chore(cli): avoid use of testutil.RandomPort() in prometheus test (#17297)

Should hopefully fix https://github.com/coder/internal/issues/282

Instead of picking a random port for the prometheus server, listen on
`:0` and read the port from the CLI stdout.

cli/agent.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/pprof"
 	"net/url"
@@ -491,8 +492,6 @@ func (r *RootCmd) workspaceAgent() *serpent.Command {
 }
 
 func ServeHandler(ctx context.Context, logger slog.Logger, handler http.Handler, addr, name string) (closeFunc func()) {
-	logger.Debug(ctx, "http server listening", slog.F("addr", addr), slog.F("name", name))
-
 	// ReadHeaderTimeout is purposefully not enabled. It caused some issues with
 	// websockets over the dev tunnel.
 	// See: https://github.com/coder/coder/pull/3730
@@ -502,9 +501,15 @@ func ServeHandler(ctx context.Context, logger slog.Logger, handler http.Handler,
 		Handler: handler,
 	}
 	go func() {
-		err := srv.ListenAndServe()
+		ln, err := net.Listen("tcp", addr)
-		if err != nil && !xerrors.Is(err, http.ErrServerClosed) {
+		if err != nil {
-			logger.Error(ctx, "http server listen", slog.F("name", name), slog.Error(err))
+			logger.Error(ctx, "http server listen", slog.F("name", name), slog.F("addr", addr), slog.Error(err))
+			return
+		}
+		defer ln.Close()
+		logger.Info(ctx, "http server listening", slog.F("addr", ln.Addr()), slog.F("name", name))
+		if err := srv.Serve(ln); err != nil && !xerrors.Is(err, http.ErrServerClosed) {
+			logger.Error(ctx, "http server serve", slog.F("addr", ln.Addr()), slog.F("name", name), slog.Error(err))
 		}
 	}()
 

cli/server_test.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"runtime"
 	"strconv"
 	"strings"
@@ -1217,24 +1218,31 @@ func TestServer(t *testing.T) {
 			t.Parallel()
 
 			ctx := testutil.Context(t, testutil.WaitLong)
-			randPort := testutil.RandomPort(t)
+			inv, _ := clitest.New(t,
-			inv, cfg := clitest.New(t,
 				"server",
 				"--in-memory",
 				"--http-address", ":0",
 				"--access-url", "http://example.com",
 				"--provisioner-daemons", "1",
 				"--prometheus-enable",
-				"--prometheus-address", ":"+strconv.Itoa(randPort),
+				"--prometheus-address", ":0",
 				// "--prometheus-collect-db-metrics", // disabled by default
 				"--cache-dir", t.TempDir(),
 			)
 
+			pty := ptytest.New(t)
+			inv.Stdout = pty.Output()
+			inv.Stderr = pty.Output()
+
 			clitest.Start(t, inv)
-			_ = waitAccessURL(t, cfg)
+
+			// Wait until we see the prometheus address in the logs.
+			addrMatchExpr := `http server listening\s+addr=(\S+)\s+name=prometheus`
+			lineMatch := pty.ExpectRegexMatchContext(ctx, addrMatchExpr)
+			promAddr := regexp.MustCompile(addrMatchExpr).FindStringSubmatch(lineMatch)[1]
 
 			testutil.Eventually(ctx, t, func(ctx context.Context) bool {
-				req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://127.0.0.1:%d/metrics", randPort), nil)
+				req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://%s/metrics", promAddr), nil)
 				if err != nil {
 					t.Logf("error creating request: %s", err.Error())
 					return false
@@ -1272,24 +1280,31 @@ func TestServer(t *testing.T) {
 			t.Parallel()
 
 			ctx := testutil.Context(t, testutil.WaitLong)
-			randPort := testutil.RandomPort(t)
+			inv, _ := clitest.New(t,
-			inv, cfg := clitest.New(t,
 				"server",
 				"--in-memory",
 				"--http-address", ":0",
 				"--access-url", "http://example.com",
 				"--provisioner-daemons", "1",
 				"--prometheus-enable",
-				"--prometheus-address", ":"+strconv.Itoa(randPort),
+				"--prometheus-address", ":0",
 				"--prometheus-collect-db-metrics",
 				"--cache-dir", t.TempDir(),
 			)
 
+			pty := ptytest.New(t)
+			inv.Stdout = pty.Output()
+			inv.Stderr = pty.Output()
+
 			clitest.Start(t, inv)
-			_ = waitAccessURL(t, cfg)
+
+			// Wait until we see the prometheus address in the logs.
+			addrMatchExpr := `http server listening\s+addr=(\S+)\s+name=prometheus`
+			lineMatch := pty.ExpectRegexMatchContext(ctx, addrMatchExpr)
+			promAddr := regexp.MustCompile(addrMatchExpr).FindStringSubmatch(lineMatch)[1]
 
 			testutil.Eventually(ctx, t, func(ctx context.Context) bool {
-				req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://127.0.0.1:%d/metrics", randPort), nil)
+				req, err := http.NewRequestWithContext(ctx, "GET", fmt.Sprintf("http://%s/metrics", promAddr), nil)
 				if err != nil {
 					t.Logf("error creating request: %s", err.Error())
 					return false