mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
204 lines
8.6 KiB
ReStructuredText
204 lines
8.6 KiB
ReStructuredText
#################
|
|
Contract Metadata
|
|
#################
|
|
|
|
.. index:: metadata, contract verification
|
|
|
|
The Solidity compiler automatically generates a JSON file, the contract
|
|
metadata, that contains information about the compiled contract. You can use
|
|
this file to query the compiler version, the sources used, the ABI and NatSpec
|
|
documentation to more safely interact with the contract and verify its source
|
|
code.
|
|
|
|
The compiler appends by default the IPFS hash of the metadata file to the end
|
|
of the bytecode (for details, see below) of each contract, so that you can
|
|
retrieve the file in an authenticated way without having to resort to a
|
|
centralized data provider. The other available options are the Swarm hash and
|
|
not appending the metadata hash to the bytecode. These can be configured via
|
|
the :ref:`Standard JSON Interface<compiler-api>`.
|
|
|
|
You have to publish the metadata file to IPFS, Swarm, or another service so
|
|
that others can access it. You create the file by using the ``solc --metadata``
|
|
command that generates a file called ``ContractName_meta.json``. It contains
|
|
IPFS and Swarm references to the source code, so you have to upload all source
|
|
files and the metadata file.
|
|
|
|
The metadata file has the following format. The example below is presented in a
|
|
human-readable way. Properly formatted metadata should use quotes correctly,
|
|
reduce whitespace to a minimum and sort the keys of all objects to arrive at a
|
|
unique formatting. Comments are not permitted and used here only for
|
|
explanatory purposes.
|
|
|
|
.. code-block:: none
|
|
|
|
{
|
|
// Required: The version of the metadata format
|
|
version: "1",
|
|
// Required: Source code language, basically selects a "sub-version"
|
|
// of the specification
|
|
language: "Solidity",
|
|
// Required: Details about the compiler, contents are specific
|
|
// to the language.
|
|
compiler: {
|
|
// Required for Solidity: Version of the compiler
|
|
version: "0.4.6+commit.2dabbdf0.Emscripten.clang",
|
|
// Optional: Hash of the compiler binary which produced this output
|
|
keccak256: "0x123..."
|
|
},
|
|
// Required: Compilation source files/source units, keys are file names
|
|
sources:
|
|
{
|
|
"myFile.sol": {
|
|
// Required: keccak256 hash of the source file
|
|
"keccak256": "0x123...",
|
|
// Required (unless "content" is used, see below): Sorted URL(s)
|
|
// to the source file, protocol is more or less arbitrary, but a
|
|
// Swarm URL is recommended
|
|
"urls": [ "bzzr://56ab..." ]
|
|
},
|
|
"destructible": {
|
|
// Required: keccak256 hash of the source file
|
|
"keccak256": "0x234...",
|
|
// Required (unless "url" is used): literal contents of the source file
|
|
"content": "contract destructible is owned { function destroy() { if (msg.sender == owner) selfdestruct(owner); } }"
|
|
}
|
|
},
|
|
// Required: Compiler settings
|
|
settings:
|
|
{
|
|
// Required for Solidity: Sorted list of remappings
|
|
remappings: [ ":g=/dir" ],
|
|
// Optional: Optimizer settings. The fields "enabled" and "runs" are deprecated
|
|
// and are only given for backwards-compatibility.
|
|
optimizer: {
|
|
enabled: true,
|
|
runs: 500,
|
|
details: {
|
|
// peephole defaults to "true"
|
|
peephole: true,
|
|
// jumpdestRemover defaults to "true"
|
|
jumpdestRemover: true,
|
|
orderLiterals: false,
|
|
deduplicate: false,
|
|
cse: false,
|
|
constantOptimizer: false,
|
|
yul: true,
|
|
// Optional: Only present if "yul" is "true"
|
|
yulDetails: {
|
|
stackAllocation: false,
|
|
optimizerSteps: "dhfoDgvulfnTUtnIf..."
|
|
}
|
|
}
|
|
},
|
|
metadata: {
|
|
// Reflects the setting used in the input json, defaults to false
|
|
useLiteralContent: true,
|
|
// Reflects the setting used in the input json, defaults to "ipfs"
|
|
bytecodeHash: "ipfs"
|
|
}
|
|
// Required for Solidity: File and name of the contract or library this
|
|
// metadata is created for.
|
|
compilationTarget: {
|
|
"myFile.sol": "MyContract"
|
|
},
|
|
// Required for Solidity: Addresses for libraries used
|
|
libraries: {
|
|
"MyLib": "0x123123..."
