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Tidy Objects::read

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m-holger 2024-10-14 11:12:14 +01:00
parent 6408c5c364
commit 486e68f2e5
1 changed files with 55 additions and 56 deletions
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111
libqpdf/QPDF_objects.cc
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@ -1489,64 +1489,63 @@ Objects::read(
} }
QPDFObjectHandle oh = read_object(description, og); QPDFObjectHandle oh = read_object(description, og);
if (!unresolved(og)) {
if (unresolved(og)) { return oh;
// Store the object in the cache here so it gets cached whether we first know the offset or
// whether we first know the object ID and generation (in which we case we would get here
// through resolve).
// Determine the end offset of this object before and after white space. We use these
// numbers to validate linearization hint tables. Offsets and lengths of objects may imply
// the end of an object to be anywhere between these values.
qpdf_offset_t end_before_space = m->file->tell();
// skip over spaces
while (true) {
char ch;
if (m->file->read(&ch, 1)) {
if (!isspace(static_cast<unsigned char>(ch))) {
m->file->seek(-1, SEEK_CUR);
break;
}
} else {
throw qpdf.damagedPDF(m->file->tell(), "EOF after endobj");
}
}
qpdf_offset_t end_after_space = m->file->tell();
if (skip_cache_if_in_xref && xref.type(og)) {
// Ordinarily, an object gets read here when resolved through xref table or stream. In
// the special case of the xref stream and linearization hint tables, the offset comes
// from another source. For the specific case of xref streams, the xref stream is read
// and loaded into the object cache very early in parsing. Ordinarily, when a file is
// updated by appending, items inserted into the xref table in later updates take
// precedence over earlier items. In the special case of reusing the object number
// previously used as the xref stream, we have the following order of events:
//
// * reused object gets loaded into the xref table
// * old object is read here while reading xref streams
// * original xref entry is ignored (since already in xref table)
//
// It is the second step that causes a problem. Even though the xref table is correct in
// this case, the old object is already in the cache and so effectively prevails over
// the reused object. To work around this issue, we have a special case for the xref
// stream (via the skip_cache_if_in_xref): if the object is already in the xref stream,
// don't cache what we read here.
//
// It is likely that the same bug may exist for linearization hint tables, but the
// existing code uses end_before_space and end_after_space from the cache, so fixing
// that would require more significant rework. The chances of a linearization hint
// stream being reused seems smaller because the xref stream is probably the highest
// object in the file and the linearization hint stream would be some random place in
// the middle, so I'm leaving that bug unfixed for now. If the bug were to be fixed, we
// could use !check_og in place of skip_cache_if_in_xref.
QTC::TC("qpdf", "QPDF skipping cache for known unchecked object");
} else {
xref.linearization_offsets(toS(og.getObj()), end_before_space, end_after_space);
update_table(og.getObj(), og.getGen(), oh.getObj());
}
} }
return oh; // Store the object in the cache here so it gets cached whether we first know the offset or
// whether we first know the object ID and generation (in which we case we would get here
// through resolve).
// Determine the end offset of this object before and after white space. We use these numbers
// to validate linearization hint tables. Offsets and lengths of objects may imply the end of
// an object to be anywhere between these values.
qpdf_offset_t end_before_space = m->file->tell();
// skip over spaces
while (true) {
char ch;
if (m->file->read(&ch, 1)) {
if (!isspace(static_cast<unsigned char>(ch))) {
m->file->seek(-1, SEEK_CUR);
break;
}
} else {
throw qpdf.damagedPDF(m->file->tell(), "EOF after endobj");
}
}
qpdf_offset_t end_after_space = m->file->tell();
if (skip_cache_if_in_xref && xref.type(og)) {
// Ordinarily, an object gets read here when resolved through xref table or stream. In the
// special case of the xref stream and linearization hint tables, the offset comes from
// another source. For the specific case of xref streams, the xref stream is read and loaded
// into the object cache very early in parsing. Ordinarily, when a file is updated by
// appending, items inserted into the xref table in later updates take precedence over
// earlier items. In the special case of reusing the object number previously used as the
// xref stream, we have the following order of events:
//
// * reused object gets loaded into the xref table
// * old object is read here while reading xref streams
// * original xref entry is ignored (since already in xref table)
//
// It is the second step that causes a problem. Even though the xref table is correct in
// this case, the old object is already in the cache and so effectively prevails over the
// reused object. To work around this issue, we have a special case for the xref stream (via
// the skip_cache_if_in_xref): if the object is already in the xref stream, don't cache what
// we read here.
//
// It is likely that the same bug may exist for linearization hint tables, but the existing
// code uses end_before_space and end_after_space from the cache, so fixing that would
// require more significant rework. The chances of a linearization hint stream being reused
// seems smaller because the xref stream is probably the highest object in the file and the
// linearization hint stream would be some random place in the middle, so I'm leaving that
// bug unfixed for now. If the bug were to be fixed, we could use !check_og in place of
// skip_cache_if_in_xref.
QTC::TC("qpdf", "QPDF skipping cache for known unchecked object");
return oh;
}
xref.linearization_offsets(toS(og.getObj()), end_before_space, end_after_space);
return {update_table(og.getObj(), og.getGen(), oh.getObj())};
} }
QPDFObject* QPDFObject*