Allow _pause() to be called when already paused

.changeset/smooth-penguins-pause.md

@@ -0,0 +1,6 @@
+---
+'openzeppelin-solidity': patch
+---
+
+Allow `_pause()` to be called when the contract is already paused, preventing denial of service in emergency scenarios.
+

contracts/utils/Pausable.sol

@@ -89,11 +89,10 @@ abstract contract Pausable is Context {
     /**
      * @dev Triggers stopped state.
      *
-     * Requirements:
-     *
-     * - The contract must not be paused.
+     * This function can be called even when the contract is already paused, which
+     * can be useful in emergency scenarios to prevent denial of service.
      */
-    function _pause() internal virtual whenNotPaused {
+    function _pause() internal virtual {
         _paused = true;
         emit Paused(_msgSender());
     }

fv/specs/Pausable.spec

@@ -23,11 +23,11 @@ rule pause(env e) {
 
     bool pausedAfter = paused();
 
-    // liveness
-    assert success <=> !pausedBefore, "works if and only if the contract was not paused before";
+    // liveness - pause() can be called even when already paused to prevent DOS
+    assert success, "pause() always succeeds, even when already paused";
 
     // effect
-    assert success => pausedAfter, "contract must be paused after a successful call";
+    assert pausedAfter, "contract must be paused after a successful call";
 }
 
 /*

test/utils/Pausable.test.js

@@ -51,8 +51,10 @@ describe('Pausable', function () {
         expect(await this.mock.drasticMeasureTaken()).to.be.true;
       });
 
-      it('reverts when re-pausing', async function () {
-        await expect(this.mock.pause()).to.be.revertedWithCustomError(this.mock, 'EnforcedPause');
+      it('allows re-pausing when already paused', async function () {
+        const tx = await this.mock.pause();
+        await expect(tx).to.emit(this.mock, 'Paused').withArgs(this.pauser);
+        expect(await this.mock.paused()).to.be.true;
       });
 
       describe('unpausing', function () {