Airdrop Issuance Hook (#75)

* add AirdropIssuanceHook

* add AirdropIssuanceHook tests

* add invokePreRedeemHook test

* improve comments

* add comment about positive rebase requirement

Author: ncitron

contracts/hooks/AirdropIssuanceHook.sol

@@ -0,0 +1,80 @@
+/*
+    Copyright 2021 Index Cooperative.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+*/
+
+pragma solidity 0.6.10;
+
+import { IAirdropModule } from "../interfaces/IAirdropModule.sol";
+import { IManagerIssuanceHook } from "../interfaces/IManagerIssuanceHook.sol";
+import { ISetToken } from "../interfaces/ISetToken.sol";
+
+/**
+ * @title AirdropIssuanceHook
+ * @author Index Coop
+ *
+ * Issuance hooks that absorbs all airdropped tokens. Useful for ensuring that rebasing tokens are fully accounted for before issuance. Only works
+ * with tokens that strictly positively rebase such as aTokens.
+ */
+contract AirdropIssuanceHook is IManagerIssuanceHook {
+
+    /* ============ State Variables ============ */
+
+    // Address of Set Protocol AirdropModule
+    IAirdropModule public airdropModule;
+
+    /* ============== Constructor ================ */
+
+    /**
+     * Sets state variables.
+     *
+     * @param   _airdropModule      address of AirdropModule
+     */
+    constructor(IAirdropModule _airdropModule) public {
+        airdropModule = _airdropModule;
+    }
+
+    /* =========== External Functions =========== */
+
+    /**
+     * Absorbs all airdropped tokens. Called by some issuance modules before issuance.
+     *
+     * @param   _setToken           address of SetToken to absorb airdrops for
+     */
+    function invokePreIssueHook(ISetToken _setToken, uint256 /* _issueQuantity */, address /* _sender */, address /* _to */) external override {
+        _sync(_setToken);
+    }
+
+    /**
+     * Absorbs all airdropped tokens. Called by some issuance modules before redemption.
+     *
+     * @param   _setToken           address of SetToken to absorb airdrops for
+     */
+    function invokePreRedeemHook(ISetToken _setToken, uint256 /* _issueQuantity */, address /* _sender */, address /* _to */) external override {
+        _sync(_setToken);
+    }
+
+    /* =========== Internal Functions ========== */
+
+    /**
+     * Absorbs all airdropped tokens. AirdropModule must be added to an initialized for the SetToken. Must have anyoneAbsorb set to true on
+     * the AirdropModule.
+     *
+     * @param   _setToken           address of SetToken to absorb airdrops for
+     */
+    function _sync(ISetToken _setToken) internal {
+        address[] memory airdrops = airdropModule.getAirdrops(_setToken);
+        airdropModule.batchAbsorb(_setToken, airdrops);
+    }
+}

contracts/interfaces/IAirdropModule.sol

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+/*
+    Copyright 2021 Index Cooperative.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+*/
+
+pragma solidity 0.6.10;
+
+import { ISetToken } from "../interfaces/ISetToken.sol";
+
+interface IAirdropModule {
+    function batchAbsorb(ISetToken _setToken, address[] memory _tokens) external;
+    function getAirdrops(ISetToken _setToken) external view returns (address[] memory);
+}