Add cross platform "get number of open file descriptor" util

go.mod

@@ -7,11 +7,11 @@ require (
 	fortio.org/dflag v1.5.2
 	fortio.org/log v1.3.0
 	fortio.org/version v1.0.2
+	golang.org/x/sys v0.5.0
 )
 
 require (
 	fortio.org/sets v1.0.2 // indirect
 	github.com/fsnotify/fsnotify v1.6.0 // indirect
 	golang.org/x/exp v0.0.0-20230224173230-c95f2b4c22f2 // indirect
-	golang.org/x/sys v0.5.0 // indirect
 )

num_fd.go

@@ -0,0 +1,57 @@
+// Fortio CLI util: number of open filedescriptor.
+//
+// (c) 2023 Fortio Authors
+// See LICENSE
+
+//go:build !windows
+// +build !windows
+
+package scli // import "fortio.org/scli"
+
+import (
+	"os"
+	"runtime"
+
+	"fortio.org/log"
+)
+
+func countDir(dir string) int {
+	f, err := os.Open(dir)
+	if err != nil {
+		log.Errf("Unable to open %s: %v", dir, err)
+		return -1
+	}
+	/* to run lsof between stage to debug that on macos /dev/fd gets opened twice somehow
+	if log.LogDebug() {
+		log.Debugf("Sleeping after open before Readdirnames")
+		time.Sleep(30 * time.Second)
+		log.Debugf("Done sleeping, calling Readdirnames")
+	}
+	*/
+	names, err := f.Readdirnames(-1)
+	if err != nil {
+		log.Errf("Unable to read %s: %v", dir, err)
+		f.Close()
+		return -1
+	}
+	if log.LogDebug() {
+		log.Debugf("Found %d names in %s: %v", len(names), dir, names)
+		// time.Sleep(60 * time.Second)
+		// log.Debugf("Done sleeping, closing dir")
+	}
+	f.Close()
+	return len(names) - 1 // for the dir we just opened
+}
+
+func NumFD() int {
+	switch runtime.GOOS {
+	case "windows":
+		log.Fatalf("Shouldn't be reached (!windows build tag)")
+		return -1 // not reached (also, not possible)
+	case "darwin":
+		return countDir("/dev/fd") - 1 // macos seems to open 2 fds to Readdirnames /dev/fd
+	default:
+		// assume everyone else has a /proc/self/fd
+		return countDir("/proc/self/fd")
+	}
+}

num_fd_windows.go

@@ -0,0 +1,42 @@
+// Fortio CLI util: number of open filedescriptor.
+//
+// (c) 2023 Fortio Authors
+// See LICENSE
+
+//go:build windows
+// +build windows
+
+package scli // import "fortio.org/scli"
+
+import (
+	"syscall"
+	"unsafe"
+
+	"fortio.org/log"
+	"golang.org/x/sys/windows"
+)
+
+var (
+	modkernel32           = windows.NewLazySystemDLL("kernel32.dll")
+	getProcessHandleCount = modkernel32.NewProc("GetProcessHandleCount")
+)
+
+func GetCurrentProcessHandleCount() int {
+	hdl, err := windows.GetCurrentProcess()
+	if err != nil {
+		log.Errf("GetCurrentProcess failed: %v", err)
+		return -1
+	}
+	count := uint32(0)
+	ret, _, err := syscall.Syscall(getProcessHandleCount.Addr(), 2, uintptr(hdl), uintptr(unsafe.Pointer(&count)), 0)
+	log.Debugf("GetProcessHandleCount = %v, %v : %v", ret, err, count)
+	if ret == 0 {
+		log.Errf("GetProcessHandleCount failed: %v", err)
+		return -1
+	}
+	return int(count)
+}
+
+func NumFD() int {
+	return GetCurrentProcessHandleCount()
+}

sampleServer/main.go

@@ -1,14 +1,26 @@
 package main
 
 import (
+	"time"
+
 	"fortio.org/cli"
+	"fortio.org/log"
 	"fortio.org/scli"
 )
 
 func main() {
 	cli.MinArgs = 2
 	cli.MaxArgs = 4
-	if !scli.ServerMain() {
+	srvStarted := scli.ServerMain()
+	time.Sleep(1 * time.Second)
+	log.Infof("FD count: %d", scli.NumFD())
+	time.Sleep(20 * time.Second)
+	log.Infof("FD count: %d", scli.NumFD())
+	// is it stable:
+	for i := 0; i < 10; i++ {
+		log.Infof("FD count: %d", scli.NumFD())
+	}
+	if !srvStarted {
 		// in reality in both case we'd start some actual server
 		return
 	}