Add transferLiquidity function back in

contracts/gas-snapshots/ccip.gas-snapshot

@@ -942,17 +942,18 @@ TokenProxy_getFee:test_GetFeeGasShouldBeZero_Revert() (gas: 16827)
 TokenProxy_getFee:test_GetFeeInvalidToken_Revert() (gas: 12658)
 TokenProxy_getFee:test_GetFeeNoDataAllowed_Revert() (gas: 15849)
 TokenProxy_getFee:test_GetFee_Success() (gas: 86702)
-USDCTokenPoolAltMechanismTests:test_LockOrBurn_PrimaryMechanism_Success() (gas: 136896)
-USDCTokenPoolAltMechanismTests:test_LockOrBurn_WhileMigrationPause_Revert() (gas: 98179)
-USDCTokenPoolAltMechanismTests:test_LockOrBurn_altMechanism_then_switchToPrimary_Success() (gas: 209580)
-USDCTokenPoolAltMechanismTests:test_LockOrBurn_onAltMechanism_Success() (gas: 143718)
+USDCTokenPoolAltMechanismTests:test_LockOrBurn_PrimaryMechanism_Success() (gas: 135152)
+USDCTokenPoolAltMechanismTests:test_LockOrBurn_WhileMigrationPause_Revert() (gas: 97433)
+USDCTokenPoolAltMechanismTests:test_LockOrBurn_altMechanism_then_switchToPrimary_Success() (gas: 207691)
+USDCTokenPoolAltMechanismTests:test_LockOrBurn_onAltMechanism_Success() (gas: 143689)
 USDCTokenPoolAltMechanismTests:test_MintOrRelease_OnAltMechanism_Success() (gas: 212557)
 USDCTokenPoolAltMechanismTests:test_MintOrRelease_altMechanism_then_switchToPrimary_Success() (gas: 423280)
 USDCTokenPoolAltMechanismTests:test_MintOrRelease_incomingMessageWithPrimaryMechanism() (gas: 270078)
 USDCTokenPoolAltMechanismTests:test_withdrawLiquidity_Success() (gas: 118475)
 USDCTokenPoolMigrationTests:test_burnLockedUSDC_invalidPermissions_Revert() (gas: 39951)
-USDCTokenPoolMigrationTests:test_lockOrBurn_then_BurnInCCTPMigration_Success() (gas: 246269)
-USDCTokenPoolMigrationTests:test_proposeCCTPMigration_withInvalidSelector_Revert() (gas: 35556)
+USDCTokenPoolMigrationTests:test_lockOrBurn_then_BurnInCCTPMigration_Success() (gas: 246296)
+USDCTokenPoolMigrationTests:test_proposeCCTPMigration_withInvalidSelector_Revert() (gas: 35644)
+USDCTokenPoolMigrationTests:test_transferLiquidity_Success() (gas: 152643)
 USDCTokenPool__validateMessage:test_ValidateInvalidMessage_Revert() (gas: 25290)
 USDCTokenPool_lockOrBurn:test_CallerIsNotARampOnRouter_Revert() (gas: 35322)
 USDCTokenPool_lockOrBurn:test_LockOrBurnWithAllowList_Revert() (gas: 30073)

contracts/src/v0.8/ccip/pools/MultiMechanismPools/IMigratableMultiMechanismTokenPool.sol

@@ -0,0 +1,5 @@
+pragma solidity ^0.8.0;
+
+interface IMigratableMultiMechanismTokenPool {
+  function withdrawLiquidity(uint64 remoteChainSelector, uint256 amount) external;
+}

contracts/src/v0.8/ccip/pools/MultiMechanismPools/MultiMechanismUSDCTokenPool.sol

@@ -3,9 +3,9 @@ pragma solidity 0.8.24;
 
 import {ILiquidityContainer} from "../../../liquiditymanager/interfaces/ILiquidityContainer.sol";
 import {ITokenMessenger} from "../USDC/ITokenMessenger.sol";
+import {IMigratableMultiMechanismTokenPool} from "./IMigratableMultiMechanismTokenPool.sol";
 
 import {Pool} from "../../libraries/Pool.sol";
-
 import {TokenPool} from "../TokenPool.sol";
 import {USDCTokenPool} from "../USDC/USDCTokenPool.sol";
 import {MultiMechanismPoolManager} from "./MultiMechanismPoolManager.sol";
@@ -28,6 +28,8 @@ contract MultiMechanismUSDCTokenPool is USDCTokenPool, MultiMechanismPoolManager
   /// balanceOf(pool) on home chain >= sum(totalSupply(mint/burn "wrapped" token) on all remote chains) should always hold
   address internal s_rebalancer;
 
