Add platform match comparer interface

Adds a new platform interface for matching and comparing platforms.
This new interface allows both filtering and ordering of platforms
to support running multiple platform and choosing the best platform.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

platforms/compare.go

@@ -0,0 +1,192 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package platforms
+
+import specs "github.com/opencontainers/image-spec/specs-go/v1"
+
+// MatchComparer is able to match and compare platforms to
+// filter and sort platforms.
+type MatchComparer interface {
+	Matcher
+
+	Less(specs.Platform, specs.Platform) bool
+}
+
+// Only returns a match comparer for a single platform
+// using default resolution logic for the platform.
+//
+// For ARMv7, will also match ARMv6 and ARMv5
+// For ARMv6, will also match ARMv5
+func Only(platform specs.Platform) MatchComparer {
+	platform = Normalize(platform)
+	if platform.Architecture == "arm" {
+		if platform.Variant == "v7" {
+			return orderedPlatformComparer{
+				matchers: []Matcher{
+					&matcher{
+						Platform: platform,
+					},
+					&matcher{
+						Platform: specs.Platform{
+							Architecture: platform.Architecture,
+							OS:           platform.OS,
+							OSVersion:    platform.OSVersion,
+							OSFeatures:   platform.OSFeatures,
+							Variant:      "v6",
+						},
+					},
+					&matcher{
+						Platform: specs.Platform{
+							Architecture: platform.Architecture,
+							OS:           platform.OS,
+							OSVersion:    platform.OSVersion,
+							OSFeatures:   platform.OSFeatures,
+							Variant:      "v5",
+						},
+					},
+				},
+			}
+		}
+		if platform.Variant == "v6" {
+			return orderedPlatformComparer{
+				matchers: []Matcher{
+					&matcher{
+						Platform: platform,
+					},
+					&matcher{
+						Platform: specs.Platform{
+							Architecture: platform.Architecture,
+							OS:           platform.OS,
+							OSVersion:    platform.OSVersion,
+							OSFeatures:   platform.OSFeatures,
+							Variant:      "v5",
+						},
+					},
+				},
+			}
+		}
+	}
+
+	return singlePlatformComparer{
+		Matcher: &matcher{
+			Platform: platform,
+		},
+	}
+}
+
+// Ordered returns a platform MatchComparer which matches any of the platforms
+// but orders them in order they are provided.
+func Ordered(platforms ...specs.Platform) MatchComparer {
+	matchers := make([]Matcher, len(platforms))
+	for i := range platforms {
+		matchers[i] = NewMatcher(platforms[i])
+	}
+	return orderedPlatformComparer{
+		matchers: matchers,
+	}
+}
+
+// Any returns a platform MatchComparer which matches any of the platforms
+// with no preference for ordering.
+func Any(platforms ...specs.Platform) MatchComparer {
+	matchers := make([]Matcher, len(platforms))
+	for i := range platforms {
+		matchers[i] = NewMatcher(platforms[i])
+	}
+	return anyPlatformComparer{
+		matchers: matchers,
+	}
+}
+
+// All is a platform MatchComparer which matches all platforms
+// with preference for ordering.
+var All MatchComparer = allPlatformComparer{}
+
+type singlePlatformComparer struct {
+	Matcher
+}
+
+func (c singlePlatformComparer) Less(p1, p2 specs.Platform) bool {
+	return c.Match(p1) && !c.Match(p2)
+}
+
+type orderedPlatformComparer struct {
+	matchers []Matcher
+}
+
+func (c orderedPlatformComparer) Match(platform specs.Platform) bool {
+	for _, m := range c.matchers {
+		if m.Match(platform) {
+			return true
+		}
+	}
+	return false
+}
+
+func (c orderedPlatformComparer) Less(p1 specs.Platform, p2 specs.Platform) bool {
+	for _, m := range c.matchers {
+		p1m := m.Match(p1)
+		p2m := m.Match(p2)
+		if p1m && !p2m {
+			return true
+		}
+		if p1m || p2m {
+			return false
+		}
+	}
+	return false
+}
+
+type anyPlatformComparer struct {
+	matchers []Matcher
+}
+
+func (c anyPlatformComparer) Match(platform specs.Platform) bool {
+	for _, m := range c.matchers {
+		if m.Match(platform) {
+			return true
+		}
+	}
+	return false
+}
+
+func (c anyPlatformComparer) Less(p1, p2 specs.Platform) bool {
+	var p1m, p2m bool
+	for _, m := range c.matchers {
+		if !p1m && m.Match(p1) {
+			p1m = true
+		}
+		if !p2m && m.Match(p2) {
+			p2m = true
+		}
+		if p1m && p2m {
+			return false
+		}
+	}
+	// If one matches, and the other does, sort match first
+	return p1m && !p2m
+}
+
+type allPlatformComparer struct{}
+
+func (allPlatformComparer) Match(specs.Platform) bool {
+	return true
+}
+
+func (allPlatformComparer) Less(specs.Platform, specs.Platform) bool {
+	return false
+}

platforms/defaults.go

@@ -22,8 +22,13 @@ import (
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-// Default returns the default specifier for the platform.
-func Default() string {
+// Default returns the default matcher for the platform.
+func Default() MatchComparer {
+	return Only(DefaultSpec())
+}
+
+// DefaultString returns the default string specifier for the platform.
+func DefaultString() string {
 	return Format(DefaultSpec())
 }
 

platforms/defaults_test.go

@@ -35,7 +35,7 @@ func TestDefault(t *testing.T) {
 		t.Fatalf("default platform not as expected: %#v != %#v", p, expected)
 	}
 
-	s := Default()
+	s := DefaultString()
 	if s != Format(p) {
 		t.Fatalf("default specifier should match formatted default spec: %v != %v", s, p)
 	}