Validate the 'expose' option

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/config/fields_schema.json

@@ -41,7 +41,10 @@
 
         "expose": {
           "type": "array",
-          "items": {"type": ["string", "number"]},
+          "items": {
+            "type": ["string", "number"],
+            "format": "expose"
+          },
           "uniqueItems": true
         },
 

compose/config/validation.py

@@ -1,6 +1,7 @@
 import json
 import logging
 import os
+import re
 import sys
 
 import six
@@ -34,6 +35,7 @@ DOCKER_CONFIG_HINTS = {
 
 
 VALID_NAME_CHARS = '[a-zA-Z0-9\._\-]'
+VALID_EXPOSE_FORMAT = r'^\d+(\/[a-zA-Z]+)?$'
 
 
 @FormatChecker.cls_checks(format="ports", raises=ValidationError)
@@ -45,6 +47,16 @@ def format_ports(instance):
     return True
 
 
+@FormatChecker.cls_checks(format="expose", raises=ValidationError)
+def format_expose(instance):
+    if isinstance(instance, six.string_types):
+        if not re.match(VALID_EXPOSE_FORMAT, instance):
+            raise ValidationError(
+                "should be of the format 'PORT[/PROTOCOL]'")
+
+    return True
+
+
 @FormatChecker.cls_checks(format="bool-value-in-mapping")
 def format_boolean_in_environment(instance):
     """
@@ -273,7 +285,7 @@ def validate_against_fields_schema(config, filename):
     _validate_against_schema(
         config,
         "fields_schema.json",
-        format_checker=["ports", "bool-value-in-mapping"],
+        format_checker=["ports", "expose", "bool-value-in-mapping"],
         filename=filename)