Lock allocation of IP address

current logic is not safe as it will allow an IP that isnt persisted to
the DB to be given out multiple times if machines joins in quick
succession.

This adds a lock around the "get ip" and machine registration and save
to DB so we ensure thiis isnt happning.

Currently this had to be done three places, which is silly, and outlined
in #294.

api.go

@@ -574,6 +574,9 @@ func (h *Headscale) handleAuthKey(
 			Str("func", "handleAuthKey").
 			Str("machine", machine.Name).
 			Msg("Authentication key was valid, proceeding to acquire IP addresses")
+
+		h.ipAllocationMutex.Lock()
+
 		ips, err := h.getAvailableIPs()
 		if err != nil {
 			log.Error().
@@ -602,6 +605,8 @@ func (h *Headscale) handleAuthKey(
 		machine.Registered = true
 		machine.RegisterMethod = RegisterMethodAuthKey
 		h.db.Save(&machine)
+
+		h.ipAllocationMutex.Unlock()
 	}
 
 	pak.Used = true

app.go

@@ -153,6 +153,8 @@ type Headscale struct {
 	oidcStateCache *cache.Cache
 
 	requestedExpiryCache *cache.Cache
+
+	ipAllocationMutex sync.Mutex
 }
 
 // Look up the TLS constant relative to user-supplied TLS client

machine.go

@@ -856,6 +856,9 @@ func (h *Headscale) RegisterMachine(
 		return nil, err
 	}
 
+	h.ipAllocationMutex.Lock()
+	defer h.ipAllocationMutex.Unlock()
+
 	ips, err := h.getAvailableIPs()
 	if err != nil {
 		log.Error().

oidc.go

@@ -317,6 +317,8 @@ func (h *Headscale) OIDCCallback(ctx *gin.Context) {
 				return
 			}
 
+			h.ipAllocationMutex.Lock()
+
 			ips, err := h.getAvailableIPs()
 			if err != nil {
 				log.Error().
@@ -338,6 +340,8 @@ func (h *Headscale) OIDCCallback(ctx *gin.Context) {
 			machine.LastSuccessfulUpdate = &now
 			machine.Expiry = &requestedTime
 			h.db.Save(&machine)
+
+			h.ipAllocationMutex.Unlock()
 		}
 
 		var content bytes.Buffer

utils.go

@@ -12,6 +12,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net"
+	"sort"
 	"strings"
 
 	"github.com/rs/zerolog/log"
@@ -157,9 +158,6 @@ func GetIPPrefixEndpoints(na netaddr.IPPrefix) (network, broadcast netaddr.IP) {
 	return
 }
 
-// TODO: Is this concurrency safe?
-// What would happen if multiple hosts were to register at the same time?
-// Would we attempt to assign the same addresses to multiple nodes?
 func (h *Headscale) getAvailableIP(ipPrefix netaddr.IPPrefix) (*netaddr.IP, error) {
 	usedIps, err := h.getUsedIPs()
 	if err != nil {
@@ -179,7 +177,7 @@ func (h *Headscale) getAvailableIP(ipPrefix netaddr.IPPrefix) (*netaddr.IP, erro
 		switch {
 		case ip.Compare(ipPrefixBroadcastAddress) == 0:
 			fallthrough
-		case containsIPs(usedIps, ip):
+		case usedIps.Contains(ip):
 			fallthrough
 		case ip.IsZero() || ip.IsLoopback():
 			ip = ip.Next()
@@ -192,24 +190,38 @@ func (h *Headscale) getAvailableIP(ipPrefix netaddr.IPPrefix) (*netaddr.IP, erro
 	}
 }
 
-func (h *Headscale) getUsedIPs() ([]netaddr.IP, error) {
+func (h *Headscale) getUsedIPs() (netaddr.IPSet, error) {
 	// FIXME: This really deserves a better data model,
 	// but this was quick to get running and it should be enough
 	// to begin experimenting with a dual stack tailnet.
 	var addressesSlices []string
 	h.db.Model(&Machine{}).Pluck("ip_addresses", &addressesSlices)
 
-	ips := make([]netaddr.IP, 0, len(h.cfg.IPPrefixes)*len(addressesSlices))
+	log.Trace().
+		Strs("addresses", addressesSlices).
+		Msg("Got allocated ip addresses from databases")
+
+	var ips netaddr.IPSetBuilder
 	for _, slice := range addressesSlices {
-		var a MachineAddresses
-		err := a.Scan(slice)
+		var machineAddresses MachineAddresses
+		err := machineAddresses.Scan(slice)
 		if err != nil {
-			return nil, fmt.Errorf("failed to read ip from database: %w", err)
+			return netaddr.IPSet{}, fmt.Errorf(
+				"failed to read ip from database: %w",
+				err,
+			)
+		}
+
+		for _, ip := range machineAddresses {
+			ips.Add(ip)
 		}
-		ips = append(ips, a...)
 	}
 
-	return ips, nil
+	log.Trace().
+		Interface("addresses", ips).
+		Msg("Parsed ip addresses that has been allocated from databases")
+
+	return netaddr.IPSet{}, nil
 }
 
 func containsString(ss []string, s string) bool {
@@ -222,16 +234,6 @@ func containsString(ss []string, s string) bool {
 	return false
 }
 
-func containsIPs(ips []netaddr.IP, ip netaddr.IP) bool {
-	for _, v := range ips {
-		if v == ip {
-			return true
-		}
-	}
-
-	return false
-}
-
 func tailNodesToString(nodes []*tailcfg.Node) string {
 	temp := make([]string, len(nodes))