mavsdk_tests: retry gz model spawn command

This should help when gzserver does not respond yet and we end up
without a model and hence can't connect later and time out.

This change also required a fix to prevent the tester to hang on
terminating all runners. By using poll instead of only read we can
prevent that and actually properly join the logger thread.

test/mavsdk_tests/mavsdk_test_runner.py

@@ -444,6 +444,10 @@ class Tester:
         for runner in self.active_runners:
             runner.set_log_filename(
                 self.determine_logfile_path(log_dir, runner.name))
+
+            # Some runners need to be started a couple of times
+            # until they succeed.
+            for _ in range(10):
                 try:
                     runner.start()
                 except TimeoutError:
@@ -452,12 +456,16 @@ class Tester:
                           .format(runner.name))
                     break
 
-            # Workaround to prevent gz not being able to communicate
-            # with gzserver. In CI it tends to take longer.
-            if os.getenv("GITHUB_WORKFLOW") and runner.name == "gzserver":
-                time.sleep(10)
+                if runner.has_started_ok():
+                    break
+
+                runner.stop
+                time.sleep(1)
+
             else:
-                time.sleep(2)
+                abort = True
+                print("Could not start runner: {}".format(runner.name))
+                break
 
         if abort:
             self.stop_runners()

test/mavsdk_tests/process_helper.py

@@ -8,6 +8,7 @@ import subprocess
 import shutil
 import threading
 import errno
+import select
 from typing import Any, Dict, List, TextIO, Optional
 
 
@@ -62,9 +63,18 @@ class Runner:
         self.thread = threading.Thread(target=self.process_output)
         self.thread.start()
 
+    def has_started_ok(self) -> bool:
+        return True
+
     def process_output(self) -> None:
         assert self.process.stdout is not None
-        while True:
+
+        poll_obj = select.poll()
+        poll_obj.register(self.process.stdout, select.POLLIN)
+
+        while not self.stop_thread.is_set():
+            poll_result = poll_obj.poll(0)
+            if poll_result:
                 line = self.process.stdout.readline()
                 if not line and \
                         (self.stop_thread.is_set() or self.poll is not None):
@@ -255,6 +265,39 @@ class GzmodelspawnRunner(Runner):
                      "--model-name", self.model,
                      "-x", "1.01", "-y", "0.98", "-z", "0.83"]
 
+    def has_started_ok(self) -> bool:
+        # The problem is that sometimes gzserver does not seem to start
+        # quickly enough and gz model spawn fails with the error:
+        # "An instance of Gazebo is not running." but still returns 0
+        # as a result.
+        # We work around this by trying to start and then check whether
+        # using has_started_ok() whether it was succesful or not.
+        timeout_s = 3
+        steps = 10
+        for _ in range(steps*timeout_s):
+            if self.verbose:
+                print("Checking if gz model spawn is done...")
+            returncode = self.process.poll()
+            if returncode is None:
+                if self.verbose:
+                    print("not done yet")
+                time.sleep(float(timeout_s)/float(steps))
+                continue
+
+            if self.verbose:
+                print("gz model spawn is done")
+            with open(self.log_filename, 'r') as f:
+                for line in f.readlines():
+                    if 'An instance of Gazebo is not running' in line:
+                        return False
+                else:
+                    return True
+
+        if self.verbose:
+            print("gzmodelspawn did not return within {}s".
+                  format(timeout_s))
+        return False
+
 
 class GzclientRunner(Runner):
     def __init__(self,