How to do progressive loading with MuPDF.
=========================================

What is progressive loading?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The idea of progressive loading is that as you download a PDF file
into a browser, you can display the pages as they appear.

MuPDF can make use of 2 different mechanisms to achieve this. The
first relies on the file being "linearized", the second relies on
the caller of MuPDF having fine control over the http fetch and on
the server supporting byte-range fetches.

For optimum performance a file should be both linearized and be
available over a byte-range supporting link, but benefits can still
be had with either one of these alone.

Progressive download using "linearized" files
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Adobe defines "linearized" PDFs as being ones that have both a
specific layout of objects and a small amount of extra
information to help avoid seeking within a file. The stated aim
is to deliver the first page of a document in advance of the whole
document downloading, whereupon subsequent pages will become
available. Adobe also refers to these as "Optimized for fast web
view" or "Web Optimized".

In fact, the standard outlines (poorly) a mechanism by which 'hints'
can be included that enable the subsequent pages to be found within
the file too. Unfortunately this is very poorly supported with
many tools, and so the hints have to be treated with suspicion.

MuPDF will attempt to use hints if they are available, but will also
use a linear search of the file to discover pages if not. This means
that the first page will be displayed quickly, and then subsequent
ones will appear with 'incomplete' renderings that improve over time
as more and more resources are gradually delivered.

Essentially the file starts with a slightly modified header, and the
first object in the file is a special one (the linearization object)
that a) indicates that the file is linearized, and b) gives some
useful information (like the number of pages in the file etc).

This object is then followed by all the objects required for the
first page, then the "hint stream", then sets of object for each
subsequent page in turn, then shared objects required for those
pages, then various other random things.

[Yes, really. While page 1 is sent with all the objects that it
uses, shared or otherwise, subsequent pages do not get shared
resources until after all the unshared page objects have been
sent.]

