daguimu opened a new pull request, #16169:
URL: https://github.com/apache/dubbo/pull/16169
## Problem
`BitList.emptyList()` returns a shared static singleton instance. Since
`BitList` is mutable (supports `add()`, `addToTailList()`, `and()`, etc.), any
caller that modifies the returned "empty" list inadvertently mutates the
singleton, contaminating all subsequent callers.
In `AbstractDirectory`, the `invokers` field is initialized with
`BitList.emptyList()`. When the first interface's invokers are added, they
pollute the shared singleton. When a second interface later calls
`BitList.emptyList()`, it receives a list already containing the first
interface's invokers, causing traffic to be routed to the wrong instances and
resulting in `NoSuchMethodError`.
## Root Cause
```java
// Shared mutable singleton - any mutation affects all callers
private static final BitList emptyList = new
BitList(Collections.emptyList());
public static <T> BitList<T> emptyList() {
return emptyList; // returns the SAME instance every time
}
```
## Fix
Return a new `BitList` instance on each call to `emptyList()`, consistent
with how mutable collections should behave:
```java
public static <T> BitList<T> emptyList() {
return new BitList<>(Collections.emptyList());
}
```
## Tests Added
- `testEmptyListReturnsIndependentInstances` — Verifies each call returns a
distinct object
- `testEmptyListMutationDoesNotAffectOtherEmptyList` — Reproduces the exact
bug: adding elements to one empty list must not affect another (the core
scenario from #16131)
- `testEmptyListTailListMutationDoesNotAffectOtherEmptyList` — Verifies tail
list mutations are also isolated
- `testEmptyListIsInitiallyEmpty` — Confirms the returned list starts truly
empty
All 26 tests in `BitListTest` pass.
## Impact
- Minimal change: removes 1 static field, modifies 1 method (3 lines changed
in production code)
- No behavioral change for callers — they get the same empty `BitList`, just
a fresh instance
- No reference equality (`==`) checks exist against `BitList.emptyList()` in
the codebase
- Fixes a critical production bug where cross-interface invoker leakage
causes `NoSuchMethodError`
Fixes #16131
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