body-drain: add -c concurrency and -idle-per-host to the client

Bounded concurrency via semaphore+WaitGroup, atomic counters, stop-on-first-
error without cancelling in-flight requests (cancel would hide the leaks).
Exposes MaxIdleConnsPerHost so pool thrash under concurrency is observable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

99-experiments/body-drain/README.md

@@ -8,6 +8,33 @@ go run ./99-experiments/body-drain -mode all
 go run ./99-experiments/body-drain -mode leak -n 200
 ```
 
+## Concurrency: `-c` and `-idle-per-host`
+
+`-c N` keeps up to N requests in flight (semaphore + WaitGroup, the same
+shape as exercise 01). `-idle-per-host` sets `Transport.MaxIdleConnsPerHost`
+(default 10; Go's own default is only **2**).
+
+```
+go run ./99-experiments/body-drain -mode drain -n 200 -c 8
+#   httptrace GotConn  : reused=192 fresh=8      ← one conn per worker, then reused
+#   server new conns   : 8
+
+go run ./99-experiments/body-drain -mode drain -n 200 -c 8 -idle-per-host 2
+#   server new conns   : 13+                     ← pool too small: conns returned
+#                                                   while 2 are already idle get CLOSED
+```
+
+That second run is the trap in exercise 01: `limit` goroutines against one
+host with the default transport means only 2 conns are ever kept, the rest are
+dialed and thrown away on every round — even though you drained the body
+correctly. On loopback the effect is small; against a TLS host each throwaway
+is a full handshake. Raise `MaxIdleConnsPerHost` to your concurrency.
+
+With `-c` the leak modes also exhaust fds N times faster, and failures no
+longer stop in-flight requests: cancelling a request context closes its body
+and would clean up the very leaks we want to see, so the client only stops
+*launching* new requests after the first error.
+
 ## Two processes: watch each side run out of fds on its own
 
 In one process the client and server share one fd limit and each leaked

99-experiments/body-drain/main.go

@@ -24,6 +24,8 @@
 // Run in one process:
 //
 //	go run ./99-experiments/body-drain -mode all
+//	go run ./99-experiments/body-drain -mode drain -n 200 -c 8               # 8 in flight, pool of 10 → still 1 host conn per worker
+//	go run ./99-experiments/body-drain -mode drain -n 200 -c 8 -idle-per-host 2   # pool too small → fresh dials even when draining
 //
 // Run as two processes (two terminals):
 //
@@ -56,6 +58,7 @@ import (
 	"os"
 	"os/signal"
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"time"
 )
@@ -153,53 +156,73 @@ func serveForever(addr string) {
 
 // ---- client ----
 
-// runClient performs n GETs in the given mode and reports what it observed.
+// clientOpts are the knobs for one client run.
-func runClient(m mode, n int, url string, headerTimeout time.Duration) {
+type clientOpts struct {
+	n             int           // total requests
+	concurrency   int           // max in-flight requests (1 = sequential)
+	idlePerHost   int           // Transport.MaxIdleConnsPerHost
+	headerTimeout time.Duration // Transport.ResponseHeaderTimeout
+}
+
+// runClient performs opts.n GETs in the given mode and reports what it observed.
+func runClient(m mode, url string, opts clientOpts) {
 	// Fresh transport per run so runs don't share a pool. Cloning keeps sane defaults.
 	tr := http.DefaultTransport.(*http.Transport).Clone()
-	tr.MaxIdleConnsPerHost = 10
+	// With concurrency > MaxIdleConnsPerHost, even `drain` mode dials fresh
+	// connections: conns that come back while the idle pool is full are
+	// closed instead of pooled. Watch `fresh` climb as -c grows past -idle-per-host.
+	tr.MaxIdleConnsPerHost = opts.idlePerHost
 	// Without this, a request whose connection the server can't accept (server
 	// out of fds) blocks forever waiting for headers.
-	tr.ResponseHeaderTimeout = headerTimeout
+	tr.ResponseHeaderTimeout = opts.headerTimeout
 	// NOTE: deliberately no Client.Timeout. With a timeout, a leaked body gets
 	// force-closed when the timer fires, which hides the leak (try adding one
 	// and watch the goroutine count drop after the timeout).
 	client := &http.Client{Transport: tr}
 
-	var reused, fresh int
+	// Counters are touched from many goroutines when -c > 1 → atomics.
+	var reused, fresh, done, failed atomic.Int64
 	trace := &httptrace.ClientTrace{
 		GotConn: func(info httptrace.GotConnInfo) {
 			if info.Reused {
-				reused++
+				reused.Add(1)
 			} else {
