MachMessageServer: handle allocations more reasonably

MachMessageServer: handle allocations more reasonably.

MachMessageServer was wasteful with allocations for request and reply
messages. It allocated new memory for each request receive and for each
reply send, and if it needed to resize an allocation for a request, it
would maintain two request allocations simultaneously. The new behavior
allocates memory for a new request only if it needs a different size
than for the previous request, and it never maintains two request
allocations simultaneously. Memory for a reply is allocated once per
method invocation and maintained, since this never needs to be resized.

One pass of the loop is now guaranteed, even if a caller specifies a
very small timeout that expires before attempting to receive a message.

An infinite looping bug that could occur when ignoring large messages
has also been fixed.

TEST=util_test MachMessageServer.*
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/49f170e6334865054e617f6e17d72c90f463ed14

[Mark Mentovai, 2014-11-25 23:02:49]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Mark Mentovai, 2014-11-30 21:45:18]

I think you can skip reviewing this as-is, I have a couple of ideas for
simplifying this entire function that I’d rather pursue.

[Robert Sesek, 2014-12-01 20:59:17]

https://codereview.chromium.org/759023004/diff/1/util/mach/mach_message_serve...
File util/mach/mach_message_server.cc (right):

https://codereview.chromium.org/759023004/diff/1/util/mach/mach_message_serve...
util/mach/mach_message_server.cc:149: if (request_scoper.size() !=
this_request_alloc) {
Rather than equality, why not just if this_request_alloc > request_scoper.size()
?

[Mark Mentovai, 2014-12-01 21:04:09]

https://codereview.chromium.org/759023004/diff/1/util/mach/mach_message_serve...
File util/mach/mach_message_server.cc (right):

https://codereview.chromium.org/759023004/diff/1/util/mach/mach_message_serve...
util/mach/mach_message_server.cc:149: if (request_scoper.size() !=
this_request_alloc) {
Robert Sesek wrote:
> Rather than equality, why not just if this_request_alloc >
request_scoper.size()
> ?

I added a comment explaining why.

[Robert Sesek, 2014-12-01 21:09:02]

LGTM

[Mark Mentovai, 2014-12-01 21:13:44]

Committed patchset #2 (id:20001) manually as
49f170e6334865054e617f6e17d72c90f463ed14 (presubmit successful).