gecko-dev/accessible/windows/msaa/MsaaIdGenerator.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=2 et sw=2 tw=80: */
/* 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/. */

#include "MsaaIdGenerator.h"

#include "mozilla/a11y/AccessibleWrap.h"
#include "mozilla/ClearOnShutdown.h"
#include "mozilla/dom/ContentChild.h"
#include "mozilla/StaticPtr.h"
#include "mozilla/Unused.h"
#include "nsDataHashtable.h"
#include "nsIXULRuntime.h"

// These constants may be adjusted to modify the proportion of the Child ID
// allocated to the content ID vs proportion allocated to the unique ID. They
// must always sum to 31, ie. the width of a 32-bit integer less the sign bit.

// NB: kNumContentProcessIDBits must be large enough to successfully hold the
// maximum permitted number of e10s content processes. If the e10s maximum
// number of content processes changes, then kNumContentProcessIDBits must also
// be updated if necessary to accommodate that new value!
static const uint32_t kNumContentProcessIDBits = 7UL;
static const uint32_t kNumUniqueIDBits = (31UL - kNumContentProcessIDBits);

static_assert(kNumContentProcessIDBits + kNumUniqueIDBits == 31,
              "Allocation of Content ID bits and Unique ID bits must sum to 31");

namespace mozilla {
namespace a11y {
namespace detail {

typedef nsDataHashtable<nsUint64HashKey, uint32_t> ContentParentIdMap;

#pragma pack(push, 1)
union MsaaID
{
  int32_t mInt32;
  uint32_t mUInt32;
  struct
  {
    uint32_t mUniqueID:kNumUniqueIDBits;
    uint32_t mContentProcessID:kNumContentProcessIDBits;
    uint32_t mSignBit:1;
  }
  mCracked;
};
#pragma pack(pop)

static uint32_t
BuildMsaaID(const uint32_t aID, const uint32_t aContentProcessID)
{
  MsaaID id;
  id.mCracked.mSignBit = 0;
  id.mCracked.mUniqueID = aID;
  id.mCracked.mContentProcessID = aContentProcessID;
  return ~id.mUInt32;
}

class MsaaIDCracker
{
public:
  explicit MsaaIDCracker(const uint32_t aMsaaID)
  {
    mID.mUInt32 = ~aMsaaID;
  }

  uint32_t GetContentProcessId()
  {
    return mID.mCracked.mContentProcessID;
  }

  uint32_t GetUniqueId()
  {
    return mID.mCracked.mUniqueID;
  }

private:
  MsaaID  mID;
};

} // namespace detail

constexpr MsaaIdGenerator::MsaaIdGenerator()
  : mIDSet(kNumUniqueIDBits)
{}

uint32_t
MsaaIdGenerator::GetID()
{
  uint32_t id = mIDSet.GetID();
  MOZ_ASSERT(id <= ((1UL << kNumUniqueIDBits) - 1UL));
  return detail::BuildMsaaID(id, ResolveContentProcessID());
}

void
MsaaIdGenerator::ReleaseID(AccessibleWrap* aAccWrap)
{
  MOZ_ASSERT(aAccWrap);
  uint32_t id = aAccWrap->GetExistingID();
  MOZ_ASSERT(id != AccessibleWrap::kNoID);
  detail::MsaaIDCracker cracked(id);
  if (cracked.GetContentProcessId() != ResolveContentProcessID()) {
    // This may happen if chrome holds a proxy whose ID was originally generated
    // by a content process. Since ReleaseID only has meaning in the process
    // that originally generated that ID, we ignore ReleaseID calls for any ID
    // that did not come from the current process.
    MOZ_ASSERT(aAccWrap->IsProxy());
    return;
  }
  mIDSet.ReleaseID(cracked.GetUniqueId());
}

bool
MsaaIdGenerator::IsChromeID(uint32_t aID)
{
  detail::MsaaIDCracker cracked(aID);
  return cracked.GetContentProcessId() == 0;
}

bool
MsaaIdGenerator::IsIDForThisContentProcess(uint32_t aID)
{
  MOZ_ASSERT(XRE_IsContentProcess());
  detail::MsaaIDCracker cracked(aID);
  return cracked.GetContentProcessId() == ResolveContentProcessID();
}

