зеркало из https://github.com/mozilla/gecko-dev.git
222 строки
10 KiB
C++
222 строки
10 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef mozilla_dom_IPCBlobUtils_h
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#define mozilla_dom_IPCBlobUtils_h
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#include "mozilla/dom/File.h"
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/*
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* Blobs and IPC
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* ~~~~~~~~~~~~~
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*
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* Simplifying, DOM Blob objects are chunks of data with a content type and a
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* size. DOM Files are Blobs with a name. They are are used in many APIs and
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* they can be cloned and sent cross threads and cross processes.
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*
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* If we see Blobs from a platform point of view, the main (and often, the only)
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* interesting part is how to retrieve data from it. This is done via
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* nsIInputStream and, except for a couple of important details, this stream is
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* used in the parent process.
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*
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* For this reason, when we consider the serialization of a blob via IPC
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* messages, the biggest effort is put in how to manage the nsInputStream
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* correctly. To serialize, we use the IPCBlob data struct: basically, the blob
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* properties (size, type, name if it's a file) and the nsIInputStream.
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*
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* Before talking about the nsIInputStream it's important to say that we have
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* different kinds of Blobs, based on the different kinds of sources. A non
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* exaustive list is:
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* - a memory buffer: MemoryBlobImpl
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* - a string: StringBlobImpl
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* - a real OS file: FileBlobImpl
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* - a temporary OS file: TemporaryBlobImpl
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* - a generic nsIInputStream: StreamBlobImpl
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* - an empty blob: EmptyBlobImpl
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* - more blobs combined together: MultipartBlobImpl
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* Each one of these implementations has a custom ::GetInternalStream method.
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* So, basically, each one has a different kind of nsIInputStream (nsFileStream,
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* nsIStringInputStream, SlicedInputStream, and so on).
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*
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* Another important point to keep in mind is that a Blob can be created on the
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* content process (for example: |new Blob([123])|) or it can be created on the
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* parent process and sent to content (a FilePicker creates Blobs and it runs on
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* the parent process).
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*
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* Child to Parent Blob Serialization
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*
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* When a document creates a blob, this can be sent, for different reasons to
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* the parent process. For instance it can be sent as part of a FormData, or it
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* can be converted to a BlobURL and broadcasted to any other existing
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* processes.
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*
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* When this happens, we use the IPCStream data struct for the serialization
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* of the nsIInputStream. This means that, if the stream is fully serializable
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* and its size is lower than 1Mb, we are able to recreate the stream completely
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* on the parent side. This happens, basically with any kind of child-to-parent
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* stream except for huge memory streams. In this case we end up using
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* PChildToParentStream. See more information in IPCStreamUtils.h.
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*
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* In order to populate IPCStream correctly, we use AutoIPCStream as documented
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* in IPCStreamUtils.h. Note that we use the 'delayed start' feature because,
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* often, the stream doesn't need to be read on the parent side.
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*
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* Parent to Child Blob Serialization
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*
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* This scenario is common when we talk about Blobs pointing to real files:
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* HTMLInputElement (type=file), or Entries API, DataTransfer and so on. But we
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* also have this scenario when a content process creates a Blob and it
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* broadcasts it because of a BlobURL or because BroadcastChannel API is used.
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*
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* The approach here is this: normally, the content process doesn't really read
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* data from the blob nsIInputStream. The content process needs to have the
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* nsIInputStream and be able to send it back to the parent process when the
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* "real" work needs to be done. This is true except for 2 usecases: FileReader
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* API and BlobURL usage. So, if we ignore these 2, normally, the parent sends a
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* blob nsIInputStream to a content process, and then, it will receive it back
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* in order to do some networking, or whatever.
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*
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* For this reason, IPCBlobUtils uses a particular protocol for serializing
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* nsIInputStream parent to child: PIPCBlobInputStream. This protocol keeps the
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* original nsIInputStream alive on the parent side, and gives its size and a
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* UUID to the child side. The child side creates a IPCBlobInputStream and that
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* is incapsulated into a StreamBlobImpl.
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*
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* The UUID is useful when the content process sends the same nsIInputStream
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* back to the parent process because, the only information it has to share is
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* the UUID. Each nsIInputStream sent via PIPCBlobInputStream, is registered
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* into the IPCBlobInputStreamStorage.
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*
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* On the content process side, IPCBlobInputStream is a special inputStream:
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* the only reliable methods are:
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* - nsIInputStream.available() - the size is shared by PIPCBlobInputStream
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* actor.
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* - nsIIPCSerializableInputStream.serialize() - we can give back this stream to
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* the parent because we know its UUID.
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* - nsICloneableInputStream.cloneable() and nsICloneableInputStream.clone() -
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* this stream can be cloned. We just need to have a reference of the
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* PIPCBlobInputStream actor and its UUID.
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* - nsIAsyncInputStream.asyncWait() - see next section.
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*
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* Any other method (read, readSegment and so on) will fail if asyncWait() is
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* not previously called (see the next section). Basically, this inputStream
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* cannot be used synchronously for any 'real' reading operation.
