Fixed bug caused by multiple concurrent builds of the same resource.

Several parts of the code request resources to be loaded/built, and these separate threads were tripping over each other and causing build failures. This has been avoided by making sure the resource isn't already building before requesting it.

src/main/java/org/qortal/api/gateway/resource/GatewayResource.java

@@ -56,7 +56,9 @@ public class GatewayResource {
         if (build != null && build == true) {
             ArbitraryDataReader reader = new ArbitraryDataReader(name, ArbitraryDataFile.ResourceIdType.NAME, service, null);
             try {
+                if (!reader.isBuilding()) {
                     reader.loadSynchronously(false);
+                }
             } catch (Exception e) {
                 // No need to handle exception, as it will be reflected in the status
             }

src/main/java/org/qortal/api/resource/ArbitraryResource.java

@@ -938,6 +938,7 @@ public class ArbitraryResource {
 			// Loop until we have data
 			while (!Controller.isStopping()) {
 				attempts++;
+				if (!arbitraryDataReader.isBuilding()) {
 					try {
 						arbitraryDataReader.loadSynchronously(rebuild);
 						break;
@@ -946,10 +947,10 @@ public class ArbitraryResource {
 							// Give up after 5 attempts
 							throw ApiExceptionFactory.INSTANCE.createCustomException(request, ApiError.INVALID_CRITERIA, "Data unavailable. Please try again later.");
 						}
-
-					Thread.sleep(3000L);
 					}
 				}
+				Thread.sleep(3000L);
+			}
 			java.nio.file.Path outputPath = arbitraryDataReader.getFilePath();
 
 			if (filepath == null || filepath.isEmpty()) {
@@ -986,7 +987,9 @@ public class ArbitraryResource {
 		if (build != null && build == true) {
 			ArbitraryDataReader reader = new ArbitraryDataReader(name, ArbitraryDataFile.ResourceIdType.NAME, service, null);
 			try {
+				if (!reader.isBuilding()) {
 					reader.loadSynchronously(false);
+				}
 			} catch (Exception e) {
 				// No need to handle exception, as it will be reflected in the status
 			}

src/main/java/org/qortal/arbitrary/ArbitraryDataReader.java

@@ -107,6 +107,10 @@ public class ArbitraryDataReader {
         return false;
     }
 
+    public boolean isBuilding() {
+        return ArbitraryDataBuildManager.getInstance().isInBuildQueue(this.createQueueItem());
+    }
+
     private ArbitraryDataBuildQueueItem createQueueItem() {
         return new ArbitraryDataBuildQueueItem(this.resourceId, this.resourceIdType, this.service, this.identifier);
     }

src/main/java/org/qortal/arbitrary/ArbitraryDataRenderer.java

@@ -6,7 +6,9 @@ import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.qortal.api.HTMLParser;
 import org.qortal.arbitrary.ArbitraryDataFile.*;
+import org.qortal.arbitrary.exception.MissingDataException;
 import org.qortal.arbitrary.misc.Service;
+import org.qortal.controller.Controller;
 
 import javax.servlet.ServletContext;
 import javax.servlet.http.HttpServletRequest;
@@ -72,8 +74,23 @@ public class ArbitraryDataRenderer {
                     return this.getLoadingResponse(service, resourceId);
                 }
 
-                // Otherwise, hang the request until the build completes
+                // Otherwise, loop until we have data
+                int attempts = 0;
+                while (!Controller.isStopping()) {
+                    attempts++;
+                    if (!arbitraryDataReader.isBuilding()) {
+                        try {
                             arbitraryDataReader.loadSynchronously(false);
+                            break;
+                        } catch (MissingDataException e) {
+                            if (attempts > 5) {
+                                // Give up after 5 attempts
+                                return ArbitraryDataRenderer.getResponse(response, 404, "Data unavailable. Please try again later.");
+                            }
+                        }
+                    }
+                    Thread.sleep(3000L);
+                }
             }
 
         } catch (Exception e) {