ipn/ipnlocal/c2n: add logallocs endpoint

This allows C2N clients to obtain sampled information about
memory allocations since the program started.

We now also force GC on logheap.

Updates tailscale/corp#18514

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -48,6 +48,7 @@
 	req("/debug/metrics"):           handleC2NDebugMetrics,
 	req("/debug/component-logging"): handleC2NDebugComponentLogging,
 	req("/debug/logheap"):           handleC2NDebugLogHeap,
+	req("/debug/logallocs"):         handleC2NDebugLogAllocs,
 	req("POST /logtail/flush"):      handleC2NLogtailFlush,
 	req("POST /sockstats"):          handleC2NSockStats,
 
@@ -175,9 +176,22 @@ func handleC2NDebugLogHeap(b *LocalBackend, w http.ResponseWriter, r *http.Reque
 		http.Error(w, "not implemented", http.StatusNotImplemented)
 		return
 	}
+	runtime.GC()
 	c2nLogHeap(w, r)
 }
 
+var c2nLogAllocs func(http.ResponseWriter, *http.Request) // non-nil on most platforms (c2n_pprof.go)
+
+func handleC2NDebugLogAllocs(b *LocalBackend, w http.ResponseWriter, r *http.Request) {
+	if c2nLogAllocs == nil {
+		// Not implemented on platforms trying to optimize for binary size or
+		// reduced memory usage.
+		http.Error(w, "not implemented", http.StatusNotImplemented)
+		return
+	}
+	c2nLogAllocs(w, r)
+}
+
 func handleC2NSSHUsernames(b *LocalBackend, w http.ResponseWriter, r *http.Request) {
 	var req tailcfg.C2NSSHUsernamesRequest
 	if r.Method == "POST" {

ipn/ipnlocal/c2n_pprof.go

@@ -14,4 +14,8 @@ func init() {
 	c2nLogHeap = func(w http.ResponseWriter, r *http.Request) {
 		pprof.WriteHeapProfile(w)
 	}
+
+	c2nLogAllocs = func(w http.ResponseWriter, r *http.Request) {
+		pprof.Lookup("allocs").WriteTo(w, 0)
+	}
 }