Advertise a minimum alignment for SharedMemory::Map().

Advertise a minimum alignment for SharedMemory::Map().

No real changes, just adds a DCHECK() and unit test for a minimum
alignment that callers can count on from Map().  Map() is already
returning addresses which are aligned to the platform page size.

Necessary for a media use case in which a wrapped shared memory
object is passed around and is expected to have a certain alignment.

BUG=none
TEST=unittests, try bots.


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=152218

[DaleCurtis, 2012-08-17 21:07:04]

jar: I figured you are most likely to know how sane this change is.  This is the
result of discussion here:

https://groups.google.com/a/chromium.org/d/topic/chromium-dev/VzuISrCbqeI/dis...

[jar (doing other things), 2012-08-17 21:32:32]

This seems plausible... but I don't think this is a definitive test.  IMO, if
you really want it to be so, you should CHECK in your media calls, as the cost
is small, and the risk is presumably high.

That said, I *think* that mmap's definition of "map memory PAGES ..." suggests
it is indeed page aligned.  I also found restructions on calls to munmap(),
requiring a page aligned argument... which presumably came straight from mmap().
 I'm pretty sure it is true... but it is nerve-rackingly understated.

Please fix the nit below.... then LGTM

https://chromiumcodereview.appspot.com/10827400/diff/1/base/shared_memory.h
File base/shared_memory.h (right):

https://chromiumcodereview.appspot.com/10827400/diff/1/base/shared_memory.h#n...
base/shared_memory.h:148: enum { kMapMinimumAlignment = 32 };
nit: use MACRO_STYLE MAP_MINIMUM_ALIGNMENT  (this is a Chromium style issue).

[DaleCurtis, 2012-08-17 21:43:46]

Thanks for review! I will indeed be CHECK() this assumption in the calling code.

https://chromiumcodereview.appspot.com/10827400/diff/1/base/shared_memory.h
File base/shared_memory.h (right):

https://chromiumcodereview.appspot.com/10827400/diff/1/base/shared_memory.h#n...
base/shared_memory.h:148: enum { kMapMinimumAlignment = 32 };
On 2012/08/17 21:32:32, jar wrote:
> nit: use MACRO_STYLE MAP_MINIMUM_ALIGNMENT  (this is a Chromium style issue).

Done.

[commit-bot: I haz the power, 2012-08-17 21:46:23]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/dalecurtis@chromium.org/10827400/7001

[commit-bot: I haz the power, 2012-08-18 02:17:28]

Change committed as 152218