Add custom platform matcher for testing purposes

This is a hack so that we can test out fetching a specific OS version
variant of an image.
Since the platforms package only does OSVersion comparison on for
"windows" we just create our own dumby OS where we can do some basic
tests.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

frontend/dockerfile/dockerfile_test.go

@@ -9439,12 +9439,12 @@ func testPlatformWithOSVersion(t *testing.T, sb integration.Sandbox) {
 
 	f := getFrontend(t, sb)
 	p1 := ocispecs.Platform{
-		OS:           "foo",
+		OS:           "internalbuildkitintegration",
 		OSVersion:    "1.2.3",
 		Architecture: "bar",
 	}
 	p2 := ocispecs.Platform{
-		OS:           "foo",
+		OS:           "internalbuildkitintegration",
 		OSVersion:    "1.1.0",
 		Architecture: "bar",
 	}

util/pull/pull.go

@@ -112,13 +112,36 @@ func (p *Puller) tryLocalResolve(ctx context.Context) error {
 	return nil
 }
 
+type simpleVersionMatcher struct {
+	platforms.MatchComparer
+	OSVersion string
+}
+
+func (m simpleVersionMatcher) Match(p ocispecs.Platform) bool {
+	if !m.MatchComparer.Match(p) {
+		return false
+	}
+	if m.OSVersion == "" || p.OSVersion == "" {
+		return true
+	}
+	return m.OSVersion == p.OSVersion
+}
+
 func (p *Puller) PullManifests(ctx context.Context, getResolver SessionResolver) (*PulledManifests, error) {
 	err := p.resolve(ctx, p.Resolver)
 	if err != nil {
 		return nil, err
 	}
 
 	platform := platforms.Only(p.Platform)
+	if p.Platform.OS == "internalbuildkitintegration" {
+		// NOTE: HACK: This is a special case for testing purposes.
+		// It is used to test that we can pull images based on a specific OS version.
+		//
+		// The default platform, currently, matcher ignores OS version unless p.OS is set to windows.
+		// Hence the need for a custom platform matcher which will include OS version.
+		platform = simpleVersionMatcher{OSVersion: p.Platform.OSVersion, MatchComparer: platform}
+	}
 
 	var mu sync.Mutex // images.Dispatch calls handlers in parallel
 	metadata := make(map[digest.Digest]ocispecs.Descriptor)