TUN-7002: Randomise first region selection

We previously always preferred region2 as the first region to connect
to if both the regions cloudflared connects to have the same number of
availabe addresses. This change randomises that choice. The first
connection, conn index: 0, can now either connect to region 1 or region
2.

More importantly, conn 0 and 2 and 1 and 3 need not belong to the same
region.

Author: sudarshan-reddy

edgediscovery/allregions/regions.go

@@ -2,6 +2,7 @@ package allregions
 
 import (
 	"fmt"
+	"math/rand"
 
 	"github.com/rs/zerolog"
 )
@@ -85,6 +86,15 @@ func (rs *Regions) AddrUsedBy(connID int) *EdgeAddr {
 // GetUnusedAddr gets an unused addr from the edge, excluding the given addr. Prefer to use addresses
 // evenly across both regions.
 func (rs *Regions) GetUnusedAddr(excluding *EdgeAddr, connID int) *EdgeAddr {
+	// If both regions have the same number of available addrs, lets randomise which one
+	// we pick. The rest of this algorithm will continue to make sure we always use addresses
+	// evenly across both regions.
+	if rs.region1.AvailableAddrs() == rs.region2.AvailableAddrs() {
+		regions := []Region{rs.region1, rs.region2}
+		firstChoice := rand.Intn(2)
+		return getAddrs(excluding, connID, &regions[firstChoice], &regions[1-firstChoice])
+	}
+
 	if rs.region1.AvailableAddrs() > rs.region2.AvailableAddrs() {
 		return getAddrs(excluding, connID, &rs.region1, &rs.region2)
 	}