->create_params.array_buffer_allocator)

    {

        delete instance->create_params.array_buffer_allocator;

    }

    

    instance->~MJSEInstance();

    free(instance);

}

void mjse_execute_script(void *minstance, char *jscript)

{

    MJSEInstance *instance = (MJSEInstance *)minstance;

    // Create a stack-allocated handle scope.

    v8::HandleScope handle_scope(instance->isolate);

    // Create a new context.

    v8::Local<v8::Context> context = v8::Context::New(instance->isolate);

    

    // Enter the context for compiling and running the hello world script.

    v8::Context::Scope context_scope(context);

    {

        // Create a string containing the JavaScript source code.

        v8::Local<v8::String> source =

          v8::String::NewFromUtf8(instance->isolate, jscript,

          v8::NewStringType::kNormal).ToLocalChecked();

              

        // Compile the source code.

        v8::Local<v8::Script> script =

          v8::Script::Compile(context, source).ToLocalChecked();

        // Run the script to get the result.

        v8::Local<v8::Value> result = script->Run(context).ToLocalChecked();

        

        // Convert the result to an UTF8 string and print it.

        v8::String::Utf8Value utf8(instance->isolate, result);

        printf("%s\n", *utf8);

    }

}

}

Main File

int main(int argc, char **argv)

{

    char *script;

    void *instance;

    instance = mjse_create_instance(argv);

    script =    "function js_add_elements(var1, var2) \

                { \

                    var var3 = parseInt(var1) + parseInt(var2); \

                    var result = 'Addition of ' + var1 + ' and ' + var2 + ' results ' + var3; \

                    return result; \

                };";

    mjse_execute_script(instance, script);

    script = "js_add_elements(2, 3);";

    mjse_execute_script(instance, script);

    mjse_destroy_instance(instance);

    return 0;

}

Now whats happening, while executing javascript first time it executes fine. But while trying to execute second time (letter in red), it gets exception.

I understand, it is creating new v8 Local context each time, and does not have js1.js details, hence getting exception. I also found in web we can have v8 Global context and refer the context while executing for second time.

However I have not been able to find, how to declare v8 global context and store / move v8 local context there and refer it during second time execution. Please let me know with a sample/example code.

Thanks

Austin 

[Ben Noordhuis]

You can make the context persistent like this:

if (instance->context == nullptr) {

v8::Local<v8::Context> context = v8::Context::New(instance->isolate);

instance->context = new v8::Global<v8::Context>(instance->isolate, context);
}

v8::Local<v8::Context> context(instance->isolate, instance->context);
v8::Context::Scope context_scope(context);

[Austin Einter]

Thanks Ben. I followed your suggestion.

I added v8::Global<v8::Context> *context; member in instance structure.

I put the code as below.

    if(instance->context == nullptr)

    {
        v8::Local<v8::Context> context = v8::Context::New(instance->isolate);
        instance->context = new v8::Global<v8::Context>(instance->isolate, context);
    }

    v8::Local<v8::Context> context(instance->isolate, instance->context);

 For last line (in red) I am getting below compilation error.

error: no matching function for call to ‘v8::Local<v8::Context>::Local(v8::Isolate*&, v8::Global<v8::Context>*&)’

Please let me know what I need to do to avoid this error.

Thanks

Austin

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[Austin Einter]

I changed the line to

v8::Local<v8::Context> context(instance->context->Get(instance->isolate));

It is compiling also working fine.

However when I execute this executable with valgrind, I get lot of "Conditional jump or move depends on uninitialised value(s)"

One such conditional jump is given below.

