daemon: parallelize tap-pool warmup

Pool warmup ran createTap calls sequentially (one per loop iteration),
so warming N taps cold took N times the per-tap cost. Each releaseTap
also fired its own ensureTapPool goroutine, racing on n.tapPool.next.

Reserve a batch of names under the lock, then run up to
maxConcurrentTapWarmup createTap RPCs in parallel — root helper already
handles each connection in its own goroutine, so multiple in-flight
priv.create_tap requests don't contend at the wire level. Add a
warming flag to dedupe concurrent ensureTapPool invocations triggered
by parallel releases.

Bail-on-first-error semantics preserved: if every goroutine in a
batch fails (e.g. host out of taps, kernel limit), the loop exits
rather than burning monotonic indices forever.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/daemon/tap_pool.go

@@ -6,6 +6,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 )
 
 const tapPoolPrefix = "tap-pool-"
@@ -16,8 +17,16 @@ type tapPool struct {
 	mu      sync.Mutex
 	entries []string
 	next    int
+	warming bool
 }
 
+// maxConcurrentTapWarmup caps the number of `priv.create_tap` RPCs the
+// warmup loop runs in parallel. Each tap creation is ~4 root-helper
+// shell-outs serialized within one RPC handler; running too many at
+// once just contends on netlink. 8 is the production sweet spot for
+// SMOKE_JOBS=8.
+const maxConcurrentTapWarmup = 8
+
 // initializeTapPool seeds the monotonic pool index from the set of
 // tap names already in use by running/stopped VMs, so newly warmed
 // pool entries don't collide with existing ones. Callers (Daemon.Open)
@@ -41,6 +50,23 @@ func (n *HostNetwork) ensureTapPool(ctx context.Context) {
 	if n.config.TapPoolSize <= 0 {
 		return
 	}
+
+	// Dedupe concurrent warmup invocations. Releases trigger a fresh
+	// ensureTapPool in a goroutine; without this, N parallel releases
+	// would each spin up their own warmup loop racing on n.tapPool.next.
+	n.tapPool.mu.Lock()
+	if n.tapPool.warming {
+		n.tapPool.mu.Unlock()
+		return
+	}
+	n.tapPool.warming = true
+	n.tapPool.mu.Unlock()
+	defer func() {
+		n.tapPool.mu.Lock()
+		n.tapPool.warming = false
+		n.tapPool.mu.Unlock()
+	}()
+
 	for {
 		select {
 		case <-ctx.Done():
@@ -51,27 +77,53 @@ func (n *HostNetwork) ensureTapPool(ctx context.Context) {
 		}
 
 		n.tapPool.mu.Lock()
-		if len(n.tapPool.entries) >= n.config.TapPoolSize {
+		deficit := n.config.TapPoolSize - len(n.tapPool.entries)
+		if deficit <= 0 {
 			n.tapPool.mu.Unlock()
 			return
 		}
-		tapName := fmt.Sprintf("%s%d", tapPoolPrefix, n.tapPool.next)
-		n.tapPool.next++
-		n.tapPool.mu.Unlock()
-
-		if err := n.createTap(ctx, tapName); err != nil {
-			if n.logger != nil {
-				n.logger.Warn("tap pool warmup failed", "tap_device", tapName, "error", err.Error())
-			}
-			return
+		batch := deficit
+		if batch > maxConcurrentTapWarmup {
+			batch = maxConcurrentTapWarmup
+		}
+		// Reserve names up front so concurrent goroutines can't collide
+		// on n.tapPool.next.
+		names := make([]string, batch)
+		for i := range names {
+			names[i] = fmt.Sprintf("%s%d", tapPoolPrefix, n.tapPool.next)
+			n.tapPool.next++
 		}
-
-		n.tapPool.mu.Lock()
-		n.tapPool.entries = append(n.tapPool.entries, tapName)
 		n.tapPool.mu.Unlock()
 
-		if n.logger != nil {
-			n.logger.Debug("tap added to idle pool", "tap_device", tapName)
+		var (
+			wg        sync.WaitGroup
+			progress  atomic.Int32
+		)
+		for _, tapName := range names {
+			wg.Add(1)
+			go func(tapName string) {
+				defer wg.Done()
+				if err := n.createTap(ctx, tapName); err != nil {
+					if n.logger != nil {
+						n.logger.Warn("tap pool warmup failed", "tap_device", tapName, "error", err.Error())
+					}
+					return
+				}
+				n.tapPool.mu.Lock()
+				n.tapPool.entries = append(n.tapPool.entries, tapName)
+				n.tapPool.mu.Unlock()
+				progress.Add(1)
+				if n.logger != nil {
+					n.logger.Debug("tap added to idle pool", "tap_device", tapName)
+				}
+			}(tapName)
+		}
+		wg.Wait()
+
+		// Whole batch failed → bail rather than burn names indefinitely
+		// (the original sequential loop bailed on first error too).
+		if progress.Load() == 0 {
+			return
 		}
 	}
 }