net/porttrack: add net.Listen wrapper to help tests allocate ports race-free

Updates tailscale/corp#27805
Updates tailscale/corp#27806
Updates tailscale/corp#37964

Change-Id: I7bb5ed7f258e840a8208e5d725c7b2f126d7ef96
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/porttrack/porttrack_test.go

@@ -0,0 +1,95 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package porttrack
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"testing"
+)
+
+func TestCollectorAndListen(t *testing.T) {
+	c := NewCollector(t)
+
+	labels := []string{"main", "plaintext", "debug"}
+	ports := make([]int, len(labels))
+
+	for i, label := range labels {
+		ln, err := Listen("tcp", c.Addr(label))
+		if err != nil {
+			t.Fatalf("Listen(%q): %v", label, err)
+		}
+		defer ln.Close()
+		p, err := c.Port(t.Context(), label)
+		if err != nil {
+			t.Fatalf("Port(%q): %v", label, err)
+		}
+		ports[i] = p
+	}
+
+	// All ports should be distinct non-zero values.
+	seen := map[int]string{}
+	for i, label := range labels {
+		if ports[i] == 0 {
+			t.Errorf("Port(%q) = 0", label)
+		}
+		if prev, ok := seen[ports[i]]; ok {
+			t.Errorf("Port(%q) = Port(%q) = %d", label, prev, ports[i])
+		}
+		seen[ports[i]] = label
+	}
+}
+
+func TestListenPassthrough(t *testing.T) {
+	ln, err := Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("Listen passthrough: %v", err)
+	}
+	defer ln.Close()
+	if ln.Addr().(*net.TCPAddr).Port == 0 {
+		t.Fatal("expected non-zero port")
+	}
+}
+
+func TestRoundTrip(t *testing.T) {
+	c := NewCollector(t)
+
+	ln, err := Listen("tcp", c.Addr("http"))
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	defer ln.Close()
+
+	// Start a server on the listener.
+	go http.Serve(ln, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}))
+
+	port, err := c.Port(t.Context(), "http")
+	if err != nil {
+		t.Fatalf("Port: %v", err)
+	}
+	resp, err := http.Get(fmt.Sprintf("http://localhost:%d/", port))
+	if err != nil {
+		t.Fatalf("http.Get: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusNoContent {
+		t.Fatalf("status = %d, want %d", resp.StatusCode, http.StatusNoContent)
+	}
+}
+
+func TestPortContextCancelled(t *testing.T) {
+	c := NewCollector(t)
+	// Nobody will ever report "never", so Port should block until ctx is done.
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+	_, err := c.Port(ctx, "never")
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("Port with cancelled context: got %v, want %v", err, context.Canceled)
+	}
+}