Check data exists before load in safe_load()

lua_eval() routine is used for evaluating lua command on a given
instance. Before run the command it reconnects to the needed instance
and runs it there. After the command run safe_load() is called to parse
the result. Found that some times safe_load() routine may fail with
AttributeError internal exception. It happens, because result value
from running command on the instance was not checked that it was not
empty. To fix it was added check on results value before safe_load()
routine use.

Check the following output from gitlab-ci job [1]:

  DEBUG: sending command: test_run:wait_fullmesh(SERVERS)
  DEBUG: test-run received command: config engine
  DEBUG: test-run's response for [config engine]
   | !!python/unicode 'engine': !!python/unicode 'memtx'
   | ...
  DEBUG: test-run received command: eval autobootstrap1 "return box.info.server"
  DEBUG: test-run's response for [eval autobootstrap1 "return box.info.server"]
   | - id: null
   |   lsn: -1
   |   ro: true
   |   uuid: e84d86bd-d2fc-43c1-b832-8146d1f02cd1
   | ...
  DEBUG: test-run received command: eval autobootstrap1 "return box.info.server"
  DEBUG: test-run's response for [eval autobootstrap1 "return box.info.server"]
   | - id: null
   |   lsn: -1
   |   ro: true
   |   uuid: e84d86bd-d2fc-43c1-b832-8146d1f02cd1
   | ...
  DEBUG: test-run received command: eval autobootstrap1 "return box.info.server"

  TarantoolInpector.handle() received the following error:
  Traceback (most recent call last):
    File "test-run/lib/inspector.py", line 100, in handle
      result = self.parser.parse_preprocessor(line)
    File "test-run/lib/preprocessor.py", line 87, in parse_preprocessor
      return self.lua_eval(name, expr[1:-1])
    File "test-run/lib/preprocessor.py", line 404, in lua_eval
      result = yaml.safe_load(result)
    File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 162, in safe_load
      return load(stream, SafeLoader)
    File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 112, in load
      loader = Loader(stream)
    File "/usr/local/lib/python2.7/site-packages/yaml/loader.py", line 34, in __init__
      Reader.__init__(self, stream)
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 87, in __init__
      self.determine_encoding()
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 126, in determine_encoding
      self.update_raw()
    File "/usr/local/lib/python2.7/site-packages/yaml/reader.py", line 183, in update_raw
      data = self.stream.read(size)
  AttributeError: 'NoneType' object has no attribute 'read'
  DEBUG: test-run's response for [eval autobootstrap1 "return box.info.server"]
   | error: AttributeError("'NoneType' object has no attribute 'read'",)
   | ...
  Kill all servers ...

  [Instance "autobootstrap1" returns with non-zero exit code: 1]

It is seen here that wait_fullmesh() routine waits for the server in
loop and after successfull 2 checks it fails on the 3rd one.

[1] - https://gitlab.com/tarantool/tarantool/-/jobs/878962286

Author: avtikhon

lib/preprocessor.py

@@ -398,9 +398,11 @@ def lua_eval(self, name, expr, silent=True):
                                            'the nonexistent server {0}'.format(
                                             repr(name)))
         self.servers[name].admin.reconnect()
-        result = self.servers[name].admin(
-            '%s%s' % (expr, self.delimiter), silent=silent
-        )
+        result = None
+        while not result:
+            result = self.servers[name].admin(
+                '%s%s' % (expr, self.delimiter), silent=silent
+            )
         result = yaml.safe_load(result)
         if not result:
             result = []