fix(whip): clean up lifecycle — proper net import and checkPortFree implementation

app/webrtc/whip_lifecycle.go

@@ -2,6 +2,7 @@ package webrtc
 
 import (
 	"fmt"
+	"net"
 
 	appcfg "github.com/datarhei/core/v16/restream/app"
 )
@@ -103,7 +104,7 @@ func (s *Subsystem) onWHIPProcessStop(id string) {
 
 // allocAdjacentPortPair finds two consecutive free loopback UDP ports
 // (V, V+1) and returns V. It retries up to allocAttempts times because
-// the kernel may hand us an odd-ended pair whose +1 neighbor is taken.
+// the kernel may hand us a port whose +1 neighbor is already taken.
 //
 // The caller owns the returned port numbers; FFmpeg will bind them
 // immediately on process start via the ConfigIO input legs.
@@ -115,14 +116,14 @@ func allocAdjacentPortPair() (int, error) {
 			lastErr = err
 			continue
 		}
-		// Verify the audio port (videoPort+1) is also free.
-		// We do a quick bind-and-release on videoPort+1.
 		audioPort := videoPort + 1
 		if audioPort > 65535 {
-			lastErr = fmt.Errorf("video port %d would require audio port > 65535", videoPort)
+			lastErr = fmt.Errorf("video port %d would push audio port above 65535", videoPort)
 			continue
 		}
-		// Try to claim videoPort+1 momentarily to verify availability.
+		// Verify the audio port (videoPort+1) is also free by attempting
+		// a momentary bind. TOCTOU race is accepted; FFmpeg will fail-fast
+		// on the actual bind and the process will restart cleanly.
 		if err := checkPortFree(audioPort); err != nil {
 			lastErr = err
 			continue
@@ -135,27 +136,15 @@ func allocAdjacentPortPair() (int, error) {
 	return 0, fmt.Errorf("after %d attempts: %w", allocAttempts, lastErr)
 }
 
-// checkPortFree does a quick UDP bind-and-release on the given loopback
-// port. Returns nil if the port appears free, non-nil if it is in use.
-// There is an inherent TOCTOU race between this check and FFmpeg's bind;
-// callers should handle the bind failure gracefully (process restart).
+// checkPortFree attempts a momentary UDP bind on the given loopback
+// port. Returns nil if the port appears available, non-nil otherwise.
 func checkPortFree(port int) error {
-	import_net_listen := func() error {
-		// Inline the check without importing net again (already in portalloc.go
-		// via the package-level Alloc function). We re-use Alloc's pattern:
-		// bind :port, get error, close.
-		//
-		// This file is in the same package so we can call the unexported helper.
-		// However, since we only have Alloc() (which uses :0), we implement
-		// this using net directly here.
-		return nil
+	c, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4(127, 0, 0, 1), Port: port})
+	if err != nil {
+		return fmt.Errorf("port %d not free: %w", port, err)
 	}
-	_ = import_net_listen
-	// Use net directly since this package already imports it via portalloc.go.
-	// We can't import "net" twice, but since this file is in the same package
-	// as portalloc.go which imports "net", the package-level net functions are
-	// available to us via the Alloc() pattern. We implement the check manually:
-	return checkUDPPortFree(port)
+	_ = c.Close()
+	return nil
 }
 
 // buildWHIPInputLegs produces the two FFmpeg input ConfigIO legs for