Docs and tests for pids_limit.

Signed-off-by: Joffrey F <joffrey@docker.com>

docs/hostconfig.md

@@ -111,11 +111,12 @@ for example:
   CPU period.
 * cpu_shares (int): CPU shares (relative weight)
 * cpuset_cpus (str): CPUs in which to allow execution (0-3, 0,1)
-* blkio_weight: Block IO weight (relative weight), accepts a weight value between 10 and 1000.
+* blkio_weight: Block IO weight (relative weight), accepts a weight value
+  between 10 and 1000.
 * blkio_weight_device: Block IO weight (relative device weight) in the form of:
   `[{"Path": "device_path", "Weight": weight}]`
-* device_read_bps: Limit read rate (bytes per second) from a device in the form of:
-  `[{"Path": "device_path", "Rate": rate}]`
+* device_read_bps: Limit read rate (bytes per second) from a device in the
+  form of: `[{"Path": "device_path", "Rate": rate}]`
 * device_write_bps: Limit write rate (bytes per second) from a device.
 * device_read_iops: Limit read rate (IO per second) from a device.
 * device_write_iops: Limit write rate (IO per second) from a device.
@@ -128,6 +129,7 @@ for example:
 * sysctls (dict): Kernel parameters to set in the container.
 * userns_mode (str): Sets the user namespace mode for the container when user
   namespace remapping option is enabled. Supported values are: `host`
+* pids_limit (int): Tune a container’s pids limit. Set -1 for unlimited.
 
 **Returns** (dict) HostConfig dictionary
 

tests/unit/utils_test.py

@@ -185,6 +185,15 @@ class HostConfigTest(base.BaseTestCase):
             InvalidVersion, lambda: create_host_config(
                 version='1.20', kernel_memory=67108864))
 
+    def test_create_host_config_with_pids_limit(self):
+        config = create_host_config(version='1.23', pids_limit=1024)
+        self.assertEqual(config.get('PidsLimit'), 1024)
+
+        with pytest.raises(InvalidVersion):
+            create_host_config(version='1.22', pids_limit=1024)
+        with pytest.raises(TypeError):
+            create_host_config(version='1.22', pids_limit='1024')
+
 
 class UlimitTest(base.BaseTestCase):
     def test_create_host_config_dict_ulimit(self):