osbuild: include context in skopedo error messages

When trying to create enough data for a full pipeline run for
#287 I struggled a bit
because it was not clear which items where faulty.

This commit adds more context to the errors. Sadly I had to
implement `assertPanicsWithErrorRegexp` because of
stretchr/testify#1304

I can contribute it upstream but for that it needs some tests
around it first :)

pkg/osbuild/skopeo_source.go

@@ -39,13 +39,12 @@ func NewSkopeoSourceItem(name, digest string, tlsVerify *bool) SkopeoSourceItem
 }
 
 func (item SkopeoSourceItem) validate() error {
-
 	if item.Image.Name == "" {
-		return fmt.Errorf("source item has empty name")
+		return fmt.Errorf("source item %#v has empty name", item)
 	}
 
 	if !skopeoDigestPattern.MatchString(item.Image.Digest) {
-		return fmt.Errorf("source item has invalid digest")
+		return fmt.Errorf("source item %#v has invalid digest", item)
 	}
 
 	return nil
@@ -63,7 +62,7 @@ func NewSkopeoSource() *SkopeoSource {
 func (source *SkopeoSource) AddItem(name, digest, image string, tlsVerify *bool) {
 	item := NewSkopeoSourceItem(name, digest, tlsVerify)
 	if !skopeoDigestPattern.MatchString(image) {
-		panic("item has invalid image id")
+		panic(fmt.Errorf("item %#v has invalid image id", image))
 	}
 	source.Items[image] = item
 }

pkg/osbuild/skopeo_source_test.go

@@ -1,13 +1,32 @@
 package osbuild
 
 import (
+	"fmt"
+	"regexp"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 
 	"github.com/osbuild/images/internal/common"
 )
 
+// needed until https://github.com/stretchr/testify/issues/1304 is fixed
+func assertPanicsWithErrorRegexp(t *testing.T, reg *regexp.Regexp, f assert.PanicTestFunc) (assertOk bool) {
+	defer func() {
+		var message interface{} // nolint: gosimple
+
+		message = recover()
+		err, ok := message.(error)
+		if !ok || err == nil {
+			assert.Fail(t, fmt.Sprintf("func %#v should return an error but got: %[1]v (type %[1]T)", f, message))
+		}
+		assertOk = assert.Regexp(t, reg, err.Error())
+	}()
+
+	f()
+	return assert.Fail(t, fmt.Sprintf("func %#v should panic but did not", f))
+}
+
 func TestNewSkopeoSource(t *testing.T) {
 	testDigest := "sha256:f29b6cd42a94a574583439addcd6694e6224f0e4b32044c9e3aee4c4856c2a50"
 	imageID := "sha256:c2ecf25cf190e76b12b07436ad5140d4ba53d8a136d498705e57a006837a720f"
@@ -33,27 +52,30 @@ func TestNewSkopeoSource(t *testing.T) {
 	assert.Nil(t, item.Image.TLSVerify)
 
 	// empty name
-	assert.Panics(t, func() {
+	expectedErr := regexp.MustCompile(`source item osbuild.SkopeoSourceItem.* has empty name`)
+	assertPanicsWithErrorRegexp(t, expectedErr, func() {
 		source.AddItem("", testDigest, imageID, nil)
 	})
 
 	// empty digest
-	assert.Panics(t, func() {
+	expectedErr = regexp.MustCompile(`source item osbuild.SkopeoSourceItem.* has invalid digest`)
+	assertPanicsWithErrorRegexp(t, expectedErr, func() {
 		source.AddItem("name", "", imageID, nil)
 	})
 
 	// empty image id
-	assert.Panics(t, func() {
+	assert.PanicsWithError(t, `item "" has invalid image id`, func() {
 		source.AddItem("name", testDigest, "", nil)
 	})
 
 	// invalid digest
-	assert.Panics(t, func() {
+	expectedErr = regexp.MustCompile(`item osbuild.SkopeoSourceItem.* has invalid digest`)
+	assertPanicsWithErrorRegexp(t, expectedErr, func() {
 		source.AddItem("name", "foo", imageID, nil)
 	})
 
 	// invalid image id
-	assert.Panics(t, func() {
+	assert.PanicsWithError(t, `item "sha256:foo" has invalid image id`, func() {
 		source.AddItem("name", testDigest, "sha256:foo", nil)
 	})
 }