control/controlclient: free memory on iOS before full netmap work

Updates tailscale/corp#18514

Change-Id: I8d0330334b030ed8692b25549a0ee887ac6d7188
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/map.go

@@ -11,6 +11,8 @@ import (
 	"maps"
 	"net"
 	"reflect"
+	"runtime"
+	"runtime/debug"
 	"slices"
 	"sort"
 	"strconv"
@@ -187,6 +189,12 @@ func (ms *mapSession) HandleNonKeepAliveMapResponse(ctx context.Context, resp *t
 	// our UpdateFullNetmap call). This is the part we tried to avoid but
 	// some field mutations (especially rare ones) aren't yet handled.
 
+	if runtime.GOOS == "ios" {
+		// Memory is tight on iOS. Free what we can while we
+		// can before this potential burst of in-use memory.
+		debug.FreeOSMemory()
+	}
+
 	nm := ms.netmap()
 	ms.lastNetmapSummary = nm.VeryConcise()
 	ms.occasionallyPrintSummary(ms.lastNetmapSummary)