Make links unique-by-alias when merging

Factor out MergeDict from merge_service_dicts to reduce complexity below limit.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

compose/config/config.py

@@ -26,6 +26,7 @@ from .sort_services import get_service_name_from_network_mode
 from .sort_services import sort_service_dicts
 from .types import parse_extra_hosts
 from .types import parse_restart_spec
+from .types import ServiceLink
 from .types import VolumeFromSpec
 from .types import VolumeSpec
 from .validation import match_named_volumes
@@ -641,51 +642,79 @@ def merge_service_dicts_from_files(base, override, version):
     return new_service
 
 
+class MergeDict(dict):
+    """A dict-like object responsible for merging two dicts into one."""
+
+    def __init__(self, base, override):
+        self.base = base
+        self.override = override
+
+    def needs_merge(self, field):
+        return field in self.base or field in self.override
+
+    def merge_field(self, field, merge_func, default=None):
+        if not self.needs_merge(field):
+            return
+
+        self[field] = merge_func(
+            self.base.get(field, default),
+            self.override.get(field, default))
+
+    def merge_mapping(self, field, parse_func):
+        if not self.needs_merge(field):
+            return
+
+        self[field] = parse_func(self.base.get(field))
+        self[field].update(parse_func(self.override.get(field)))
+
+    def merge_sequence(self, field, parse_func):
+        def parse_sequence_func(seq):
+            return to_mapping((parse_func(item) for item in seq), 'merge_field')
+
+        if not self.needs_merge(field):
+            return
+
+        merged = parse_sequence_func(self.base.get(field, []))
+        merged.update(parse_sequence_func(self.override.get(field, [])))
+        self[field] = [item.repr() for item in merged.values()]
+
+    def merge_scalar(self, field):
+        if self.needs_merge(field):
+            self[field] = self.override.get(field, self.base.get(field))
+
+
 def merge_service_dicts(base, override, version):
-    d = {}
+    md = MergeDict(base, override)
 
-    def merge_field(field, merge_func, default=None):
-        if field in base or field in override:
-            d[field] = merge_func(
-                base.get(field, default),
-                override.get(field, default))
-
-    def merge_mapping(mapping, parse_func):
-        if mapping in base or mapping in override:
-            merged = parse_func(base.get(mapping, None))
-            merged.update(parse_func(override.get(mapping, None)))
-            d[mapping] = merged
-
-    merge_mapping('environment', parse_environment)
-    merge_mapping('labels', parse_labels)
-    merge_mapping('ulimits', parse_ulimits)
+    md.merge_mapping('environment', parse_environment)
+    md.merge_mapping('labels', parse_labels)
+    md.merge_mapping('ulimits', parse_ulimits)
+    md.merge_sequence('links', ServiceLink.parse)
 
     for field in ['volumes', 'devices']:
-        merge_field(field, merge_path_mappings)
+        md.merge_field(field, merge_path_mappings)
 
     for field in [
         'depends_on',
         'expose',
         'external_links',
-        'links',
         'ports',
         'volumes_from',
     ]:
-        merge_field(field, operator.add, default=[])
+        md.merge_field(field, operator.add, default=[])
 
     for field in ['dns', 'dns_search', 'env_file']:
-        merge_field(field, merge_list_or_string)
+        md.merge_field(field, merge_list_or_string)
 
-    for field in set(ALLOWED_KEYS) - set(d):
-        if field in base or field in override:
-            d[field] = override.get(field, base.get(field))
+    for field in set(ALLOWED_KEYS) - set(md):
+        md.merge_scalar(field)
 
     if version == V1:
-        legacy_v1_merge_image_or_build(d, base, override)
+        legacy_v1_merge_image_or_build(md, base, override)
     else:
-        merge_build(d, base, override)
+        merge_build(md, base, override)
 
-    return d
+    return dict(md)
 
 
 def merge_build(output, base, override):
@@ -919,6 +948,10 @@ def to_list(value):
         return value
 
 
+def to_mapping(sequence, key_field):
+    return {getattr(item, key_field): item for item in sequence}
+
+
 def has_uppercase(name):
     return any(char in string.ascii_uppercase for char in name)
 

compose/config/types.py

@@ -168,3 +168,22 @@ class VolumeSpec(namedtuple('_VolumeSpec', 'external internal mode')):
     @property
     def is_named_volume(self):
         return self.external and not self.external.startswith(('.', '/', '~'))
+
+
+class ServiceLink(namedtuple('_ServiceLink', 'target alias')):
+
+    @classmethod
+    def parse(cls, link_spec):
+        target, _, alias = link_spec.partition(':')
+        if not alias:
+            alias = target
+        return cls(target, alias)
+
+    def repr(self):
+        if self.target == self.alias:
+            return self.target
+        return '{s.target}:{s.alias}'.format(s=self)
+
+    @property
+    def merge_field(self):
+        return self.alias