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LibPDF: Improve stream parsing

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When parsing streams we rely on a /Length item being defined in the
stream's dictionary to know how much data comprises the stream. Its
value is usually a direct value, but it can be indirect. There was
however a contradiction in the code: the condition that allowed it to
read and use the /Length value required it to be a direct value, but the
actual code using the value would have worked with indirect ones. This
meant that indirect /Length values triggered the fallback, "manual"
stream parsing code.

On the other hand, this latter code was also buggy, because it relied on
the "endstream" keyword to appear on a separate line, which isn't always
the case.

This commit both fixes the bug in the manual stream parsing scenario,
while also allowing for indirect /Length values to be used to parse
streams more directly and avoid the manual approach. The main caveat to
this second change is that for a brief period of time the Document is
not able to resolve references (i.e., before the xref table itself is
not parsed). Any parsing happening before that (e..g, the linearization
dictionary) must therefore use the manual stream parsing approach.
This commit is contained in:
Rodrigo Tobar 2023-02-06 00:05:33 +08:00 • committed by Andreas Kling
parent 8670526f2a
commit a533ea7ae6
3 changed files with 14 additions and 11 deletions
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18
Userland/Libraries/LibPDF/Parser.cpp
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@ -446,7 +446,7 @@ PDFErrorOr<NonnullRefPtr<StreamObject>> Parser::parse_stream(NonnullRefPtr<DictO
ReadonlyBytes bytes;
auto maybe_length = dict->get(CommonNames::Length);
if (maybe_length.has_value() && (!maybe_length->has<Reference>())) {
if (maybe_length.has_value() && m_document->can_resolve_refefences()) {
// The PDF writer has kindly provided us with the direct length of the stream
m_reader.save();
auto length = TRY(m_document->resolve_to<int>(maybe_length.value()));
@ -457,17 +457,13 @@ PDFErrorOr<NonnullRefPtr<StreamObject>> Parser::parse_stream(NonnullRefPtr<DictO
} else {
// We have to look for the endstream keyword
auto stream_start = m_reader.offset();
while (true) {
m_reader.move_until([&](auto) { return m_reader.matches_eol(); });
auto potential_stream_end = m_reader.offset();
m_reader.consume_eol();
if (!m_reader.matches("endstream"))
continue;
bytes = m_reader.bytes().slice(stream_start, potential_stream_end - stream_start);
break;
while (!m_reader.matches("endstream")) {
m_reader.consume();
m_reader.move_until('e');
}
auto stream_end = m_reader.offset();
m_reader.consume_eol();
bytes = m_reader.bytes().slice(stream_start, stream_end - stream_start);
}
m_reader.move_by(9);