[StorageReading] Invoke external viewer contract via static delegatecall

contracts/StorageAccessible.sol

@@ -3,6 +3,10 @@ pragma solidity ^0.5.2;
 
 /// @title StorageAccessible - generic base contract that allows callers to access all internal storage.
 contract StorageAccessible {
+    bytes4 public constant SIMULATE_DELEGATECALL_INTERNAL_SELECTOR = bytes4(
+        keccak256("simulateDelegatecallInternal(address,bytes)")
+    );
+
     /**
      * @dev Reads `length` bytes of storage in the currents contract
      * @param offset - the offset in the current contract's storage in words to start reading from
@@ -23,4 +27,41 @@ contract StorageAccessible {
         }
         return result;
     }
+
+    /**
+     * @dev Performs a delegetecall on a targetContract in the context of self.
+     * Internally reverts execution to avoid side effects (making it static). Catches revert and returns encoded result as bytes.
+     * @param targetContract Address of the contract containing the code to execute.
+     * @param calldataPayload Calldata that should be sent to the target contract (encoded method name and arguments).
+     */
+    function simulateDelegatecall(
+        address targetContract,
+        bytes memory calldataPayload
+    ) public returns (bytes memory) {
+        bytes memory innerCall = abi.encodeWithSelector(
+            SIMULATE_DELEGATECALL_INTERNAL_SELECTOR,
+            targetContract,
+            calldataPayload
+        );
+        (, bytes memory response) = address(this).call(innerCall);
+        return response;
+    }
+
+    /**
+     * @dev Performs a delegetecall on a targetContract in the context of self.
+     * Internally reverts execution to avoid side effects (making it static). Returns encoded result as revert message.
+     * @param targetContract Address of the contract containing the code to execute.
+     * @param calldataPayload Calldata that should be sent to the target contract (encoded method name and arguments).
+     */
+    function simulateDelegatecallInternal(
+        address targetContract,
+        bytes memory calldataPayload
+    ) public returns (bytes memory) {
+        (, bytes memory response) = targetContract.delegatecall(
+            calldataPayload
+        );
+        assembly {
+            revert(add(response, 0x20), mload(response))
+        }
+    }
 }

contracts/test/StorageAccessibleWrapper.sol

@@ -48,3 +48,22 @@ contract StorageAccessibleWrapper is StorageAccessible {
         foobar = FooBar({foo: foo_, bar: bar_});
     }
 }
+
+
+/**
+ * Defines reader methods on StorageAccessibleWrapper that can be later executed
+ * in the context of a previously deployed instance
+ */
+contract ExternalStorageReader {
+    // Needs same storage layout as the contract it is reading from
+    uint256 foo;
+
+    function getFoo() public view returns (uint256) {
+        return foo;
+    }
+
+    function setAndGetFoo(uint256 foo_) public returns (uint256) {
+        foo = foo_;
+        return foo;
+    }
+}

test/storage_accessible.js

@@ -1,51 +1,83 @@
 const StorageAccessibleWrapper = artifacts.require('StorageAccessibleWrapper')
+const ExternalStorageReader = artifacts.require('ExternalStorageReader')
 
 contract('StorageAccessible', () => {
   const fromHex = string => parseInt(string, 16)
   const keccak = numbers => web3.utils.soliditySha3(
     ...numbers.map(v => ({ type: 'uint256', value: v }))
   )
 
-  it('can read statically sized words', async () => {
-    const instance = await StorageAccessibleWrapper.new()
-    await instance.setFoo(42)
+  describe('getStorageAt', async () => {
+    it('can read statically sized words', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setFoo(42)
 
-    assert.equal(42, await instance.getStorageAt(await instance.SLOT_FOO(), 1))
-  })
-  it('can read fields that are packed into single storage slot', async () => {
-    const instance = await StorageAccessibleWrapper.new()
-    await instance.setBar(7)
-    await instance.setBam(13)
+      assert.equal(42, await instance.getStorageAt(await instance.SLOT_FOO(), 1))
+    })
+    it('can read fields that are packed into single storage slot', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setBar(7)
+      await instance.setBam(13)
 
