This commit adds a new function named SerializeSize to the public API for
MsgTx, TxOut, and TxIn which can be used to determine how many bytes the
serialized data would take without having to actually serialize it.
The following benchmark shows the difference between using the new
function to get the serialize size for a typical transaction and
serializing into a temporary buffer and taking the length of it:
Bufffer: BenchmarkTxSerializeSizeBuffer 200000 7050 ns/op
New: BenchmarkTxSerializeSizeNew 100000000 18 ns/op
This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27.
Most variable length integers are smaller numbers, so this commit reverses
the order of the if checks in the writeVarInt to assume smaller numbers
are more common.
This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27
Several of the bitcoin data structures contain variable length entries,
many of which have well-defined maximum limits. However, there are still
a few cases, such as variable length strings and number of transactions
which don't have clearly defined maximum limits. Instead they are only
limited by the maximum size of a message.
In order to efficiently decode messages, space is pre-allocated for the
slices which hold these variable length pieces as to avoid needing to
dynamically grow the backing arrays. Due to this however, it was
previously possible to claim extremely high slice lengths which exceed
available memory (or maximum allowed slice lengths).
This commit imposes limits to all of these cases based on calculating
the maximum possible number of elements that could fit into a message
and using those as sane upper limits.
The variable length string case was found (and tests added to hit it) by
drahn@ which prompted an audit to find all cases.
It is technically possible for the Read method on a reader to return zero
bytes read with a nil error even though that behavior is "discouraged" by
the interface documenation. This commit switches the read of the first
byte to use io.ReadFull which will always error in this case.
