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SafeCore now stores only recent state roots
The safe core now only tracks a fixed number of most recent state roots. The number of state roots to track can be set on construction. The safe core has a ring buffer that tracks the last `n` state roots and from `n + 1` starts overwriting the oldest state root whenever a new state root is committed. I created a separate contract for the circular buffer in order to decouple the concept of a circular buffer and the concept of a state root storage. There were three options of how to add the circular buffer logic to the state root: 1. Deploy the buffer as a separate contract (on core construction) and call to that contract for storage. 2. Use dependency injection and pass the address of the buffer when constructing the core. 3. Extend the buffer with the core to make the logic available. 4. Make the buffer a library. Option 4. was quickly ruled out, as the storage part of the buffer is essential to its logic and inner workings. Options 1. and 2. are somewhat similar. They very well decouple the contracts and make testing easy. From an architectural point of view, option 2. is the easy choice. However, in solidity it does cost actual money to split contracts and make external calls. So that's the drawback. Option 3. puts everything into a single contract and makes all calls to the buffer internal, which is good from an economical point of view. The drawback is of cours that the contracts are very tightly coupled and testing is a pain, as a wrapper contract must be written for the buffer to "externalize" its methods. After a discussion with @benjaminbollen and @pgev, I was convinced to choose the economically more viable approach over the better architectural approach. Core now extends the buffer and makes `internal` calls to it. I decided to leave the contract name in front of all the method names to make it clear to the reader of the contract when the buffer is used. I added tests for the circular buffer and also added a test specific to the state roots to the safe core. Fixes OpenST#504
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| pragma solidity ^0.5.0; | ||
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| // Copyright 2017 OpenST Ltd. | ||
| // | ||
| // 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. | ||
| // | ||
| // ---------------------------------------------------------------------------- | ||
| // | ||
| // http://www.simpletoken.org/ | ||
| // | ||
| // ---------------------------------------------------------------------------- | ||
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| contract CircularBufferUint { | ||
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| /** Storage */ | ||
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| /** | ||
| * The circular buffer that stores the latest `items.length` items. Once | ||
| * `items.length` items were stored, items will be overwritten starting at | ||
| * zero. | ||
| */ | ||
| uint256[] private items; | ||
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| /** | ||
| * The current index in the items array. The index increases up to | ||
| * `items.length - 1` and then resets to zero in an endless loop. This | ||
| * means that a new item will always overwrite the oldest item. | ||
| */ | ||
| uint256 private index; | ||
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| /* Constructor */ | ||
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| /** | ||
| * @notice Create a new buffer with the size `_maxItems`. | ||
| * | ||
| * @param _maxItems Defines how many items this buffer stores before | ||
| * overwriting older items. | ||
| */ | ||
| constructor(uint256 _maxItems) public { | ||
| require( | ||
| _maxItems > 0, | ||
| "The max number of items to store in a circular buffer must be greater than 0." | ||
| ); | ||
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| items.length = _maxItems; | ||
| } | ||
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| /* Internal functions */ | ||
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| /** | ||
| * @notice Store a new item in the circular buffer. | ||
| * | ||
| * @param _item The item to store in the circular buffer. | ||
| * | ||
| * @return overwrittenItem_ The item that was in the circular buffer's | ||
| * position where the new item is now stored. The | ||
| * overwritten item is no longer available in the | ||
| * circular buffer. | ||
| */ | ||
| function store(uint256 _item) internal returns(uint256 overwrittenItem_) { | ||
| nextIndex(); | ||
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| /* | ||
| * Retrieve the old item from the circular buffer before overwriting it | ||
| * with the new item. | ||
| */ | ||
| overwrittenItem_ = items[index]; | ||
| items[index] = _item; | ||
| } | ||
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| /** | ||
| * @notice Get the most recent item that was stored in the circular buffer. | ||
| * | ||
| * @return head_ The most recently stored item. | ||
| */ | ||
| function head() internal view returns(uint256 head_) { | ||
| head_ = items[index]; | ||
| } | ||
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| /* Private functions */ | ||
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| /** | ||
| * @notice Updates the index of the circular buffer to point to the next | ||
| * slot of where to store an item. Resets to zero if it gets to the | ||
| * end of the array that represents the circular. | ||
| */ | ||
| function nextIndex() private { | ||
| index++; | ||
| if (index == items.length) { | ||
| index = 0; | ||
| } | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,64 @@ | ||
| pragma solidity ^0.5.0; | ||
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| // Copyright 2017 OpenST Ltd. | ||
| // | ||
| // 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. | ||
| // | ||
| // ---------------------------------------------------------------------------- | ||
| // | ||
| // http://www.simpletoken.org/ | ||
| // | ||
| // ---------------------------------------------------------------------------- | ||
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| import "../../lib/CircularBufferUint.sol"; | ||
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| contract TestCircularBufferUint is CircularBufferUint { | ||
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| /* Constructor */ | ||
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| /** | ||
| * @notice Create a new test buffer with the size `_maxItems`. | ||
| * | ||
| * @param _maxItems Defines how many items this test buffer stores before | ||
| * overwriting older items. | ||
| */ | ||
| constructor(uint256 _maxItems) public CircularBufferUint(_maxItems) {} | ||
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| /* Internal functions */ | ||
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| /** | ||
| * @notice Store a new item in the circular test buffer. | ||
| * | ||
| * @param _item The item to store in the circular test buffer. | ||
| * | ||
| * @return overwrittenItem_ The item that was in the circular test buffer's | ||
| * position where the new item is now stored. The | ||
| * overwritten item is no longer available in the | ||
| * circular test buffer. | ||
| */ | ||
| function storeExternal(uint256 _item) external returns(uint256 overwrittenItem_) { | ||
| overwrittenItem_ = CircularBufferUint.store(_item); | ||
| } | ||
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| /** | ||
| * @notice Get the most recent item that was stored in the circular test | ||
| * buffer. | ||
| * | ||
| * @return head_ The most recently stored item. | ||
| */ | ||
| function headExternal() external view returns(uint256 head_) { | ||
| head_ = CircularBufferUint.head(); | ||
| } | ||
| } |
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