|
|
}
|
|
},
|
|
// Required: Generated information about the contract.
|
|
output:
|
|
{
|
|
// Required: ABI definition of the contract
|
|
abi: [ ... ],
|
|
// Required: NatSpec user documentation of the contract
|
|
userdoc: [ ... ],
|
|
// Required: NatSpec developer documentation of the contract
|
|
devdoc: [ ... ],
|
|
}
|
|
}
|
|
|
|
.. warning::
|
|
Since the bytecode of the resulting contract contains the metadata hash by default, any
|
|
change to the metadata might result in a change of the bytecode. This includes
|
|
changes to a filename or path, and since the metadata includes a hash of all the
|
|
sources used, a single whitespace change results in different metadata, and
|
|
different bytecode.
|
|
|
|
.. note::
|
|
The ABI definition above has no fixed order. It can change with compiler versions.
|
|
Starting from Solidity version 0.5.12, though, the array maintains a certain
|
|
order.
|
|
|
|
.. _encoding-of-the-metadata-hash-in-the-bytecode:
|
|
|
|
Encoding of the Metadata Hash in the Bytecode
|
|
=============================================
|
|
|
|
Because we might support other ways to retrieve the metadata file in the future,
|
|
the mapping ``{"ipfs": <IPFS hash>, "solc": <compiler version>}`` is stored
|
|
`CBOR <https://tools.ietf.org/html/rfc7049>`_-encoded. Since the mapping might
|
|
contain more keys (see below) and the beginning of that
|
|
encoding is not easy to find, its length is added in a two-byte big-endian
|
|
encoding. The current version of the Solidity compiler usually adds the following
|
|
to the end of the deployed bytecode::
|
|
|
|
0xa2
|
|
0x64 'i' 'p' 'f' 's' 0x58 0x22 <34 bytes IPFS hash>
|
|
0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding>
|
|
0x00 0x33
|
|
|
|
So in order to retrieve the data, the end of the deployed bytecode can be checked
|
|
to match that pattern and use the IPFS hash to retrieve the file.
|
|
|
|
Whereas release builds of solc use a 3 byte encoding of the version as shown
|
|
above (one byte each for major, minor and patch version number), prerelease builds
|
|
will instead use a complete version string including commit hash and build date.
|
|
|
|
.. note::
|
|
The CBOR mapping can also contain other keys, so it is better to fully
|
|
decode the data instead of relying on it starting with ``0xa264``.
|
|
For example, if any experimental features that affect code generation
|
|
are used, the mapping will also contain ``"experimental": true``.
|
|
|
|
.. note::
|
|
The compiler currently uses the IPFS hash of the metadata by default, but
|
|
it may also use the bzzr1 hash or some other hash in the future, so do
|
|
not rely on this sequence to start with ``0xa2 0x64 'i' 'p' 'f' 's'``. We
|
|
might also add additional data to this CBOR structure, so the best option
|
|
is to use a proper CBOR parser.
|
|
|
|
|
|
Usage for Automatic Interface Generation and NatSpec
|
|
====================================================
|
|
|
|
The metadata is used in the following way: A component that wants to interact
|
|
with a contract (e.g. Mist or any wallet) retrieves the code of the contract,
|
|
from that the IPFS/Swarm hash of a file which is then retrieved. That file
|
|
is JSON-decoded into a structure like above.
|
|
|
|
The component can then use the ABI to automatically generate a rudimentary
|
|
user interface for the contract.
|
|
|
|
Furthermore, the wallet can use the NatSpec user documentation to display a confirmation message to the user
|
|
whenever they interact with the contract, together with requesting
|
|
authorization for the transaction signature.
|
|
|
|
For additional information, read :doc:`Ethereum Natural Language Specification (NatSpec) format <natspec-format>`.
|
|
|
|
Usage for Source Code Verification
|
|
==================================
|
|
|
|
In order to verify the compilation, sources can be retrieved from IPFS/Swarm
|
|
via the link in the metadata file.
|
|
The compiler of the correct version (which is checked to be part of the "official" compilers)
|
|
is invoked on that input with the specified settings. The resulting
|
|
bytecode is compared to the data of the creation transaction or ``CREATE`` opcode data.
|
|
This automatically verifies the metadata since its hash is part of the bytecode.
|
|
Excess data corresponds to the constructor input data, which should be decoded
|
|
according to the interface and presented to the user.
|
|
|
|
In the repository `source-verify <https://github.com/ethereum/source-verify>`_
|
|
(`npm package <https://www.npmjs.com/package/source-verify>`_) you can see
|
|
example code that shows how to use this feature.
|