Add memcached_exist() function

[ccocchi]

This PR add the function memcached_exist() to libmemcached. This is a port from the function introduced in version 0.53 of libmemcached.

The wrapper expose this method through the exist method, that returns nil in case the key exist or raises Memcached::NotFound is it doesn't.

This is a performance gain for store using memcached gem as client and implementing the exist? method. For example, using this store, here are the results :

Testing with
ruby 1.9.3p125 (2012-02-16 revision 34643) [x86_64-darwin10.8.0]

Before patch:
exist:miss:libm                      0.060000   0.040000   0.100000 (  0.209961)
exist:hit:libm                       0.050000   0.050000   0.100000 (  0.177668)

After patch:
exist:miss:libm                      0.030000   0.030000   0.060000 (  0.114763)
exist:hit:libm                       0.020000   0.030000   0.050000 (  0.105897)

[evan]

Great! Just need a couple things.

• We need to add the exist method to the benchmark script.
• The Memcached::Rails wrapper needs to delegate to your method instead of get_orig.
• Can you verify that your code gives the performance you expect after making the above change?
• Is there any reason to prefer exist() vs. exist?(). We should be consistent.
• We need a CHANGELOG entry for the release.

Thanks!

[ccocchi]

I added the method to the benchmark and to the wrapper, here are the results :

Before
exist-missing: libm:ascii              0.230000   0.200000   0.430000 (  0.875489)
exist-missing: libm:ascii:pipeline     0.240000   0.260000   0.500000 (  0.984436)
exist-missing: libm:ascii:udp          0.190000   0.130000   0.320000 (  0.843025)
exist-missing: libm:bin                0.260000   0.300000   0.560000 (  1.040943)
exist-missing: libm:bin:buffer         0.250000   0.340000   0.590000 (  1.066506)

exist: libm:ascii                      0.200000   0.210000   0.410000 (  0.847553)
exist: libm:ascii:pipeline             0.210000   0.250000   0.460000 (  0.858453)
exist: libm:ascii:udp                  0.160000   0.110000   0.270000 (  0.644295)
exist: libm:bin                        0.200000   0.270000   0.470000 (  0.904462)
exist: libm:bin:buffer                 0.210000   0.310000   0.520000 (  0.958519)

After
exist-missing: libm:ascii              0.120000   0.180000   0.300000 (  0.658207)
exist-missing: libm:ascii:pipeline     0.130000   0.270000   0.400000 (  0.718886)
exist-missing: libm:ascii:udp          0.080000   0.110000   0.190000 (  0.433517)
exist-missing: libm:bin                0.110000   0.190000   0.300000 (  0.633672)
exist-missing: libm:bin:buffer         0.130000   0.260000   0.390000 (  0.710204)

exist: libm:ascii                      0.080000   0.180000   0.260000 (  0.588192)
exist: libm:ascii:pipeline             0.130000   0.260000   0.390000 (  0.668358)
exist: libm:ascii:udp                  0.050000   0.100000   0.150000 (  0.325765)
exist: libm:bin                        0.080000   0.170000   0.250000 (  0.574753)
exist: libm:bin:buffer                 0.060000   0.130000   0.190000 (  0.352050)

About the method name, I chose exist over exist?, because for me, when you suffix your method with an interrogation mark, you expect a boolean response. Memcached#exist only expose memcached_exist from the lib and therefore can raise a NotFound error. Then in the wrapper (or other applications) you can handled this error as you want, returning trusly / falsy values here, and I think that's where the exist? method belongs.

But this is only my point view, it can be changed ;)

[evan]

Looks good to me and I agree with your reasoning about the name.

Add a comment documenting the new method in memcached.rb and I will merge and release.

[ccocchi]

Done