fix: ensure each ProcessorSlotChain gets independent slot instances

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/DefaultSlotChainBuilder.java

@@ -46,9 +46,39 @@ public ProcessorSlotChain build() {
                 continue;
             }
 
-            chain.addLast((AbstractLinkedProcessorSlot<?>) slot);
+            // Always create a new instance of the slot to ensure each chain has its own
+            // copy with independent next pointers. This is necessary because singleton slots
+            // (isSingleton=true in @Spi annotation) are shared across all build() calls.
+            // If a singleton slot is directly used in multiple chains, modifying its 'next'
+            // pointer when building one chain will affect all other chains that use it.
+            // See: https://github.com/alibaba/Sentinel/issues/3007
+            AbstractLinkedProcessorSlot<?> newSlot = newSlot(slot);
+            chain.addLast(newSlot);
         }
 
         return chain;
     }
+
+    /**
+     * Create a new instance of the given slot.
+     * <p>
+     * This method creates a fresh instance of the slot's concrete class via reflection,
+     * ensuring that each chain gets its own independent copy of every slot. This prevents
+     * the issue where singleton slots shared across multiple chains would have their
+     * {@code next} pointer overwritten by the last chain built.
+     * </p>
+     *
+     * @param slot the slot to create a new instance of
+     * @return a new instance of the slot, or the original slot if creation fails
+     */
+    @SuppressWarnings("unchecked")
+    private static AbstractLinkedProcessorSlot<?> newSlot(ProcessorSlot slot) {
+        try {
+            return slot.getClass().getDeclaredConstructor().newInstance();
+        } catch (Exception e) {
+            RecordLog.warn("Failed to create new instance of ProcessorSlot("
+                + slot.getClass().getCanonicalName() + "), using original instance", e);
+            return (AbstractLinkedProcessorSlot<?>) slot;
+        }
+    }
 }