js/xpconnect/src/XPCString.cpp
author Kyle Huey <khuey@kylehuey.com>
Tue, 11 Aug 2015 06:10:46 -0700
changeset 257246 a13c1f26e351dd6251da641fe7a9eb53790fc2d0
parent 236396 02f2f4c75007651c63bbc0791d9a58dea88f545f
child 301499 232a786cf87fb0f68f9bda6e9316515524d2ac8a
permissions -rw-r--r--
Bug 1179909: Refactor stable state handling. r=smaug This is motivated by three separate but related problems: 1. Our concept of recursion depth is broken for things that run from AfterProcessNextEvent observers (e.g. Promises). We decrement the recursionDepth counter before firing observers, so a Promise callback running at the lowest event loop depth has a recursion depth of 0 (whereas a regular nsIRunnable would be 1). This is a problem because it's impossible to distinguish a Promise running after a sync XHR's onreadystatechange handler from a top-level event (since the former runs with depth 2 - 1 = 1, and the latter runs with just 1). 2. The nsIThreadObserver mechanism that is used by a lot of code to run "after" the current event is a poor fit for anything that runs script. First, the order the observers fire in is the order they were added, not anything fixed by spec. Additionally, running script can cause the event loop to spin, which is a big source of pain here (bholley has some nasty bug caused by this). 3. We run Promises from different points in the code for workers and main thread. The latter runs from XPConnect's nsIThreadObserver callbacks, while the former runs from a hardcoded call to run Promises in the worker event loop. What workers do is particularly problematic because it means we can't get the right recursion depth no matter what we do to nsThread. The solve this, this patch does the following: 1. Consolidate some handling of microtasks and all handling of stable state from appshell and WorkerPrivate into CycleCollectedJSRuntime. 2. Make the recursionDepth counter only available to CycleCollectedJSRuntime (and its consumers) and remove it from the nsIThreadInternal and nsIThreadObserver APIs. 3. Adjust the recursionDepth counter so that microtasks run with the recursionDepth of the task they are associated with. 4. Introduce the concept of metastable state to replace appshell's RunBeforeNextEvent. Metastable state is reached after every microtask or task is completed. This provides the semantics that bent and I want for IndexedDB, where transactions autocommit at the end of a microtask and do not "spill" from one microtask into a subsequent microtask. This differs from appshell's RunBeforeNextEvent in two ways: a) It fires between microtasks, which was the motivation for starting this. b) It no longer ensures that we're at the same event loop depth in the native event queue. bent decided we don't care about this. 5. Reorder stable state to happen after microtasks such as Promises, per HTML. Right now we call the regular thread observers, including appshell, before the main thread observer (XPConnect), so stable state tasks happen before microtasks.

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/* vim: set ts=8 sts=4 et sw=4 tw=99: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*
 * Infrastructure for sharing DOMString data with JSStrings.
 *
 * Importing an nsAString into JS:
 * If possible (GetSharedBufferHandle works) use the external string support in
 * JS to create a JSString that points to the readable's buffer.  We keep a
 * reference to the buffer handle until the JSString is finalized.
 *
 * Exporting a JSString as an nsAReadable:
 * Wrap the JSString with a root-holding XPCJSReadableStringWrapper, which roots
 * the string and exposes its buffer via the nsAString interface, as
 * well as providing refcounting support.
 */

#include "nscore.h"
#include "nsString.h"
#include "nsStringBuffer.h"
#include "jsapi.h"
#include "xpcpublic.h"

using namespace JS;

// static
void
XPCStringConvert::FreeZoneCache(JS::Zone* zone)
{
    // Put the zone user data into an AutoPtr (which will do the cleanup for us),
    // and null out the user data (which may already be null).
    nsAutoPtr<ZoneStringCache> cache(static_cast<ZoneStringCache*>(JS_GetZoneUserData(zone)));
    JS_SetZoneUserData(zone, nullptr);
}

// static
void
XPCStringConvert::ClearZoneCache(JS::Zone* zone)
{
    ZoneStringCache* cache = static_cast<ZoneStringCache*>(JS_GetZoneUserData(zone));
    if (cache) {
        cache->mBuffer = nullptr;
        cache->mString = nullptr;
    }
}

// static
void
XPCStringConvert::FinalizeLiteral(const JSStringFinalizer* fin, char16_t* chars)
{
}

const JSStringFinalizer XPCStringConvert::sLiteralFinalizer =
    { XPCStringConvert::FinalizeLiteral };

// static
void
XPCStringConvert::FinalizeDOMString(const JSStringFinalizer* fin, char16_t* chars)
{
    nsStringBuffer* buf = nsStringBuffer::FromData(chars);
    buf->Release();
}

const JSStringFinalizer XPCStringConvert::sDOMStringFinalizer =
    { XPCStringConvert::FinalizeDOMString };

// convert a readable to a JSString, copying string data
// static
bool
XPCStringConvert::ReadableToJSVal(JSContext* cx,
                                  const nsAString& readable,
                                  nsStringBuffer** sharedBuffer,
                                  MutableHandleValue vp)
{
    *sharedBuffer = nullptr;

    uint32_t length = readable.Length();

    if (readable.IsLiteral()) {
        JSString* str = JS_NewExternalString(cx,
                                             static_cast<const char16_t*>(readable.BeginReading()),
                                             length, &sLiteralFinalizer);
        if (!str)
            return false;
        vp.setString(str);
        return true;
    }

    nsStringBuffer* buf = nsStringBuffer::FromString(readable);
    if (buf) {
        bool shared;
        if (!StringBufferToJSVal(cx, buf, length, vp, &shared))
            return false;
        if (shared)
            *sharedBuffer = buf;
        return true;
    }

    // blech, have to copy.
    JSString* str = JS_NewUCStringCopyN(cx, readable.BeginReading(), length);
    if (!str)
        return false;
    vp.setString(str);
    return true;
}

namespace xpc {

bool
NonVoidStringToJsval(JSContext* cx, nsAString& str, MutableHandleValue rval)
{
    nsStringBuffer* sharedBuffer;
    if (!XPCStringConvert::ReadableToJSVal(cx, str, &sharedBuffer, rval))
      return false;

    if (sharedBuffer) {
        // The string was shared but ReadableToJSVal didn't addref it.
        // Move the ownership from str to jsstr.
        str.ForgetSharedBuffer();
    }
    return true;
}

} // namespace xpc