Sergey Bugaev f03e452085 LibC: Sync file position when dropping read ahead buffer ...

When we flush a FILE, we behave differently depending on whether we reading from
the file or writing to it:

* If we're writing, we actually write out the buffered data.
* If we're reading, we just drop the buffered (read ahead) data.

After flushing, there should be no additional buffered state stdio keeps about a
FILE, compared to what is true about the underlying file. This includes file
position (offset). When flushing writes, this is taken care of automatically,
but dropping the buffer is not enough to achieve that when reading. This commit
fixes that by seeking back explicitly in that case.

One way the problem manifested itself was upon fseek(SEEK_CUR) calls, as the
position of the underlying file was oftentimes different to the logical position
of the FILE. Since FILE::seek() already calls FILE::flush() prior to actually
modifying the position, fixing FILE::flush() to sync the positions is enough to
fix that issue.

2020-05-22 18:58:36 +02:00

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