PR: Wait until connection file is valid

ipykernel/tests/test_embed_kernel.py

@@ -6,6 +6,7 @@
 import os
 import sys
 import time
+import json
 
 from contextlib import contextmanager
 from subprocess import Popen, PIPE
@@ -28,39 +29,56 @@ def setup_kernel(cmd):
     -------
     kernel_manager: connected KernelManager instance
     """
-    kernel = Popen([sys.executable, '-c', cmd], stdout=PIPE, stderr=PIPE)
-    connection_file = os.path.join(
-        paths.jupyter_runtime_dir(),
-        'kernel-%i.json' % kernel.pid,
-    )
-    # wait for connection file to exist, timeout after 5s
-    tic = time.time()
-    while not os.path.exists(connection_file) \
-        and kernel.poll() is None \
-        and time.time() < tic + SETUP_TIMEOUT:
-        time.sleep(0.1)
-
-    if kernel.poll() is not None:
-        o,e = kernel.communicate()
-        e = py3compat.cast_unicode(e)
-        raise IOError("Kernel failed to start:\n%s" % e)
-
-    if not os.path.exists(connection_file):
-        if kernel.poll() is None:
-            kernel.terminate()
-        raise IOError("Connection file %r never arrived" % connection_file)
 
-    client = BlockingKernelClient(connection_file=connection_file)
-    client.load_connection_file()
-    client.start_channels()
-    client.wait_for_ready()
+    def connection_file_ready(connection_file):
+        """Check if connection_file is a readable json file."""
+        if not os.path.exists(connection_file):
+            return False
+        try:
+            with open(connection_file) as f:
+                json.load(f)
+            return True
+        except ValueError:
+            return False
 
+    kernel = Popen([sys.executable, '-c', cmd], stdout=PIPE, stderr=PIPE)
     try:
-        yield client
+        connection_file = os.path.join(
+            paths.jupyter_runtime_dir(),
+            'kernel-%i.json' % kernel.pid,
+        )
+        # wait for connection file to exist, timeout after 5s
+        tic = time.time()
+        while not connection_file_ready(connection_file) \
+            and kernel.poll() is None \
+            and time.time() < tic + SETUP_TIMEOUT:
+            time.sleep(0.1)
+
+        # Wait 100ms for the writing to finish
+        time.sleep(0.1)
+
+        if kernel.poll() is not None:
+            o,e = kernel.communicate()
+            e = py3compat.cast_unicode(e)
+            raise IOError("Kernel failed to start:\n%s" % e)
+
+        if not os.path.exists(connection_file):
+            if kernel.poll() is None:
+                kernel.terminate()
+            raise IOError("Connection file %r never arrived" % connection_file)
+
+        client = BlockingKernelClient(connection_file=connection_file)
+        client.load_connection_file()
+        client.start_channels()
+        client.wait_for_ready()
+        try:
+            yield client
+        finally:
+            client.stop_channels()
     finally:
-        client.stop_channels()
         kernel.terminate()
 
+
 def test_embed_kernel_basic():
     """IPython.embed_kernel() is basically functional"""
     cmd = '\n'.join([