共享内存扩容是不是会导致原有数据丢失？

在发布者想要发布数据的时候，但此时共享内存的block大小不足以放下这个msg的时候，会出现扩容，会重新创建一个共享内存。

bool Segment::AcquireBlockToWrite(std::size_t msg_size,
                              WritableBlock* writable_block) {
RETURN_VAL_IF_NULL(writable_block, false);
if (!init_ && !OpenOrCreate()) {
AERROR << "create shm failed, can't write now.";
return false;
}

bool result = true;
if (state_->need_remap()) {
result = Remap();
}

if (msg_size > conf_.ceiling_msg_size()) {
AINFO << "msg_size: " << msg_size
      << " larger than current shm_buffer_size: "
      << conf_.ceiling_msg_size() << " , need recreate.";
result = Recreate(msg_size);       // 重新创建共享内存
}

if (!result) {
AERROR << "segment update failed.";
return false;
}

uint32_t index = GetNextWritableBlockIndex();
writable_block->index = index;
writable_block->block = &blocks_[index];
writable_block->buf = block_buf_addrs_[index];
return true;
}

在Recreate函数中重新创建新的共享内存，扩充block的大小

bool Segment::Recreate(const uint64_t& msg_size) {
init_ = false;
state_->set_need_remap(true);
Reset();
Remove();
conf_.Update(msg_size);
return OpenOrCreate();
}

假设原有共享内存上存在订阅者还未来得及消费的数据 A 、B 、 C，此时的扩容会导致原有数据的映射丢失，旧block被重置了（没有将旧的数据库拷贝到新扩容的block上）。

订阅者想要消费数据的时候，发现共享内存需要remap，然后remap后就是新的共享内存block，无法找到之前的A 、B 、 C数据。

bool Segment::AcquireBlockToRead(ReadableBlock* readable_block) {
RETURN_VAL_IF_NULL(readable_block, false);
if (!init_ && !OpenOnly()) {
AERROR << "failed to open shared memory, can't read now.";
return false;
}

auto index = readable_block->index;
if (index >= conf_.block_num()) {
AERROR << "invalid block_index[" << index << "].";
return false;
}

bool result = true;
if (state_->need_remap()) {
result = Remap();                   // 需要remap，remap后没有了之前的A  B  C数据
}

if (!result) {
AERROR << "segment update failed.";
return false;
}

if (!blocks_[index].TryLockForRead()) {
return false;
}
readable_block->block = blocks_ + index;
readable_block->buf = block_buf_addrs_[index];
return true;
}

ApolloAuto / apollo

共享内存扩容是不是会导致原有数据丢失？ #15131