Reading from a GConverterInputStream with both input_buffer and
converted_buffer non-empty would return bogus data (the data from
converted_buffer would essentially get skipped over, though the
returned nread reflected what the count would be if it hadn't been).
This was never noticed before because (a) it can't happen if all of
your reads are at least as large as either the internal buffer size or
the remaining length of the stream (which covers most real-world use),
and (b) it can't happen if all of your reads are 1 byte (which covers
most of tests/converter-test). (And (c) it only happens for some
converters/input streams.) But this was happening occasionally in
libsoup when content-sniffing a gzipped response, because the
SoupContentSnifferStream would first read 512 bytes (to sniff), and
then pass through larger reads after that.
Fixed and added a test to converter-test.
https://bugzilla.gnome.org/show_bug.cgi?id=676478
Implement GPollableInputStream in GMemoryInputStream and
GConverterInputStream, and likewise implement GPollableOutputStream in
the corresponding output streams.
https://bugzilla.gnome.org/show_bug.cgi?id=673997
The loop was using a GConverterResult variable where it meant to use a
gssize, and since GConverterResult was ending up as an unsigned type,
this meant the (res < 0) check always failed.
The name buffer_availabile was kinda confusing, so its been renamed
to buffer_data_size() to match buffer_data().
Also I added a comment to buffer_ensure_space because its behaviour
wasn't obvious.
