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cmd/utils: use eth DNS tree for snap discovery (#22808)

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This removes auto-configuration of the snap.*.ethdisco.net DNS discovery tree.
Since measurements have shown that > 75% of nodes in all.*.ethdisco.net support
snap, we have decided to retire the dedicated index for snap and just use the eth
tree instead.

The dial iterators of eth and snap now use the same DNS tree in the default configuration,
so both iterators should use the same DNS discovery client instance. This ensures that
the record cache and rate limit are shared. Records will not be requested multiple times.

While testing the change, I noticed that duplicate DNS requests do happen even
when the client instance is shared. This is because the two iterators request the tree
root, link tree root, and first levels of the tree in lockstep. To avoid this problem, the
change also adds a singleflight.Group instance in the client. When one iterator
attempts to resolve an entry which is already being resolved, the singleflight object
waits for the existing resolve call to finish and returns the entry to both places.
This commit is contained in:
Felix Lange
2021-05-04 11:29:32 +02:00
committed by GitHub
parent 640d2c5e30
commit b8040a430e
7 changed files with 48 additions and 39 deletions
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51
p2p/dnsdisc/client.go
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@ -32,15 +32,17 @@ import (
"github.com/ethereum/go-ethereum/p2p/enode"
"github.com/ethereum/go-ethereum/p2p/enr"
lru "github.com/hashicorp/golang-lru"
"golang.org/x/sync/singleflight"
"golang.org/x/time/rate"
)
// Client discovers nodes by querying DNS servers.
type Client struct {
cfg Config
clock mclock.Clock
entries *lru.Cache
ratelimit *rate.Limiter
cfg Config
clock mclock.Clock
entries *lru.Cache
ratelimit *rate.Limiter
singleflight singleflight.Group
}
// Config holds configuration options for the client.
@ -135,17 +137,20 @@ func (c *Client) NewIterator(urls ...string) (enode.Iterator, error) {
// resolveRoot retrieves a root entry via DNS.
func (c *Client) resolveRoot(ctx context.Context, loc *linkEntry) (rootEntry, error) {
txts, err := c.cfg.Resolver.LookupTXT(ctx, loc.domain)
c.cfg.Logger.Trace("Updating DNS discovery root", "tree", loc.domain, "err", err)
if err != nil {
return rootEntry{}, err
}
for _, txt := range txts {
if strings.HasPrefix(txt, rootPrefix) {
return parseAndVerifyRoot(txt, loc)
e, err, _ := c.singleflight.Do(loc.str, func() (interface{}, error) {
txts, err := c.cfg.Resolver.LookupTXT(ctx, loc.domain)
c.cfg.Logger.Trace("Updating DNS discovery root", "tree", loc.domain, "err", err)
if err != nil {
return rootEntry{}, err
}
}
return rootEntry{}, nameError{loc.domain, errNoRoot}
for _, txt := range txts {
if strings.HasPrefix(txt, rootPrefix) {
return parseAndVerifyRoot(txt, loc)
}
}
return rootEntry{}, nameError{loc.domain, errNoRoot}
})
return e.(rootEntry), err
}
func parseAndVerifyRoot(txt string, loc *linkEntry) (rootEntry, error) {
@ -168,17 +173,21 @@ func (c *Client) resolveEntry(ctx context.Context, domain, hash string) (entry,
if err := c.ratelimit.Wait(ctx); err != nil {
return nil, err
}
cacheKey := truncateHash(hash)
if e, ok := c.entries.Get(cacheKey); ok {
return e.(entry), nil
}
e, err := c.doResolveEntry(ctx, domain, hash)
if err != nil {
return nil, err
}
c.entries.Add(cacheKey, e)
return e, nil
ei, err, _ := c.singleflight.Do(cacheKey, func() (interface{}, error) {
e, err := c.doResolveEntry(ctx, domain, hash)
if err != nil {
return nil, err
}
c.entries.Add(cacheKey, e)
return e, nil
})
e, _ := ei.(entry)
return e, err
}
// doResolveEntry fetches an entry via DNS.