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Document version field in CBOR metadata.

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Daniel Kirchner 2019-05-08 16:12:54 +02:00
parent a6a28ac475
commit aa4ee3a155
1 changed files with 11 additions and 6 deletions
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docs/metadata.rst
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@ -124,30 +124,35 @@ Encoding of the Metadata Hash in the Bytecode
=============================================
Because we might support other ways to retrieve the metadata file in the future,
the mapping ``{"bzzr0": <Swarm hash>}`` is stored
the mapping ``{"bzzr0": <Swarm 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::
0xa1 0x65 'b' 'z' 'z' 'r' '0' 0x58 0x20 <32 bytes swarm hash> 0x00 0x29
0xa2
0x65 'b' 'z' 'z' 'r' '0' 0x58 0x20 <32 bytes swarm hash>
0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding>
0x00 0x32
So in order to retrieve the data, the end of the deployed bytecode can be checked
to match that pattern and use the Swarm 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 ``0xa165``.
decode the data instead of relying on it starting with ``0xa265``.
For example, if any experimental features that affect code generation
are used, the mapping will also contain ``"experimental": true``.
Furthermore, we are planning to add the compiler version to the mapping
to ease source-verification and scanning for bugs.
.. note::
The compiler currently uses the "swarm version 0" hash of the metadata,
but this might change in the future, so do not rely on this sequence
to start with ``0xa1 0x65 'b' 'z' 'z' 'r' '0'``. We might also
to start with ``0xa2 0x65 'b' 'z' 'z' 'r' '0'``. We might also
add additional data to this CBOR structure, so the
best option is to use a proper CBOR parser.