-    const data = await instance.getStorageAt(await instance.SLOT_BAR(), 1)
+      const data = await instance.getStorageAt(await instance.SLOT_BAR(), 1)
 
-    assert.equal(7, fromHex(data.slice(34, 66)))
-    assert.equal(13, fromHex(data.slice(18, 34)))
-  })
-  it('can read arrays in one go', async () => {
-    const instance = await StorageAccessibleWrapper.new()
-    const slot = await instance.SLOT_BAZ()
-    await instance.setBaz([1, 2])
+      assert.equal(7, fromHex(data.slice(34, 66)))
+      assert.equal(13, fromHex(data.slice(18, 34)))
+    })
+    it('can read arrays in one go', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      const slot = await instance.SLOT_BAZ()
+      await instance.setBaz([1, 2])
 
-    const length = await instance.getStorageAt(slot, 1)
-    assert.equal(fromHex(length), 2)
+      const length = await instance.getStorageAt(slot, 1)
+      assert.equal(fromHex(length), 2)
 
-    const data = await instance.getStorageAt(keccak([slot]), length)
-    assert.equal(1, fromHex(data.slice(2, 66)))
-    assert.equal(2, fromHex(data.slice(66, 130)))
-  })
-  it('can read mappings', async () => {
-    const instance = await StorageAccessibleWrapper.new()
-    await instance.setQuxKeyValue(42, 69)
-    assert.equal(69, await instance.getStorageAt(keccak([42, await instance.SLOT_QUX()]), 1))
+      const data = await instance.getStorageAt(keccak([slot]), length)
+      assert.equal(1, fromHex(data.slice(2, 66)))
+      assert.equal(2, fromHex(data.slice(66, 130)))
+    })
+    it('can read mappings', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setQuxKeyValue(42, 69)
+      assert.equal(69, await instance.getStorageAt(keccak([42, await instance.SLOT_QUX()]), 1))
+    })
+
+    it('can read structs', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setFoobar(19, 21)
+
+      const packed = await instance.getStorageAt(await instance.SLOT_FOOBAR(), 10)
+      assert.equal(19, fromHex(packed.slice(2, 66)))
+      assert.equal(21, fromHex(packed.slice(66, 130)))
+    })
   })
 
-  it('can read structs', async () => {
-    const instance = await StorageAccessibleWrapper.new()
-    await instance.setFoobar(19, 21)
+  describe('simulateDelegatecall', async () => {
+    it('can invoke a function in the context of a previously deployed contract', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setFoo(42)
+
+      // Deploy and use reader contract to access foo
+      const reader = await ExternalStorageReader.new()
+      const getFooCall = reader.contract.methods.getFoo().encodeABI()
+      const result = await instance.simulateDelegatecall.call(reader.address, getFooCall)
+      assert.equal(42, fromHex(result))
+    })
+
+    it('can simulate a function with side effects (without executing)', async () => {
+      const instance = await StorageAccessibleWrapper.new()
+      await instance.setFoo(42)
+
+      // Deploy and use reader contract to simulate setting foo
+      const reader = await ExternalStorageReader.new()
+      const replaceFooCall = reader.contract.methods.setAndGetFoo(69).encodeABI()
+      let result = await instance.simulateDelegatecall.call(reader.address, replaceFooCall)
+      assert.equal(69, fromHex(result))
 
-    const packed = await instance.getStorageAt(await instance.SLOT_FOOBAR(), 10)
-    assert.equal(19, fromHex(packed.slice(2, 66)))
-    assert.equal(21, fromHex(packed.slice(66, 130)))
+      // Make sure foo is not actually changed
+      const getFooCall = reader.contract.methods.getFoo().encodeABI()
+      result = await instance.simulateDelegatecall.call(reader.address, getFooCall)
+      assert.equal(42, fromHex(result))
+    })
   })
 })