==5158== Conditional jump or move depends on uninitialised value(s)
==5158==    at 0x8F0868: v8::internal::DeclarationScope::CheckConflictingVarDeclarations() (in /home/user/mjse)
==5158==    by 0x4D8F68: v8::internal::PreParser::PreParseFunction(v8::internal::AstRawString const*, v8::internal::FunctionKind, v8::internal::FunctionLiteral::FunctionType, v8::internal::DeclarationScope*, int*, v8::internal::ProducedPreparseData**, int) (in /home/user/mjse)
==5158==    by 0x4B7935: v8::internal::Parser::SkipFunction(v8::internal::AstRawString const*, v8::internal::FunctionKind, v8::internal::FunctionLiteral::FunctionType, v8::internal::DeclarationScope*, int*, int*, v8::internal::ProducedPreparseData**) (in /home/user/mjse)
==5158==    by 0x4AB715: v8::internal::Parser::ParseFunctionLiteral(v8::internal::AstRawString const*, v8::internal::Scanner::Location, v8::internal::FunctionNameValidity, v8::internal::FunctionKind, int, v8::internal::FunctionLiteral::FunctionType, v8::internal::LanguageMode, v8::internal::ZoneList<v8::internal::AstRawString const*>*) (in /home/user/mjse)
==5158==    by 0x4D1EFA: v8::internal::ParserBase<v8::internal::Parser>::ParseHoistableDeclaration(int, v8::base::Flags<v8::internal::ParseFunctionFlag, int>, v8::internal::ZoneList<v8::internal::AstRawString const*>*, bool) (in /home/user/mjse)
==5158==    by 0x4AA266: v8::internal::Parser::DoParseProgram(v8::internal::Isolate*, v8::internal::ParseInfo*) (in /home/user/mjse)
==5158==    by 0x4A9958: v8::internal::Parser::ParseProgram(v8::internal::Isolate*, v8::internal::ParseInfo*) (in /home/user/mjse)
==5158==    by 0x4D5759: v8::internal::parsing::ParseProgram(v8::internal::ParseInfo*, v8::internal::Isolate*) (in /home/user/mjse)
==5158==    by 0x22A835: v8::internal::(anonymous namespace)::CompileToplevel(v8::internal::ParseInfo*, v8::internal::Isolate*, v8::internal::IsCompiledScope*) (in /home/user/mjse)
==5158==    by 0x22C128: v8::internal::Compiler::GetSharedFunctionInfoForScript(v8::internal::Isolate*, v8::internal::Handle<v8::internal::String>, v8::internal::Compiler::ScriptDetails const&, v8::ScriptOriginOptions, v8::Extension*, v8::internal::ScriptData*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason, v8::internal::NativesFlag) (in /home/user/mjse)
==5158==    by 0x1F0EE4: v8::ScriptCompiler::CompileUnboundInternal(v8::Isolate*, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)
==5158==    by 0x1F15D6: v8::ScriptCompiler::Compile(v8::Local<v8::Context>, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)
==5158== 

What could be the issue?

Thanks

Austin

[Jakob Kummerow]

Could be a bug in valgrind, or a bug in V8. Getting a line number would be helpful, along with the exact version of V8. At a quick glance, I don't see any uninitialized variables in DeclarationScope::CheckConflictingVarDeclarations.

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-users/CANXt1k_oVrTPsa6US7Ux6Q25ut%2B-jiF5XD9swpgQKHJPcVG21Q%40mail.gmail.com.

[Austin Einter]

One more observation, if I do not keep any argument in my javascript then I do not see conditional jump. I changed my first javascript to as given below, with this the conditional jump is not shown in valgrind report.

js1.js

function js_add_elements() 
{ 
    return 10; 
}

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-users/CAKSzg3SmJ0xBcw_dy2Ec3VDRhv%3DeoYM%3D6rN8uhAKqCfO5Y-A_A%40mail.gmail.com.

[Austin Einter]

Hi Jakob

Thanks for response.

Version: 7.7.0 (candidate)

(Got the version with printf("Version: %s", v8::V8::GetVersion());)

Line number, valgrind output does not show. Probably I need to rebuild v8 in debug mode.

I enabled --track-origin option in valgrind while executing. With this I get below report. Please see the text in red. The uninitialised value originates from those functions.