+  event LiquidityTransferred(address indexed from, uint64 indexed remoteChainSelector, uint256 amount);
+
   error LanePausedForCCTPMigration(uint64 remoteChainSelector);
 
   constructor(
@@ -48,7 +50,11 @@ contract MultiMechanismUSDCTokenPool is USDCTokenPool, MultiMechanismPoolManager
   {
     // If the alternative mechanism (L/R) for chains which have it enabled
     if (shouldUseAltMechForOutgoingMessage(lockOrBurnIn.remoteChainSelector)) {
-      return _altMechOutgoingMessage(lockOrBurnIn);
+      if (s_proposedUSDCMigrationChain != 0 && s_proposedUSDCMigrationChain == lockOrBurnIn.remoteChainSelector) {
+        revert LanePausedForCCTPMigration(s_proposedUSDCMigrationChain);
+      } else {
+        return _altMechOutgoingMessage(lockOrBurnIn);
+      }
     } else {
       // Otherwise, use native CCTP functionality
       return super.lockOrBurn(lockOrBurnIn);
@@ -107,17 +113,6 @@ contract MultiMechanismUSDCTokenPool is USDCTokenPool, MultiMechanismPoolManager
     return Pool.LockOrBurnOutV1({destTokenAddress: getRemoteToken(lockOrBurnIn.remoteChainSelector), destPoolData: ""});
   }
 
-  /// @notice Circle's migration policies requires a pausable supply-lock during the migration process.
-  /// This override adds an additional condition to valid lockOrBurns that ensures the destination chain
-  /// is not currently undergoing a migration by reverting, as well as all other validations
-  function _validateLockOrBurn(Pool.LockOrBurnInV1 memory lockOrBurnIn) internal override {
-    if (s_proposedUSDCMigrationChain != 0 && s_proposedUSDCMigrationChain == lockOrBurnIn.remoteChainSelector) {
-      revert LanePausedForCCTPMigration(s_proposedUSDCMigrationChain);
-    }
-
-    super._validateLockOrBurn(lockOrBurnIn);
-  }
-
   /// @notice Gets LiquidityManager, can be address(0) if none is configured.
   /// @return The current liquidity manager.
   function getRebalancer() external view returns (address) {
@@ -158,4 +153,23 @@ contract MultiMechanismUSDCTokenPool is USDCTokenPool, MultiMechanismPoolManager
     i_token.safeTransfer(msg.sender, amount);
     emit ILiquidityContainer.LiquidityRemoved(msg.sender, amount);
   }
+
+  /// @notice This function can be used to transfer liquidity from an older version of the pool to this pool. To do so
+  /// this pool will have to be set as the rebalancer in the older version of the pool. This allows it to transfer the
+  /// funds in the old pool to the new pool.
+  /// @dev When upgrading a LockRelease pool, this function can be called at the same time as the pool is changed in the
+  /// TokenAdminRegistry. This allows for a smooth transition of both liquidity and transactions to the new pool.
+  /// Alternatively, when no multicall is available, a portion of the funds can be transferred to the new pool before
+  /// changing which pool CCIP uses, to ensure both pools can operate. Then the pool should be changed in the
+  /// TokenAdminRegistry, which will activate the new pool. All new transactions will use the new pool and its
+  /// liquidity. Finally, the remaining liquidity can be transferred to the new pool using this function one more time.
+  /// @param from The address of the old pool.
+  /// @param amount The amount of liquidity to transfer.
+  function transferLiquidity(address from, uint64 remoteChainSelector, uint256 amount) external onlyOwner {
+    IMigratableMultiMechanismTokenPool(from).withdrawLiquidity(remoteChainSelector, amount);
+
+    s_lockedTokensByChainSelector[remoteChainSelector] += amount;
+
+    emit LiquidityTransferred(from, remoteChainSelector, amount);
+  }
 }

