Support HealtchCheck on tcp layer，avoid to produce needless http access log

README.markdown

@@ -70,8 +70,20 @@ http {
             ngx.log(ngx.ERR, "failed to spawn health checker: ", err)
             return
         end
+
+        -- Example of HealthCheck for Tcp
+        local ok, err = hc.spawn_checker{
+            shm = "healthcheck",  -- defined by "lua_shared_dict"
+            upstream = "foo.com", -- defined by "upstream"
+            type = "tcp",
+            interval = 2000,  -- run the check cycle every 2 sec
+            timeout = 1000,   -- 1 sec is the timeout for network operations
+            fall = 3,  -- # of successive failures before turning a peer down
+            rise = 2,  -- # of successive successes before turning a peer up
+            concurrency = 10,  -- concurrency level for test requests
+        }
 
-        -- Just call hc.spawn_checker() for more times here if you have
+        -- Just call hc.spawn_checker() for more times here if you have
         -- more upstream groups to monitor. One call for one upstream group.
         -- They can all share the same shm zone without conflicts but they
         -- need a bigger shm zone for obvious reasons.

lib/resty/upstream/healthcheck.lua

@@ -228,6 +228,12 @@ local function check_peer(ctx, id, peer, is_backup)
             errlog("failed to connect to ", name, ": ", err)
         end
         return peer_fail(ctx, is_backup, id, peer)
+    else 
+        if ctx.type == "tcp" then
+           peer_ok(ctx, is_backup, id, peer)
+           sock:close()
+           return
+        end    
     end
 
     local bytes, err = sock:send(req)
@@ -524,12 +530,12 @@ function _M.spawn_checker(opts)
         return nil, "\"type\" option required"
     end
 
-    if typ ~= "http" then
-        return nil, "only \"http\" type is supported right now"
+    if typ ~= "http" and typ ~= "tcp" then
+        return nil, "only \"http\" or \"tcp\" type is supported right now"
     end
 
     local http_req = opts.http_req
-    if not http_req then
+    if typ == "http" and not http_req then
         return nil, "\"http_req\" option required"
     end
 
@@ -602,6 +608,7 @@ function _M.spawn_checker(opts)
     end
 
     local ctx = {
+        type = typ,
         upstream = u,
         primary_peers = preprocess_peers(ppeers),
         backup_peers = preprocess_peers(bpeers),