When creating a CID parser the location of the 'StartData' or '/sfnts'
tokens needs to be known. However, the token parser requires that the
entire document be in memory and flattening the entire stream into memory is
to be avoided.
To avoid forcing the entire stream into memory, previously this code would
scan through the stream looking for 'StartData' or '/sfnts' as strings.
However, these strings could have been in a comment or string token, so the
stream would be read into memory up to that point and the parser run to
check that these strings were actually tokens. This forced a parser restart
from the beginning each time; as a result, data with many 'StartData'
non-tokens would take n^2 time to check.
* src/cid/cidparse.c (cid_parser_new): Change algorithm to make the initial
scan look for the last possible 'StartData' or '/sfnts' string in the
stream. The stream is read forward instead of backward as a typical normal
CID font will have one 'StartData' toward the beginning of the data and it
it much faster to read the data from beginning to end instead of end to
beginning. For memory-based fonts the limit is set to the end of the stream
since the stream is already in memory. Then the parser is run once to look
for 'StartData' or '/sfnts' tokens. If they are found the parser is re-set
to reflect this new information.
Reported as
https://issues.chromium.org/issues/40201695
17545d4bf7