Proxy: Add MASQUE inbound (IETF CONNECT-IP server, RFC 9484) (#6844)

Completes https://github.com/XTLS/Xray-core/pull/6807 and https://github.com/XTLS/Xray-core/pull/6810

proxy/masque/pool.go

@@ -0,0 +1,80 @@
+package masque
+
+import (
+	"net/netip"
+	"sync"
+
+	"github.com/xtls/xray-core/common/errors"
+)
+
+type addressPool struct {
+	mu     sync.Mutex
+	prefix netip.Prefix
+	server netip.Addr
+	first  netip.Addr
+	last   netip.Addr
+	next   netip.Addr
+	used   map[netip.Addr]struct{}
+}
+
+func newAddressPool(address netip.Prefix) (*addressPool, error) {
+	server := address.Addr()
+	if server.Is4In6() || server.Zone() != "" {
+		return nil, errors.New("invalid address ", address)
+	}
+	prefix := address.Masked()
+	last := lastAddr(prefix)
+	if server == prefix.Addr() || server.Is4() && server == last {
+		return nil, errors.New("address ", address, " is not a host address")
+	}
+	if server.Is4() {
+		last = last.Prev()
+	}
+	first := prefix.Addr().Next()
+	if first == last {
+		return nil, errors.New("address ", address, " leaves no addresses to assign")
+	}
+	return &addressPool{
+		prefix: prefix,
+		server: server,
+		first:  first,
+		last:   last,
+		next:   first,
+		used:   make(map[netip.Addr]struct{}),
+	}, nil
+}
+
+func lastAddr(prefix netip.Prefix) netip.Addr {
+	b := prefix.Addr().AsSlice()
+	for i := prefix.Bits(); i < len(b)*8; i++ {
+		b[i/8] |= 1 << (7 - i%8)
+	}
+	addr, _ := netip.AddrFromSlice(b)
+	return addr
+}
+
+func (p *addressPool) allocate() (netip.Addr, bool) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	for addr := p.next; ; {
+		next := addr.Next()
+		if addr == p.last {
+			next = p.first
+		}
+		if _, found := p.used[addr]; !found && addr != p.server {
+			p.used[addr] = struct{}{}
+			p.next = next
+			return addr, true
+		}
+		if next == p.next {
+			return netip.Addr{}, false
+		}
+		addr = next
+	}
+}
+
+func (p *addressPool) release(addr netip.Addr) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	delete(p.used, addr)
+}