==7113==
==7113== Conditional jump or move depends on uninitialised value(s)
==7113==    at 0x8F0868: v8::internal::DeclarationScope::CheckConflictingVarDeclarations() (in /home/user/mjse)
==7113==    by 0x4D8F68: v8::internal::PreParser::PreParseFunction(v8::internal::AstRawString const*, v8::internal::FunctionKind, v8::internal::FunctionLiteral::FunctionType, v8::internal::DeclarationScope*, int*, v8::internal::ProducedPreparseData**, int) (in /home/user/mjse)
==7113==    by 0x4B7935: v8::internal::Parser::SkipFunction(v8::internal::AstRawString const*, v8::internal::FunctionKind, v8::internal::FunctionLiteral::FunctionType, v8::internal::DeclarationScope*, int*, int*, v8::internal::ProducedPreparseData**) (in /home/user/mjse)
==7113==    by 0x4AB715: v8::internal::Parser::ParseFunctionLiteral(v8::internal::AstRawString const*, v8::internal::Scanner::Location, v8::internal::FunctionNameValidity, v8::internal::FunctionKind, int, v8::internal::FunctionLiteral::FunctionType, v8::internal::LanguageMode, v8::internal::ZoneList<v8::internal::AstRawString const*>*) (in /home/user/mjse)
==7113==    by 0x4D1EFA: v8::internal::ParserBase<v8::internal::Parser>::ParseHoistableDeclaration(int, v8::base::Flags<v8::internal::ParseFunctionFlag, int>, v8::internal::ZoneList<v8::internal::AstRawString const*>*, bool) (in /home/user/mjse)
==7113==    by 0x4AA266: v8::internal::Parser::DoParseProgram(v8::internal::Isolate*, v8::internal::ParseInfo*) (in /home/user/mjse)
==7113==    by 0x4A9958: v8::internal::Parser::ParseProgram(v8::internal::Isolate*, v8::internal::ParseInfo*) (in /home/user/mjse)
==7113==    by 0x4D5759: v8::internal::parsing::ParseProgram(v8::internal::ParseInfo*, v8::internal::Isolate*) (in /home/user/mjse)
==7113==    by 0x22A835: v8::internal::(anonymous namespace)::CompileToplevel(v8::internal::ParseInfo*, v8::internal::Isolate*, v8::internal::IsCompiledScope*) (in /home/user/mjse)
==7113==    by 0x22C128: v8::internal::Compiler::GetSharedFunctionInfoForScript(v8::internal::Isolate*, v8::internal::Handle<v8::internal::String>, v8::internal::Compiler::ScriptDetails const&, v8::ScriptOriginOptions, v8::Extension*, v8::internal::ScriptData*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason, v8::internal::NativesFlag) (in /home/user/mjse)
==7113==    by 0x1F0EE4: v8::ScriptCompiler::CompileUnboundInternal(v8::Isolate*, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)
==7113==    by 0x1F15D6: v8::ScriptCompiler::Compile(v8::Local<v8::Context>, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)


==7113==  Uninitialised value was created by a heap allocation
==7113==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7113==    by 0x555D7D: v8::internal::AllocWithRetry(unsigned long) (in /home/user/mjse)
==7113==    by 0x5A2C14: v8::internal::AccountingAllocator::AllocateSegment(unsigned long) (in /home/user/mjse)
==7113==    by 0x5A2EAA: v8::internal::Zone::NewExpand(unsigned long) (in /home/user/mjse)
==7113==    by 0x8E6E32: v8::internal::AstValueFactory::NewConsString() (in /home/user/mjse)
==7113==    by 0x4A624B: v8::internal::ParseInfo::GetOrCreateAstValueFactory() (in /home/user/mjse)
==7113==    by 0x4A91C8: v8::internal::Parser::Parser(v8::internal::ParseInfo*) (in /home/user/mjse)
==7113==    by 0x4D574B: v8::internal::parsing::ParseProgram(v8::internal::ParseInfo*, v8::internal::Isolate*) (in /home/user/mjse)
==7113==    by 0x22A835: v8::internal::(anonymous namespace)::CompileToplevel(v8::internal::ParseInfo*, v8::internal::Isolate*, v8::internal::IsCompiledScope*) (in /home/user/mjse)
==7113==    by 0x22C128: v8::internal::Compiler::GetSharedFunctionInfoForScript(v8::internal::Isolate*, v8::internal::Handle<v8::internal::String>, v8::internal::Compiler::ScriptDetails const&, v8::ScriptOriginOptions, v8::Extension*, v8::internal::ScriptData*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason, v8::internal::NativesFlag) (in /home/user/mjse)
==7113==    by 0x1F0EE4: v8::ScriptCompiler::CompileUnboundInternal(v8::Isolate*, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)
==7113==    by 0x1F15D6: v8::ScriptCompiler::Compile(v8::Local<v8::Context>, v8::ScriptCompiler::Source*, v8::ScriptCompiler::CompileOptions, v8::ScriptCompiler::NoCacheReason) (in /home/user/mjse)
==7113== 

On Thu, Jun 27, 2019 at 6:11 PM Jakob Kummerow <jkum...@chromium.org> wrote:

To view this discussion on the web visit https://groups.google.com/d/msgid/v8-users/CAKSzg3SmJ0xBcw_dy2Ec3VDRhv%3DeoYM%3D6rN8uhAKqCfO5Y-A_A%40mail.gmail.com.