This changes the p2p protocol handlers to delay message decoding. It's
the first part of a larger change that will delay decoding all the way
through message processing. For responses, we delay the decoding until
it is confirmed that the response matches an active request and does not
exceed its limits.
In order to make this work, all messages have been changed to use
rlp.RawList instead of a slice of the decoded item type. For block
bodies specifically, the decoding has been delayed all the way until
after verification of the response hash.
The role of p2p/tracker.Tracker changes significantly in this PR. The
Tracker's original purpose was to maintain metrics about requests and
responses in the peer-to-peer protocols. Each protocol maintained a
single global Tracker instance. As of this change, the Tracker is now
always active (regardless of metrics collection), and there is a
separate instance of it for each peer. Whenever a response arrives, it
is first verified that a request exists for it in the tracker. The
tracker is also the place where limits are kept.
This PR modifies how the metrics library handles `Enabled`: previously,
the package `init` decided whether to serve real metrics or just
dummy-types.
This has several drawbacks:
- During pkg init, we need to determine whether metrics are enabled or
not. So we first hacked in a check if certain geth-specific
commandline-flags were enabled. Then we added a similar check for
geth-env-vars. Then we almost added a very elaborate check for
toml-config-file, plus toml parsing.
- Using "real" types and dummy types interchangeably means that
everything is hidden behind interfaces. This has a performance penalty,
and also it just adds a lot of code.
This PR removes the interface stuff, uses concrete types, and allows for
the setting of Enabled to happen later. It is still assumed that
`metrics.Enable()` is invoked early on.
The somewhat 'heavy' operations, such as ticking meters and exp-decay,
now checks the enable-flag to prevent resource leak.
The change may be large, but it's mostly pretty trivial, and from the
last time I gutted the metrics, I ensured that we have fairly good test
coverage.
---------
Co-authored-by: Felix Lange <fjl@twurst.com>
* eth/protocols, prp/tracker: add support for req/rep rtt tracking
* p2p/tracker: sanity cap the number of pending requests
* pap/tracker: linter <3
* p2p/tracker: disable entire tracker if no metrics are enabled