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*
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* When the parent receives the serialization of a IPCBlobInputStream, it is
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* able to retrieve the correct nsIInputStream using the UUID and
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* IPCBlobInputStreamStorage.
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*
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* Parent to Child Streams, FileReader and BlobURL
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* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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*
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* The FileReader and BlobURL scenarios are described here.
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*
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* When content process needs to read data from a Blob sent from the parent
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* process, it must do it asynchronously using IPCBlobInputStream as a
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* nsIAsyncInputStream stream. This happens calling
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* IPCBlobInputStream.asyncWait(). At that point, the child actor will send a
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* StreamNeeded() IPC message to the parent side. When this is received, the
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* parent retrieves the 'real' stream from IPCBlobInputStreamStorage using the
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* UUID, it will serialize the 'real' stream, and it will send it to the child
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* side.
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*
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* When the 'real' stream is received (RecvStreamReady()), the asyncWait
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* callback will be executed and, from that moment, any IPCBlobInputStream
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* method will be forwarded to the 'real' stream ones. This means that the
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* reading will be available.
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*
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* DOM-File Thread
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* ~~~~~~~~~~~~~~~
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*
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* IPCBlobInputStreamChild actor can be created in any thread (sort of) and
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* their top-level IPDL protocol is PBackground. These actors are wrapped by 1
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* or more IPCBlobInputStream objects in order to expose nsIInputStream
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* interface and be thread-safe.
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*
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* But IPDL actors are not thread-safe and any SendFoo() method must be executed
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* on the owning thread. This means that this thread must be kept alive for the
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* life-time of the IPCBlobInputStream.
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*
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* In doing this, there are 2 main issues:
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* a. if a remote Blob is created on a worker (because of a
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* BroadcastChannel/MessagePort for instance) and it sent to the main-thread
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* via PostMessage(), we have to keep that worker alive.
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* b. if the remote Blob is created on the main-thread, any SendFoo() has to be
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* executed on the main-thread. This is true also when the inputStream is
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* used on another thread (note that nsIInputStream could do I/O and usually
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* they are used on special I/O threads).
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*
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* In order to avoid this, IPCBlobInputStreamChild are 'migrated' to a DOM-File
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* thread. This is done in this way:
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*
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* 1. If IPCBlobInputStreamChild actor is not already owned by DOM-File thread,
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* it calls Send__delete__ in order to inform the parent side that we don't
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* need this IPC channel on the current thread.
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* 3. IPCBlobInputStreamParent::Recv__delete__ is called on the parent side and
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* the parent actor is deleted. Doing this we don't remove the UUID from
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* IPCBlobInputStreamStorage.
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* 4. When IPCBlobInputStreamChild::ActorDestroy() is called, we are sure that
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* the IPC channel is completely released. IPCBlobInputStreamThread is be
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* used to assign IPCBlobInputStreamChild actor to the DOM-File thread.
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* IPCBlobInputStreamThread::GetOrCreate() creates the DOM-File thread if it
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* doesn't exist yet and it initializes PBackground on it if needed.
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* 5. IPCBlobInputStreamChild is reused on the DOM-File thread for the creation
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* of a new IPCBlobInputStreamParent actor on the parent side. Doing this,
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* IPCBlobInputStreamChild will now be owned by the DOM-File thread.
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* 6. When the new IPCBlobInputStreamParent actor is created, it will receive
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* the same UUID of the previous parent actor. The nsIInputStream will be
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* retrieved from IPCBlobInputStreamStorage.
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* 7. In order to avoid leaks, IPCBlobInputStreamStorage will monitor child
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* processes and in case one of them dies, it will release the
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* nsIInputStream objects belonging to that process.
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*
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* If any API wants to retrieve a 'real inputStream when the migration is in
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* progress, that operation is stored in a pending queue and processed at the
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* end of the migration.
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*/
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namespace mozilla {
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namespace ipc {
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class PBackgroundChild;
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class PBackgroundParent;
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}
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namespace dom {
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class IPCBlob;
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class nsIContentChild;
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class nsIContentParent;
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namespace IPCBlobUtils {
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already_AddRefed<BlobImpl>
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Deserialize(const IPCBlob& aIPCBlob);
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// These 4 methods serialize aBlobImpl into aIPCBlob using the right manager.
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nsresult
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Serialize(BlobImpl* aBlobImpl, nsIContentChild* aManager, IPCBlob& aIPCBlob);
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nsresult
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Serialize(BlobImpl* aBlobImpl, mozilla::ipc::PBackgroundChild* aManager,
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IPCBlob& aIPCBlob);
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nsresult
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Serialize(BlobImpl* aBlobImpl, nsIContentParent* aManager, IPCBlob& aIPCBlob);
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nsresult
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Serialize(BlobImpl* aBlobImpl, mozilla::ipc::PBackgroundParent* aManager,
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IPCBlob& aIPCBlob);
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} // IPCBlobUtils
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} // dom namespace
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} // mozilla namespace
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#endif // mozilla_dom_IPCBlobUtils_h
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