-				fresh++
+				fresh.Add(1)
 			}
 		},
 	}
 
-	goroutinesBefore := runtime.NumGoroutine()
+	// stop is set on the first failure so we launch no further requests.
-	start := time.Now()
+	// Deliberately NOT a cancellable request context: cancelling a request's
+	// ctx closes its body/conn, which would clean up the leaks we're here
+	// to observe. In-flight requests are left alone.
+	var stop atomic.Bool
+	ctx := httptrace.WithClientTrace(context.Background(), trace)
 
-	done := 0
+	var firstErr sync.Once
-	for i := 0; i < n; i++ {
+	doOne := func(i int) {
-		req, err := http.NewRequestWithContext(httptrace.WithClientTrace(context.Background(), trace), http.MethodGet, url, nil)
+		req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 		if err != nil {
 			panic(err)
 		}
 		resp, err := client.Do(req)
 		if err != nil {
+			failed.Add(1)
 			// This is where `leak` eventually dies. Which error you get tells
 			// you which side ran out of fds:
 			//   dial tcp ...: socket: too many open files  → THIS process (client)
 			//   timeout awaiting response headers          → the server can't accept (it's out of fds)
+			firstErr.Do(func() {
 				fmt.Printf("  request %d failed: %v\n", i, err)
 				var ne net.Error
 				if errors.As(err, &ne) && ne.Timeout() {
-				fmt.Println("  (timeout: the server probably can't accept — check its log for 'accept4: too many open files')")
+					fmt.Println("  (timeout: the server probably can't accept — check its log for 'accept: too many open files')")
 				}
-			break
+				stop.Store(true)
+			})
+			return
 		}
-		done++
+		done.Add(1)
 
 		switch m {
 		case modeDrain:
@@ -212,13 +235,33 @@ func runClient(m mode, n int, url string, headerTimeout time.Duration) {
 		}
 	}
 
+	goroutinesBefore := runtime.NumGoroutine()
+	start := time.Now()
+
+	// Bounded concurrency: a buffered channel as a semaphore + WaitGroup.
+	// Same shape as exercise 01's CheckAll.
+	sem := make(chan struct{}, opts.concurrency)
+	var wg sync.WaitGroup
+	for i := 0; i < opts.n && !stop.Load(); i++ {
+		sem <- struct{}{} // blocks while `concurrency` requests are in flight
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			defer func() { <-sem }()
+			doOne(i)
+		}(i)
+	}
+	wg.Wait()
+
 	elapsed := time.Since(start)
 	// Give the transport a moment to settle bookkeeping (put conn back to idle etc.).
 	time.Sleep(50 * time.Millisecond)
 	goroutinesAfter := runtime.NumGoroutine()
 
-	fmt.Printf("  requests           : %d/%d ok in %v\n", done, n, elapsed.Round(time.Millisecond))
+	fmt.Printf("  requests           : %d/%d ok, %d failed, in %v (concurrency %d)\n",
-	fmt.Printf("  httptrace GotConn  : reused=%d fresh=%d\n", reused, fresh)
+		done.Load(), opts.n, failed.Load(), elapsed.Round(time.Millisecond), opts.concurrency)
+	fmt.Printf("  httptrace GotConn  : reused=%d fresh=%d (MaxIdleConnsPerHost=%d)\n",
+		reused.Load(), fresh.Load(), opts.idlePerHost)
 	fmt.Printf("  goroutines         : before=%d after=%d (delta %+d)\n",
 		goroutinesBefore, goroutinesAfter, goroutinesAfter-goroutinesBefore)
 
@@ -236,7 +279,14 @@ func main() {
 	modeFlag := flag.String("mode", "all", "client: drain | close | leak | all")
 	n := flag.Int("n", 20, "client: number of requests per mode")
 	headerTimeout := flag.Duration("header-timeout", 0, "client: ResponseHeaderTimeout (0 = none; try 3s when the server is the one out of fds)")
+	concurrency := flag.Int("c", 1, "client: max requests in flight (1 = sequential)")
+	idlePerHost := flag.Int("idle-per-host", 10, "client: Transport.MaxIdleConnsPerHost (set below -c to watch drain mode stop reusing)")
 	flag.Parse()
+	if *concurrency < 1 {
+		fmt.Fprintln(os.Stderr, "-c must be >= 1")
+		os.Exit(2)
+	}
+	opts := clientOpts{n: *n, concurrency: *concurrency, idlePerHost: *idlePerHost, headerTimeout: *headerTimeout}
 
 	modes := []mode{modeDrain, modeClose, modeLeak}
 	if *modeFlag != "all" {
@@ -250,7 +300,7 @@ func main() {
 	case "client":
 		for _, m := range modes {
 			fmt.Printf("== mode=%s (client only, target %s) ==\n", m, *url)
-			runClient(m, *n, *url, *headerTimeout)
+			runClient(m, *url, opts)
 			fmt.Println()
 		}
 		if len(modes) == 1 && modes[0] == modeLeak {
@@ -265,7 +315,7 @@ func main() {
 				os.Exit(1)
 			}
 			fmt.Printf("== mode=%s ==\n", m)
-			runClient(m, *n, srv.URL(), *headerTimeout)
+			runClient(m, srv.URL(), opts)
 			time.Sleep(50 * time.Millisecond)
 			srv.report()
 			fmt.Println()