handle empty run image in metadata

client/src/main/java/dev/snowdrop/buildpack/BuilderImage.java

@@ -187,14 +187,19 @@ private ImageReference getRunImageFromMetadataJSON() throws BuildpackException {
 
             // if caller did not set runImage, and metadata is absent, error.
             if(ri==null){
-                throw new Exception("No runImage specified, and builderImage is missing metadata declaration");
+                throw new Exception("No runImage specified, and builderImage is missing metadata declaration :: "+this.metadataJson);
             }
 
-            // remap docker.io references back to docker.io
-            // if (ri.startsWith("index.docker.io/")) {
-            //     ri = ri.substring("index.docker.io/".length());
-            //     ri = "docker.io/" + ri;
-            // }
+            //first observed with ubi9 builder, stack/runImage/image is empty string
+            //image is present in the images array, so fallback to that.
+            if(ri.trim().isEmpty()){
+                //fallback to looking for runImage in the images array.. 
+                String fbImg = JsonUtils.getValue(root, "images/0/image");
+                if(fbImg==null || fbImg.trim().isEmpty()){
+                    throw new Exception("No runImage specified, and builderImage is missing metadata declaration :: (fbImage:"+fbImg+") "+this.metadataJson);
+                }
+                ri = fbImg;
+            }
 
             return new ImageReference(ri);
         } catch (Exception e) {

client/src/main/java/dev/snowdrop/buildpack/utils/JsonUtils.java

@@ -12,14 +12,23 @@ public static String getValue(JsonNode root, String path) {
             path = path.substring(1);
         }
         String[] parts = path.split("/");
-        JsonNode next = root.get(parts[0]);
+        JsonNode next = null;
+        if(!root.isArray()){
+          next = root.get(parts[0]);
+        }else{
+          try{
+            next = root.get(Integer.parseInt(parts[0]));  
+          }catch(NumberFormatException nfe){
+            throw new IllegalArgumentException("Invalid path, expected array index but got: "+parts[0]+" for path: "+path);
+          }
+        }
         if (next != null && parts.length > 1) {
           return getValue(next, path.substring(path.indexOf("/") + 1));
         }
         if (next == null) {
           return null;
-        }
-        return next.asText();
+        }   
+        return next.asText();       
       }
    public static List<String> getArray(JsonNode root, String path) {
         if (path.startsWith("/")) {

client/src/test/java/dev/snowdrop/buildpack/utils/JsonUtilsTest.java

@@ -2,6 +2,8 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.fail;
 
 import java.util.List;
@@ -60,4 +62,54 @@ void testJsonUtils() {
             fail(e);
         }
     }
+
+    @Test
+    void testGetValueArrayIndexTraversal() throws Exception {
+        // JSON with nested arrays: /name/0/thing/2/name style paths
+        String json = "{\"items\":[{\"name\":\"first\",\"tags\":[\"a\",\"b\",\"c\"]},{\"name\":\"second\",\"tags\":[\"x\",\"y\",\"z\"]}]}";
+
+        ObjectMapper om = new ObjectMapper();
+        om.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        JsonNode root = om.readTree(json);
+
+        // Access object field then array index then field
+        String name = JsonUtils.getValue(root, "items/0/name");
+        assertEquals("first", name);
+
+        String secondName = JsonUtils.getValue(root, "/items/1/name");
+        assertEquals("second", secondName);
+
+        // Access object field then array index then array index (nested arrays)
+        String tag = JsonUtils.getValue(root, "items/0/tags/2");
+        assertEquals("c", tag);
+
+        String lastTag = JsonUtils.getValue(root, "/items/1/tags/0");
+        assertEquals("x", lastTag);
+    }
+
+    @Test
+    void testGetValueArrayIndexOutOfBounds() throws Exception {
+        String json = "{\"items\":[{\"name\":\"only\"}]}";
+
+        ObjectMapper om = new ObjectMapper();
+        om.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        JsonNode root = om.readTree(json);
+
+        // Out-of-bounds index returns null
+        String val = JsonUtils.getValue(root, "items/5/name");
+        assertNull(val);
+    }
+
+    @Test
+    void testGetValueArrayNonNumericIndexThrows() throws Exception {
+        String json = "{\"items\":[{\"name\":\"only\"}]}";
+
+        ObjectMapper om = new ObjectMapper();
+        om.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
+        JsonNode root = om.readTree(json);
+
+        // Non-numeric path segment against an array should throw
+        assertThrows(IllegalArgumentException.class, () ->
+            JsonUtils.getValue(root, "items/notANumber/name"));
+    }
 }