page: add caching

[fhs, 2009-08-09 19:52:36]

The qlock in cache.c was necessary because moving through pages really fast
(hold down right/left arrow key) results in this:

% page net.pdf
reading through pdf...
bmp: bad magic number, not a BMP file
warning: couldn't read image: Success
fatal: readimage error Success
% 7673: signal: sys: write on closed pipe

I guess ghostscript is outputting nonsense when we try to draw two pages at
nearly the same time.

[russcox_gmail.com, 2009-08-09 20:41:11]

thanks very much for doing this.

the qlock/qunlock is not enough.  All the graphics ops
need to be locked between the two procs.  The way to 
do this is to set display->locking = 1 in main and
then use lockdisplay/unlockdisplay in raproc around
the call to _cachedpage.  Similarly, instead of

switch(alt(alts)) {

I think you need

unlockdisplay();
a = alt(alts);
lockdisplay();
switch(a) {

in view.c.  that will unlock the display while
waiting for input, so that the readahead proc
can do things instead.

look at the calls to lockdisplay and unlockdisplay
in the plan 9 page for more examples: they were
added explicitly for the cache.

[russcox_gmail.com, 2009-08-10 00:15:29]

codebot: apply

page: add caching from Plan 9

[old.codebot, 2009-08-10 00:16:38]

# codebot: apply

applied.

[fhs, 2009-08-10 02:43:53]

On 2009/08/10 00:16:38, codebot wrote:
> # codebot: apply
> 
> applied.

Looks like cache.c got added to the wrong place: cmd/page/cache.c instead of
src/cmd/page/cache.c. The patch had the correct path.