mirror of
https://github.com/ethereum/solidity
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147 lines
6.2 KiB
ReStructuredText
147 lines
6.2 KiB
ReStructuredText
#################
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Contract Metadata
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#################
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.. index:: metadata, contract verification
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The Solidity compiler automatically generates a JSON file, the
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contract metadata, that contains information about the current contract.
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It can be used to query the compiler version, the sources used, the ABI
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and NatSpec documentation in order to more safely interact with the contract
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and to verify its source code.
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The compiler appends a Swarm hash of the metadata file to the end of the
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bytecode (for details, see below) of each contract, so that you can retrieve
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the file in an authenticated way without having to resort to a centralized
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data provider.
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Of course, you have to publish the metadata file to Swarm (or some other service)
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so that others can access it. The file can be output by using ``solc --metadata``
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and the file will be called ``ContractName_meta.json``.
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It will contain Swarm references to the source code, so you have to upload
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all source files and the metadata file.
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The metadata file has the following format. The example below is presented in a
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human-readable way. Properly formatted metadata should use quotes correctly,
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reduce whitespace to a minimum and sort the keys of all objects to arrive at a
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unique formatting.
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Comments are of course also not permitted and used here only for explanatory purposes.
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.. code-block:: none
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{
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// Required: The version of the metadata format
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version: "1",
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// Required: Source code language, basically selects a "sub-version"
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// of the specification
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language: "Solidity",
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// Required: Details about the compiler, contents are specific
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// to the language.
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compiler: {
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// Required for Solidity: Version of the compiler
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version: "0.4.6+commit.2dabbdf0.Emscripten.clang",
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// Optional: Hash of the compiler binary which produced this output
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keccak256: "0x123..."
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},
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// Required: Compilation source files/source units, keys are file names
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sources:
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{
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"myFile.sol": {
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// Required: keccak256 hash of the source file
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"keccak256": "0x123...",
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// Required (unless "content" is used, see below): Sorted URL(s)
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// to the source file, protocol is more or less arbitrary, but a
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// Swarm URL is recommended
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"urls": [ "bzzr://56ab..." ]
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},
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"mortal": {
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// Required: keccak256 hash of the source file
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"keccak256": "0x234...",
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// Required (unless "url" is used): literal contents of the source file
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"content": "contract mortal is owned { function kill() { if (msg.sender == owner) selfdestruct(owner); } }"
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}
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},
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// Required: Compiler settings
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settings:
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{
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// Required for Solidity: Sorted list of remappings
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remappings: [ ":g/dir" ],
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// Optional: Optimizer settings (enabled defaults to false)
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optimizer: {
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enabled: true,
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runs: 500
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},
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// Required for Solidity: File and name of the contract or library this
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// metadata is created for.
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compilationTarget: {
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"myFile.sol": "MyContract"
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},
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// Required for Solidity: Addresses for libraries used
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libraries: {
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"MyLib": "0x123123..."
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}
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},
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// Required: Generated information about the contract.
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output:
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{
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// Required: ABI definition of the contract
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abi: [ ... ],
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// Required: NatSpec user documentation of the contract
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userdoc: [ ... ],
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// Required: NatSpec developer documentation of the contract
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devdoc: [ ... ],
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}
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}
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.. note::
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Note the ABI definition above has no fixed order. It can change with compiler versions.
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.. note::
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Since the bytecode of the resulting contract contains the metadata hash, any change to
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the metadata will result in a change of the bytecode. Furthermore, since the metadata
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includes a hash of all the sources used, a single whitespace change in any of the source
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codes will result in a different metadata, and subsequently a different bytecode.
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Encoding of the Metadata Hash in the Bytecode
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=============================================
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Because we might support other ways to retrieve the metadata file in the future,
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the mapping ``{"bzzr0": <Swarm hash>}`` is stored
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`CBOR <https://tools.ietf.org/html/rfc7049>`_-encoded. Since the beginning of that
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encoding is not easy to find, its length is added in a two-byte big-endian
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encoding. The current version of the Solidity compiler thus adds the following
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to the end of the deployed bytecode::
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0xa1 0x65 'b' 'z' 'z' 'r' '0' 0x58 0x20 <32 bytes swarm hash> 0x00 0x29
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So in order to retrieve the data, the end of the deployed bytecode can be checked
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to match that pattern and use the Swarm hash to retrieve the file.
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Usage for Automatic Interface Generation and NatSpec
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====================================================
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The metadata is used in the following way: A component that wants to interact
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with a contract (e.g. Mist) retrieves the code of the contract, from that
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the Swarm hash of a file which is then retrieved.
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That file is JSON-decoded into a structure like above.
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The component can then use the ABI to automatically generate a rudimentary
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user interface for the contract.
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Furthermore, Mist can use the userdoc to display a confirmation message to the user
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whenever they interact with the contract.
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Additional information about Ethereum Natural Specification (NatSpec) can be found `here <https://github.com/ethereum/wiki/wiki/Ethereum-Natural-Specification-Format>`_.
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Usage for Source Code Verification
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==================================
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In order to verify the compilation, sources can be retrieved from Swarm
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via the link in the metadata file.
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The compiler of the correct version (which is checked to be part of the "official" compilers)
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is invoked on that input with the specified settings. The resulting
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bytecode is compared to the data of the creation transaction or ``CREATE`` opcode data.
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This automatically verifies the metadata since its hash is part of the bytecode.
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Excess data corresponds to the constructor input data, which should be decoded
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according to the interface and presented to the user.
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