The Hint Stream
~~~~~~~~~~~~~~~

Adobe intended Hint Stream to be useful to facilitate the display
of subsequent pages, but it has never used it. Consequently you
can't trust people to write it properly - indeed Adobe outputs
something that doesn't quite conform to the spec.

Consequently very few people actually use it. MuPDF will use it
after sanity checking the values, and should cope with illegal/
incorrect values.

So how does MuPDF handle progressive loading?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

MuPDF has made various extensions to its mechanisms for handling
progressive loading.

  + Progressive streams

    At its lowest level MuPDF reads file data from a fz_stream,
    using the fz_open_document_with_stream call. (fz_open_document
    is implemented by calling this). We have extended the fz_stream
    slightly, giving the system a way to ask for meta information
    (or perform meta operations) on a stream.

    Using this mechanism MuPDF can query:

      + whether a stream is progressive or not (i.e. whether the
        entire stream is accessible immediately)
      + what the length of a stream should ultimately be (which an
        http fetcher should know from the Content-Length header),

    With this information MuPDF can decide whether to use its normal
    object reading code, or whether to make use of a linearized
    object. Knowing the length enables us to check with the length
    value given in the linearized object - if these differ, the
    assumption is that an incremental save has taken place, thus the
    file is no longer linearized.

    When data is pulled from a progressive stream, if we attempt to
    read data that is not currently available, the stream should
    throw a FZ_ERROR_TRYLATER error. This particular error code
    will be interpreted by the caller as an indication that it
    should retry the parsing of the current objects at a later time.

    When a MuPDF call is made on a progressive stream, such as
    fz_open_document_with_stream, or fz_load_page, the caller should
    be prepared to handle a FZ_ERROR_TRYLATER error as meaning that
    more data is required before it can continue. No indication is
    directly given as to exactly how much more data is required, but
    as the caller will be implementing the progressive fz_stream
    that it has passed into MuPDF to start with, it can reasonably
    be expected to figure out an estimate for itself.

  + Cookie

    Once a page has been loaded, if its contents are to be 'run'
    as normal (using e.g. fz_run_page) any error (such as failing
    to read a font, or an image, or even a content stream belonging
    to the page) will result in a rendering that aborts with an
    FZ_ERROR_TRYLATER error. The caller can catch this and display
    a placeholder instead.

    If each pages data was entirely self-contained and sent in
    sequence this would perhaps be acceptable, with each page
    appearing one after the other. Unfortunately, the linearization
    procedure as laid down by Adobe does NOT do this: objects shared
    between multiple pages (other than the first) are not sent with
    the pages themselves, but rather AFTER all the pages have been
    sent.

    This means that a document that has a title page, then contents
    that share a font used on pages 2 onwards, will not be able to
    correctly display page 2 until after the font has arrived in
    the file, which will not be until all the page data has been
    sent.

    To mitigate against this, MuPDF provides a way whereby callers
    can indicate that they are prepared to accept an 'incomplete'
    rendering of the file (perhaps with missing images, or with
    substitute fonts).

    Callers prepared to tolerate such renderings should set the
    'incomplete_ok' flag in the cookie, then call fz_run_page etc
    as normal. If a FZ_ERROR_TRYLATER error is thrown at any point
    during the page rendering, the error will be swallowed, the
    'incomplete' field in the cookie will become non-zero and
    rendering will continue. When control returns to the caller
    the caller can check the value of the 'incomplete' field and
    know that the rendering it received is not authoritative.

Progressive loading using byte range requests
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If the caller has control over the http fetch, then it is possible
to use byte range requests to fetch the document 'out of order'.
This enables non-linearized files to be progressively displayed as
they download, and fetches complete renderings of pages earlier than
would otherwise be the case. This process requires no changes within
MuPDF itself, but rather in the way the progressive stream learns
from the attempts MuPDF makes to fetch data.

Consider for example, an attempt to fetch a hypothetical file from
a server.

 + The initial http request for the document is sent with a "Range:"
   header to pull down the first (say) 4k of the file.

 + As soon as we get the header in from this initial request, we can
   respond to meta stream operations to give the length, and whether
   byte requests are accepted.

   - If the header indicates that byte ranges are acceptable the
     stream proceeds to go into a loop fetching chunks of the file
     at a time (not necessarily in-order). Otherwise the server
     will ignore the Range: header, and just serve the whole file.

   - If the header indicates a content-length, the stream returns
     that.

 + MuPDF can then decide how to proceed based upon these flags and
   whether the file is linearized or not. (If the file contains a
   linearized object, and the content length matches, then the file
   is considered to be linear, otherwise it is not).

   If the file is linear:

   - we proceed to read objects out of the file as it downloads.
     This will provide us the first page and all its resources. It
     will also enable us to read the hint streams (if present).

   - Once we have read the hint streams, we unpack (and sanity
     check) them to give us a map of where in the file each object
     is predicted to live, and which objects are required for each
     page. If any of these values are out of range, we treat the
     file as if there were no hint streams.

   - If we have hints, any attempt to load a subsequent page will
     cause MuPDF to attempt to read exactly the objects required.
     This will cause a sequence of seeks in the fz_stream followed
     by reads. If the stream does not have the data to satisfy that
     request yet, the stream code should remember the location that
     was fetched (and fetch that block in the background so that
     future retries will succeed) and should raise an
     FZ_ERROR_TRYLATER error.

     [Typically therefore when we jump to a page in a linear file
     on a byte request capable link, we will quickly see a rough
     rendering, which will improve fairly fast as images and fonts
     arrive.]

   - Regardless of whether we have hints or byte requests, on every
     fz_load_page call MuPDF will attempt to process more of the
     stream (that is assumed to be being downloaded in the
     background). As linearized files are guaranteed to have pages
     in order, pages will gradually become available. In the absence
     of byte requests and hints however, we have no way of getting
     resources early, so the renderings for these pages will remain
     incomplete until much more of the file has arrived.

     [Typically therefore when we jump to a page in a linear file
     on a non byte request capable link, we will see a rough
     rendering for that page as soon as data arrives for it (which
     will typically take much longer than would be the case with
     byte range capable downloads), and that will improve much more
     slowly as images and fonts may not appear until almost the
     whole file has arrived.]

   - When the whole file has arrived, then we will attempt to read
     the outlines for the file.

   For a non-linearized PDF on a byte request capable stream:

   - MuPDF will immediately seek to the end of the file to attempt
     to read the trailer. This will fail with a FZ_ERROR_TRYLATER
     due to the data not being here yet, but the stream code should
     remember that this data is required and it should be prioritized
     in the background fetch process.

   - Repeated attempts to open the stream should eventually succeed
     therefore. As MuPDF jumps through the file trying to read first
     the xrefs, then the page tree objects, then the page contents
     themselves etc, the background fetching process will be driven
     by the attempts to read the file in the foreground.

     [Typically therefore the opening of a non-linearized file will
     be slower than a linearized one, as the xrefs/page trees for a
     non-linear file can be 20%+ of the file data. Once past this
     initial point however, pages and data can be pulled from the
     file almost as fast as with a linearized file.]

   For a non-linearized PDF on a non-byte request capable stream:

   - MuPDF will immediately seek to the end of the file to attempt
     to read the trailer. This will fail with a FZ_ERROR_TRYLATER
     due to the data not being here yet. Subsequent retries will
     continue to fail until the whole file has arrived, whereupon
     the whole file will be instantly available.

     [This is the worst case situation - nothing at all can be
     displayed until the entire file has downloaded.]

  A typical structure for a fetcher process (see curl-stream.c in
  mupdf-curl as an example) might therefore look like this:

 + We consider the file as an (initially empty) buffer which we are
   filling by making requests. In order to ensure that we make
   maximum use of our download link, we ensure that whenever
   one request finishes, we immediately launch another. Further, to
   avoid the overheads for the request/response headers being too
   large, we may want to divide the file into 'chunks', perhaps 4 or 32k
   in size.

 + We can then have a receiver process that sits there in a loop
   requesting chunks to fill this buffer. In the absence of
   any other impetus the receiver should request the next 'chunk'
   of data from the file that it does not yet have, following the last
   fill point. Initially we start the fill point at the beginning of
   the file, but this will move around based on the requests made of
   the progressive stream.

 + Whenever MuPDF attempts to read from the stream, we check to see if
   we have data for this area of the file already. If we do, we can
   return it. If not, we remember this as the next "fill point" for our
   receiver process and throw a FZ_ERROR_TRYLATER error.
