Allow config authors to always use Prefix

Previously an unset Location was only allowed for wildcarded Prefixes.
This commit will allow configs with emtpy location or empty prefix, so
long as the other is valid.

Remove wildcard prefix check for empty location in rewriteReference.

Fixes:
1. #1191 (comment)
2. #1191 (comment)

Co-authored-by: Miloslav Trmač <mitr@redhat.com>
Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

docs/containers-registries.conf.5.md

@@ -82,8 +82,7 @@ location = "internal-registry-for-example.net/bar"
 requests for the image `example.com/foo/myimage:latest` will actually work with the
 `internal-registry-for-example.net/bar/myimage:latest` image.
 
-With a `prefix` containing a wildcard in the format: "*.example.com" for subdomain matching,
-the location can be empty. In such a case,
+With any valid `prefix`, the location can be emtpy. In such a case,
 prefix matching will occur, but no reference rewrite will occur. The
 original requested image string will be used as-is. But other settings like
 `insecure` / `blocked` / `mirrors` will be applied to matching images.
@@ -92,6 +91,10 @@ Example: Given
 ```
 prefix = "*.example.com"
 ```
+OR
+```
+prefix = "blah.example.com"
+```
 requests for the image `blah.example.com/foo/myimage:latest` will be used
 as-is. But other settings like insecure/blocked/mirrors will be applied to matching images
 

pkg/sysregistriesv2/system_registries_v2.go

@@ -75,14 +75,7 @@ func (e *Endpoint) rewriteReference(ref reference.Named, prefix string) (referen
 	}
 	// In the case of an empty `location` field, simply return the original
 	// input ref as-is.
-	//
-	// FIXME: already validated in postProcessRegistries, so check can probably
-	// be dropped.
-	// https://github.com/containers/image/pull/1191#discussion_r610621608
 	if e.Location == "" {
-		if prefix[:2] != "*." {
-			return nil, fmt.Errorf("invalid prefix '%v' for empty location, should be in the format: *.example.com", prefix)
-		}
 		return ref, nil
 	}
 	newNamedRef = e.Location + refString[prefixLen:]
@@ -357,21 +350,27 @@ func (config *V2RegistriesConf) postProcessRegistries() error {
 			return err
 		}
 
-		if reg.Prefix == "" {
-			if reg.Location == "" {
-				return &InvalidRegistries{s: "invalid condition: both location and prefix are unset"}
-			}
-			reg.Prefix = reg.Location
-		} else {
+		// Prefix and Location cannot both be empty.
+		// Either one at least must be set.
+		if reg.Prefix != "" {
 			reg.Prefix, err = parseLocation(reg.Prefix)
 			if err != nil {
 				return err
 			}
-			// FIXME: allow config authors to always use Prefix.
-			// https://github.com/containers/image/pull/1191#discussion_r610622495
-			if reg.Prefix[:2] != "*." && reg.Location == "" {
-				return &InvalidRegistries{s: "invalid condition: location is unset and prefix is not in the format: *.example.com"}
+			if reg.Location != "" {
+				reg.Location, err = parseLocation(reg.Location)
+				if err != nil {
+					return err
+				}
+			}
+		} else if reg.Location != "" {
+			reg.Prefix, err = parseLocation(reg.Location)
+			if err != nil {
+				return err
 			}
+			reg.Location = reg.Prefix
+		} else {
+			return &InvalidRegistries{s: "invalid condition: both location and prefix are unset"}
 		}
 
 		// make sure mirrors are valid

pkg/sysregistriesv2/system_registries_v2_test.go

@@ -291,7 +291,7 @@ func TestFindRegistry(t *testing.T) {
 
 	registries, err := GetRegistries(sys)
 	assert.Nil(t, err)
-	assert.Equal(t, 19, len(registries))
+	assert.Equal(t, 20, len(registries))
 
 	reg, err := FindRegistry(sys, "simple-prefix.com/foo/bar:latest")
 	assert.Nil(t, err)
@@ -383,6 +383,12 @@ func TestFindRegistry(t *testing.T) {
 	assert.Equal(t, "empty-prefix.com", reg.Prefix)
 	assert.Equal(t, "empty-prefix.com", reg.Location)
 
+	reg, err = FindRegistry(sys, "empty-location.set-prefix.example.com/namespace/repo")
+	assert.Nil(t, err)
+	assert.NotNil(t, reg)
+	assert.NotNil(t, "empty-location.set-prefix.example.com", reg.Prefix)
+	assert.Equal(t, "", reg.Location)
+
 	_, err = FindRegistry(&types.SystemContext{SystemRegistriesConfPath: "testdata/this-does-not-exist.conf"}, "example.com")
 	assert.Error(t, err)
 }
@@ -579,6 +585,14 @@ func TestRewriteReferenceSuccess(t *testing.T) {
 		{"sub.example.io/library/prefix/image", "*.example.io", "example.com", "example.com/library/prefix/image"},
 		{"another.sub.example.io:5000/library/prefix/image:latest", "*.sub.example.io", "example.com", "example.com:5000/library/prefix/image:latest"},
 		{"foo.bar.io/ns1/ns2/ns3/ns4", "*.bar.io", "omg.bbq.com/roflmao", "omg.bbq.com/roflmao/ns1/ns2/ns3/ns4"},
+		// Empty location with non-wildcard prefix examples. Essentially, no
+		// rewrite occurs and original reference is used as-is.
+		{"registry.com/foo", "registry.com", "", "registry.com/foo"},
+		{"abc.internal.registry.com/foo:bar", "abc.internal.registry.com", "", "abc.internal.registry.com/foo:bar"},
+		{"abc.internal.registry.com/foo/bar:baz", "abc.internal.registry.com", "", "abc.internal.registry.com/foo/bar:baz"},
+		{"alien.vs.predator.foobar.io:5000/foo@sha256:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "alien.vs.predator.foobar.io", "",
+			"alien.vs.predator.foobar.io:5000/foo@sha256:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"},
+		{"alien.vs.predator.foobar.io:5000/omg:bbq", "alien.vs.predator.foobar.io", "", "alien.vs.predator.foobar.io:5000/omg:bbq"},
 		// Empty location with wildcard prefix examples. Essentially, no
 		// rewrite occurs and original reference is used as-is.
 		{"abc.internal.registry.com/foo:bar", "*.internal.registry.com", "", "abc.internal.registry.com/foo:bar"},
@@ -602,7 +616,6 @@ func TestRewriteReferenceFailedDuringParseNamed(t *testing.T) {
 		{"example.com/foo/image:latest", "example.com/foo", "example.com/path/"},
 		{"example.com/foo@sha256:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
 			"example.com/foo", "example.com"},
-		{"example.com:5000/image:latest", "example.com", ""},
 		{"example.com:5000/image:latest", "example.com", "example.com:5000"},
 		// Malformed prefix
 		{"example.com/foo/image:latest", "example.com//foo", "example.com/path"},

pkg/sysregistriesv2/testdata/find-registry.conf

@@ -60,3 +60,6 @@ prefix="*.com"
 
 [[registry]]
 prefix="*.foobar.io"
+
+[[registry]]
+prefix="empty-location.set-prefix.example.com"