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When we meet a broken PDF file, we attempt to repair it. We do this by
reading tokens from the file and attempting to interpret them as a
normal PDF stream.
Unfortunately, if the file is corrupt enough so that we start to read
from the middle of a stream, and we happen to hit an '(' character,
we can go into string reading mode. We can then end up skipping over
vast swathes of file that we could otherwise repair.
We fix this here by using a new version of the pdf_lex function that
refuses to ever return a string. This means we may take more time
over skipping things than we did before, but are less likely to
skip stuff.
We also tweak other parts of the pdf repair logic here. If we hit a
badly formed piece of data, clear the num/gen we have stored so that
the next plausible piece we get does not get assigned to a random
object number.
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The 0 null object is leaked if a document refers to 0 0 obj before
requiring a delayed reparation (seen e.g. with 3324.pdf.asan.3.2585).
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At https://code.google.com/p/sumatrapdf/issues/detail?id=2436 , there's
a document with an empty xref section which since recently causes a
repair to be triggered. Repairs then stop when pdf_repair_obj_stms fails
on an object which isn't even required for the document to render. Such
broken object streams should rather be ignored same as broken objects
are ignored in pdf_init_document.
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Currently, if we spot a bad xref as we are reading a PDF in, we can
repair that PDF by doing a long exhaustive read of the file. This
reconstructs the information that was in the xref, and the file can
be opened (and later saved) as normal.
If we hit an object that is not in the expected place however, we
cannot trigger a repair at that point - so xrefs with duff offsets
in (within the bounds of the file) will never be repaired.
This commit solves that by triggering a repair (just once) whenever
we fail to parse an object in the expected place.
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fz_read used to return a negative value on errors. With the
introduction of fz_try/fz_catch, it throws an error instead and
always returns non-negative values. This removes the pointless
checks.
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We are testing this using a new -p flag to mupdf that sets a bitrate at
which data will appear to arrive progressively as time goes on. For
example:
mupdf -p 102400 pdf_reference17.pdf
Details of the scheme used here are presented in docs/progressive.txt
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For historical reasons lots of the code uses "xref" when talking about
a pdf document. Now pdf_xref is a separate type this has become
confusing, so replace 'xref' with 'doc' for clarity.
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Remove the fz_context field to avoid the structure growing.
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* at one place, code returns from inside an fz_try which borks up the
error stack
* pdf_load_xref wrongly assumes that at least one non-empty xref has
been read if there were no errors thrown during parsing
* pdf_repair_xref skips integers when object numbers are out of range
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