bool
MsaaIdGenerator::IsIDForContentProcess(uint32_t aID,
                                       dom::ContentParentId aIPCContentProcessId)
{
  MOZ_ASSERT(XRE_IsParentProcess());
  detail::MsaaIDCracker cracked(aID);
  return cracked.GetContentProcessId() ==
           GetContentProcessIDFor(aIPCContentProcessId);
}

bool
MsaaIdGenerator::IsSameContentProcessFor(uint32_t aFirstID, uint32_t aSecondID)
{
  detail::MsaaIDCracker firstCracked(aFirstID);
  detail::MsaaIDCracker secondCracked(aSecondID);
  return firstCracked.GetContentProcessId() ==
           secondCracked.GetContentProcessId();
}

uint32_t
MsaaIdGenerator::ResolveContentProcessID()
{
  if (XRE_IsParentProcess()) {
    return 0;
  }

  dom::ContentChild* contentChild = dom::ContentChild::GetSingleton();
  uint32_t result = contentChild->GetMsaaID();

  MOZ_ASSERT(result);
  return result;
}

/**
 * Each dom::ContentParent has a 64-bit ID. This ID is monotonically increasing
 * with each new content process, so those IDs are effectively single-use. OTOH,
 * MSAA requires 32-bit IDs. Since we only allocate kNumContentProcessIDBits for
 * the content process ID component, the MSAA content process ID value must be
 * reusable. sContentParentIdMap holds the current associations between
 * dom::ContentParent IDs and the MSAA content parent IDs that have been
 * allocated to them.
 */
static StaticAutoPtr<detail::ContentParentIdMap> sContentParentIdMap;

static const uint32_t kBitsPerByte = 8UL;
// Set sContentProcessIdBitmap[0] to 1 to reserve the Chrome process's id
static uint64_t sContentProcessIdBitmap[(1UL << kNumContentProcessIDBits) /
                                        (sizeof(uint64_t) * kBitsPerByte)] = {1ULL};
static const uint32_t kBitsPerElement = sizeof(sContentProcessIdBitmap[0]) *
                                        kBitsPerByte;

uint32_t
MsaaIdGenerator::GetContentProcessIDFor(dom::ContentParentId aIPCContentProcessID)
{
  MOZ_ASSERT(XRE_IsParentProcess() && NS_IsMainThread());
  if (!sContentParentIdMap) {
    sContentParentIdMap = new detail::ContentParentIdMap();
    ClearOnShutdown(&sContentParentIdMap);
  }

  uint32_t value = 0;
  if (sContentParentIdMap->Get(aIPCContentProcessID, &value)) {
    return value;
  }

  uint32_t index = 0;
  for (; index < ArrayLength(sContentProcessIdBitmap); ++index) {
    if (sContentProcessIdBitmap[index] == UINT64_MAX) {
      continue;
    }
    uint32_t bitIndex = CountTrailingZeroes64(~sContentProcessIdBitmap[index]);
    MOZ_ASSERT(!(sContentProcessIdBitmap[index] & (1ULL << bitIndex)));
    MOZ_ASSERT(bitIndex != 0 || index != 0);
    sContentProcessIdBitmap[index] |= (1ULL << bitIndex);
    value = index * kBitsPerElement + bitIndex;
    break;
  }

  // If we run out of content process IDs, we're in trouble
  MOZ_RELEASE_ASSERT(index < ArrayLength(sContentProcessIdBitmap));

  sContentParentIdMap->Put(aIPCContentProcessID, value);
  return value;
}

void
MsaaIdGenerator::ReleaseContentProcessIDFor(dom::ContentParentId aIPCContentProcessID)
{
  MOZ_ASSERT(XRE_IsParentProcess() && NS_IsMainThread());
  if (!sContentParentIdMap) {
    // Since Content IDs are generated lazily, ContentParent might attempt
    // to release an ID that was never allocated to begin with.
    return;
  }

  Maybe<uint32_t> mapping = sContentParentIdMap->GetAndRemove(aIPCContentProcessID);
  if (!mapping) {
    // Since Content IDs are generated lazily, ContentParent might attempt
    // to release an ID that was never allocated to begin with.
    return;
  }

  uint32_t index = mapping.ref() / kBitsPerElement;
  MOZ_ASSERT(index < ArrayLength(sContentProcessIdBitmap));

  uint64_t mask = 1ULL << (mapping.ref() % kBitsPerElement);
  MOZ_ASSERT(sContentProcessIdBitmap[index] & mask);

  sContentProcessIdBitmap[index] &= ~mask;
}

} // namespace a11y
} // namespace mozilla