les: implement client connection logic (#16899)

This PR implements les.freeClientPool. It also adds a simulated clock
in common/mclock, which enables time-sensitive tests to run quickly
and still produce accurate results, and package common/prque which is
a generalised variant of prque that enables removing elements other
than the top one from the queue.

les.freeClientPool implements a client database that limits the
connection time of each client and manages accepting/rejecting
incoming connections and even kicking out some connected clients. The
pool calculates recent usage time for each known client (a value that
increases linearly when the client is connected and decreases
exponentially when not connected). Clients with lower recent usage are
preferred, unknown nodes have the highest priority. Already connected
nodes receive a small bias in their favor in order to avoid accepting
and instantly kicking out clients.

Note: the pool can use any string for client identification. Using
signature keys for that purpose would not make sense when being known
has a negative value for the client. Currently the LES protocol
manager uses IP addresses (without port address) to identify clients.

1 files changed, 57 insertions, 0 deletions

diff --git a/common/prque/prque.go b/common/prque/prque.go
new file mode 100755
index 000000000..9fd31a2e5
--- /dev/null
+++ b/common/prque/prque.go
@@ -0,0 +1,57 @@
+// This is a duplicated and slightly modified version of "gopkg.in/karalabe/cookiejar.v2/collections/prque".
+
+package prque
+
+import (
+	"container/heap"
+)
+
+// Priority queue data structure.
+type Prque struct {
+	cont *sstack
+}
+
+// Creates a new priority queue.
+func New(setIndex setIndexCallback) *Prque {
+	return &Prque{newSstack(setIndex)}
+}
+
+// Pushes a value with a given priority into the queue, expanding if necessary.
+func (p *Prque) Push(data interface{}, priority int64) {
+	heap.Push(p.cont, &item{data, priority})
+}
+
+// Pops the value with the greates priority off the stack and returns it.
+// Currently no shrinking is done.
+func (p *Prque) Pop() (interface{}, int64) {
+	item := heap.Pop(p.cont).(*item)
+	return item.value, item.priority
+}
+
+// Pops only the item from the queue, dropping the associated priority value.
+func (p *Prque) PopItem() interface{} {
+	return heap.Pop(p.cont).(*item).value
+}
+
+// Remove removes the element with the given index.
+func (p *Prque) Remove(i int) interface{} {
+	if i < 0 {
+		return nil
+	}
+	return heap.Remove(p.cont, i)
+}
+
+// Checks whether the priority queue is empty.
+func (p *Prque) Empty() bool {
+	return p.cont.Len() == 0
+}
+
+// Returns the number of element in the priority queue.
+func (p *Prque) Size() int {
+	return p.cont.Len()
+}
+
+// Clears the contents of the priority queue.
+func (p *Prque) Reset() {
+	*p = *New(p.cont.setIndex)
+}