network: response stacking for all commands

This change refactors most of memcached's networking frontend code. The
goals with this change:

- Decouple buffer memory from client connections where plausible,
reducing memory usage for currently inactive TCP sessions.
- Allow easy response stacking for all protocols. Previously every
binary command generates a syscall during response. This is no longer
true when multiple commands are read off the wire at once. The new meta
protocol and most text protocol commands are similar.
- Reduce code complexity for network handling. Remove automatic buffer
adjustments, error checking, and so on.

This is accomplished by removing the iovec, msg, and "write buffer"
structures from connection objects. A `mc_resp` object is now used to
represent an individual response. These objects contain small iovec's
and enough detail to be late-combined on transmit. As a side effect,
UDP mode now works with extstore :)

Adding to the iovec always had a remote chance of memory failure, so
every call had to be checked for an error. Now once a response object
is allocated, most manipulations can happen without any checking. This
is both a boost to code robustness and performance for some hot paths.

This patch is the first in a series, focusing on the client response.

1 files changed, 2 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index a602550..16fc0a7 100644
--- a/cache.h
+++ b/cache.h
@@ -55,6 +55,8 @@ typedef struct {
     size_t bufsize;
     /** The capacity of the list of elements */
     int freetotal;
+    /** Total malloc'ed objects */
+    int total;
     /** The current number of free elements */
     int freecurr;
     /** The constructor to be called each time we allocate more memory */