Fix TowerJsonGenerator thread-safety issue (#6619) [ci fast]

Add synchronized to writeObject method to prevent concurrent
modification of the shared stack field when multiple executor
threads serialize JSON simultaneously.

Signed-off-by: Paolo Di Tommaso <[email protected]>

Author: pditommaso

plugins/nf-tower/src/main/io/seqera/tower/plugin/TowerJsonGenerator.groovy

@@ -40,7 +40,7 @@ import org.apache.groovy.json.internal.CharBuf
 @CompileStatic
 class TowerJsonGenerator extends DefaultJsonGenerator {
 
-    List<String> stack = new ArrayList<>(10)
+    private List<String> stack = new ArrayList<>(10)
     Map<String,Integer> scheme
 
     static TowerJsonGenerator create(Map<String,Integer> scheme) {
@@ -74,8 +74,8 @@ class TowerJsonGenerator extends DefaultJsonGenerator {
     }
 
     @Override
-    protected void writeObject(String key, Object object, CharBuf buffer) {
-        final pos=stack.size()
+    protected synchronized void writeObject(String key, Object object, CharBuf buffer) {
+        final pos = stack.size()
         if(key) stack.add(pos, key)
         final fqn = stack.join('.')
         try {