GODT-1523: Reduce unnecessary shell executions. Inspired by @kortschak.

internal/store/ulimit_default.go

@@ -15,46 +15,55 @@
 // You should have received a copy of the GNU General Public License
 // along with Proton Mail Bridge. If not, see <https://www.gnu.org/licenses/>.
 
+//go:build !windows
+// +build !windows
+
 package store
 
 import (
-	"os"
 	"runtime"
-
-	"golang.org/x/sys/unix"
+	"syscall"
 )
 
+func getCurrentFDLimit() (int, error) {
+	var limits syscall.Rlimit
+	err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limits)
+	if err != nil {
+		return 0, err
+	}
+	return int(limits.Cur), nil
+}
+
+func countOpenedFDs(limit int) int {
+	openedFDs := 0
+
+	for i := 0; i < limit; i++ {
+		_, _, err := syscall.Syscall(syscall.SYS_FCNTL, uintptr(i), uintptr(syscall.F_GETFL), 0)
+		if err == 0 {
+			openedFDs++
+		}
+	}
+
+	return openedFDs
+}
+
 func isFdCloseToULimit() bool {
 	if runtime.GOOS != "darwin" && runtime.GOOS != "linux" {
 		return false
 	}
 
-	var fdPath string
-	switch runtime.GOOS {
-	case "darwin":
-		fdPath = "/dev/fd"
-	case "linux":
-		fdPath = "/proc/self/fd"
-	}
-	f, err := os.Open(fdPath)
+	limit, err := getCurrentFDLimit()
 	if err != nil {
-		log.Warn("isFdCloseToULimit: ", err)
+		log.WithError(err).Error("Cannot get current FD limit")
 		return false
 	}
-	d, err := f.ReadDir(-1)
-	if err != nil {
-		log.Warn("isFdCloseToULimit: ", err)
-		return false
-	}
-	fd := len(d) - 1
 
-	var lim unix.Rlimit
-	err = unix.Getrlimit(unix.RLIMIT_NOFILE, &lim)
-	if err != nil {
-		log.Print(err)
-	}
-	ulimit := lim.Max
+	openedFDs := countOpenedFDs(limit)
 
-	log.Info("File descriptor check: num goroutines ", runtime.NumGoroutine(), " fd ", fd, " ulimit ", ulimit)
-	return fd >= int(0.95*float64(ulimit))
+	log.
+		WithField("noGoroutines", runtime.NumCgoCall()).
+		WithField("noFDs", openedFDs).
+		WithField("limitFD", limit).
+		Info("File descriptor check")
+	return openedFDs >= int(0.95*float64(limit))
 }

internal/store/ulimit_windows.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2022 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge. If not, see <https://www.gnu.org/licenses/>.
+
+//go:build windows
+// +build windows
+
+package store
+
+func isFdCloseToULimit() bool { return false }