Auto merge of #2517 - micbou:remove-healthy-check, r=Valloric

[READY] Remove healthy check

This PR removes the code that, if the server is not yet healthy, tries in a separate thread to send the same request at different intervals until it gets a response or reaches 5 failed attempts.

Main issue with that code is that it blocks Vim if a synchronous request (`semantic_completion_available`, `run_completer_command`, etc.)  is sent while the server is not up. See issue #2071.

Other issues are that it's hard to understand how this code behaves (it combines threads and retries), it becomes useless once the server is healthy (because `SERVER_HEALTHY` is cached and never reset), it depends on arbitrary parameters (why 5 retries with a delay of 0.5s multiplied by 1.5 at each attempt?), and it has a package dependency `retries`.

Now that we catch all server exceptions (PR #2453) and define a very short connection timeout (PR #2514), we can just drop this code.

This change gives a huge improvement in startup time on Windows (don't worry, more improvements are coming for all platforms). The reason is that `requests` (probably `urllib3`) tries for ~1s to check if the server is healthy on Windows while it's almost instantaneous on other platforms.
<table>
  <tr>
    <th rowspan="2">Platform</th>
    <th colspan="2">First run (ms)</th>
    <th colspan="2">Subsequent runs (ms)</th>
  </tr>
  <tr>
    <td>Before</td>
    <td>After</td>
    <td>Before</td>
    <td>After</td>
  </tr>
  <tr>
    <td>Ubuntu 16.04 64-bit</td>
    <td>228</td>
    <td>248</td>
    <td>176</td>
    <td>151</td>
  </tr>
  <tr>
    <td>macOS 10.12</td>
    <td>433</td>
    <td>417</td>
    <td>263</td>
    <td>263</td>
  </tr>
  <tr>
    <td>Windows 10 64-bit</td>
    <td>1861</td>
    <td>844</td>
    <td>814</td>
    <td>292</td>
  </tr>
</table>

These results were obtained by running the `prof.py` script from [this branch](https://github.com/micbou/YouCompleteMe/tree/profiling-startup). The difference between first run and subsequent runs is Python bytecode generation (`*.pyc` files).

Fixes #2071.

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python/ycm/client/base_request.py

@@ -30,7 +30,6 @@
 import json
 from future.utils import native
 from base64 import b64decode, b64encode
-from retries import retries
 from requests_futures.sessions import FuturesSession
 from ycm.unsafe_thread_pool_executor import UnsafeThreadPoolExecutor
 from ycm import vimsupport
@@ -102,43 +101,20 @@ def _TalkToHandlerAsync( data,
                            handler,
                            method,
                            timeout = _READ_TIMEOUT_SEC ):
-    def SendRequest( data, handler, method, timeout ):
-      request_uri = _BuildUri( handler )
-      if method == 'POST':
-        sent_data = _ToUtf8Json( data )
-        return BaseRequest.session.post(
-            request_uri,
-            data = sent_data,
-            headers = BaseRequest._ExtraHeaders( method,
-                                                 request_uri,
-                                                 sent_data ),
-            timeout = ( _CONNECT_TIMEOUT_SEC, timeout ) )
-      if method == 'GET':
-        return BaseRequest.session.get(
-            request_uri,
-            headers = BaseRequest._ExtraHeaders( method, request_uri ),
-            timeout = ( _CONNECT_TIMEOUT_SEC, timeout ) )
-
-    @retries( 5, delay = 0.5, backoff = 1.5 )
-    def DelayedSendRequest( data, handler, method ):
-      request_uri = _BuildUri( handler )
-      if method == 'POST':
-        sent_data = _ToUtf8Json( data )
-        return requests.post(
-            request_uri,
-            data = sent_data,
-            headers = BaseRequest._ExtraHeaders( method,
-                                                 request_uri,
-                                                 sent_data ) )
-      if method == 'GET':
-        return requests.get(
-            request_uri,
-            headers = BaseRequest._ExtraHeaders( method, request_uri ) )
-
-    if not _CheckServerIsHealthyWithCache():
-      return _EXECUTOR.submit( DelayedSendRequest, data, handler, method )
-
-    return SendRequest( data, handler, method, timeout )
+    request_uri = _BuildUri( handler )
+    if method == 'POST':
+      sent_data = _ToUtf8Json( data )
+      return BaseRequest.session.post(
+          request_uri,
+          data = sent_data,
+          headers = BaseRequest._ExtraHeaders( method,
+                                               request_uri,
+                                               sent_data ),
+          timeout = ( _CONNECT_TIMEOUT_SEC, timeout ) )
+    return BaseRequest.session.get(
+        request_uri,
+        headers = BaseRequest._ExtraHeaders( method, request_uri ),
+        timeout = ( _CONNECT_TIMEOUT_SEC, timeout ) )
 
 
   @staticmethod
@@ -271,31 +247,6 @@ def _BuildUri( handler ):
                                                 handler ) ) )
 
 
-SERVER_HEALTHY = False
-
-
-def _CheckServerIsHealthyWithCache():
-  global SERVER_HEALTHY
-
-  def _ServerIsHealthy():
-    request_uri = _BuildUri( 'healthy' )
-    response = requests.get( request_uri,
-                             headers = BaseRequest._ExtraHeaders(
-                                 'GET', request_uri, bytes( b'' ) ) )
-    _ValidateResponseObject( response )
-    response.raise_for_status()
-    return response.json()
-
-  if SERVER_HEALTHY:
-    return True
-
-  try:
-    SERVER_HEALTHY = _ServerIsHealthy()
-    return SERVER_HEALTHY
-  except:
-    return False
-
-
 def MakeServerException( data ):
   if data[ 'exception' ][ 'TYPE' ] == UnknownExtraConf.__name__:
     return UnknownExtraConf( data[ 'exception' ][ 'extra_conf_file' ] )