contracts/src/v0.8/ccip/pools/TokenPool.sol

@@ -157,7 +157,7 @@ abstract contract TokenPool is IPoolV1, OwnerIsCreator {
   /// @param lockOrBurnIn The input to validate.
   /// @dev This function should always be called before executing a lock or burn. Not doing so would allow
   /// for various exploits.
-  function _validateLockOrBurn(Pool.LockOrBurnInV1 memory lockOrBurnIn) internal virtual {
+  function _validateLockOrBurn(Pool.LockOrBurnInV1 memory lockOrBurnIn) internal {
     if (!isSupportedToken(lockOrBurnIn.localToken)) revert InvalidToken(lockOrBurnIn.localToken);
     if (IRMN(i_rmnProxy).isCursed(bytes16(uint128(lockOrBurnIn.remoteChainSelector)))) revert CursedByRMN();
     _checkAllowList(lockOrBurnIn.originalSender);

contracts/src/v0.8/ccip/test/pools/MultiMechanismUSDCTokenPool.t.sol

@@ -54,6 +54,7 @@ contract USDCTokenPoolSetup is BaseTest {
   Router internal s_router;
 
   MultiMechanismUSDCTokenPool internal s_usdcTokenPool;
+  MultiMechanismUSDCTokenPool internal s_usdcTokenPoolTransferLiquidity;
   address[] internal s_allowedList;
 
   function setUp() public virtual override {
@@ -71,6 +72,9 @@ contract USDCTokenPoolSetup is BaseTest {
     s_usdcTokenPool =
       new MultiMechanismUSDCTokenPool(s_mockUSDC, s_token, new address[](0), address(s_mockRMN), address(s_router));
 
+    s_usdcTokenPoolTransferLiquidity =
+      new MultiMechanismUSDCTokenPool(s_mockUSDC, s_token, new address[](0), address(s_mockRMN), address(s_router));
+
     usdcToken.grantMintAndBurnRoles(address(s_mockUSDC));
     usdcToken.grantMintAndBurnRoles(address(s_usdcTokenPool));
 
@@ -592,4 +596,48 @@ contract USDCTokenPoolMigrationTests is USDCTokenPoolSetup {
     vm.expectRevert(abi.encodeWithSelector(USDCBridgeMigrator.ExistingMigrationProposal.selector));
     s_usdcTokenPool.burnLockedUSDC();
   }
+
+  function test_transferLiquidity_Success() public {
+    // Set as the OWNER so we can provide liquidity
+    vm.startPrank(OWNER);
+    s_usdcTokenPoolTransferLiquidity.setRebalancer(OWNER);
+
+    s_token.approve(address(s_usdcTokenPoolTransferLiquidity), type(uint256).max);
+
+    uint256 liquidityAmount = 1e9;
+
+    // Provide 1000 USDC as liquidity
+    s_usdcTokenPoolTransferLiquidity.provideLiquidity(DEST_CHAIN_SELECTOR, liquidityAmount);
+
+    // Set the new token pool as the rebalancer
+    s_usdcTokenPoolTransferLiquidity.setRebalancer(address(s_usdcTokenPool));
+
+    vm.expectEmit();
+    emit ILiquidityContainer.LiquidityRemoved(address(s_usdcTokenPool), liquidityAmount);
+
+    vm.expectEmit();
+    emit MultiMechanismUSDCTokenPool.LiquidityTransferred(
+      address(s_usdcTokenPoolTransferLiquidity), DEST_CHAIN_SELECTOR, liquidityAmount
+    );
+
+    s_usdcTokenPool.transferLiquidity(address(s_usdcTokenPoolTransferLiquidity), DEST_CHAIN_SELECTOR, liquidityAmount);
+
+    assertEq(
+      s_usdcTokenPool.getLockedTokensForChain(DEST_CHAIN_SELECTOR),
+      liquidityAmount,
+      "Tokens locked for dest chain doesn't match expected amount in storage"
+    );
+
+    assertEq(
+      s_usdcTokenPoolTransferLiquidity.getLockedTokensForChain(DEST_CHAIN_SELECTOR),
+      0,
+      "Tokens locked for dest chain in old token pool doesn't match expected amount in storage"
+    );
+
+    assertEq(
+      s_token.balanceOf(address(s_usdcTokenPool)),
+      liquidityAmount,
+      "Liquidity amount of tokens should be new in new pool, but aren't"
+    );
+  }
 }