Update

vendor/tailscale.com/metrics/fds_linux.go

@@ -1,4 +1,4 @@
-// Copyright (c) Tailscale Inc & AUTHORS
+// Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
 package metrics
@@ -6,6 +6,7 @@ package metrics
 import (
 	"io/fs"
 	"sync"
+	"syscall"
 
 	"go4.org/mem"
 	"tailscale.com/util/dirwalk"
@@ -32,10 +33,44 @@ func (c *counter) count(name mem.RO, de fs.DirEntry) error {
 	return nil
 }
 
-func currentFDs() int {
+// procSelfFDFd returns a file descriptor for the /proc/self/fd
+// directory, kept open for the process lifetime so that
+// currentFDsFast can fstat it without allocating.
+var procSelfFDFd = sync.OnceValues(func() (int, error) {
+	return syscall.Open("/proc/self/fd", syscall.O_RDONLY|syscall.O_CLOEXEC, 0)
+})
+
+// currentFDsFast returns the number of open file descriptors and
+// whether it was able to determine it. It uses the fact that as of
+// Linux 6.2 (torvalds/linux@f1f1f2569901), the stat size of the
+// /proc/self/fd directory is the number of open file descriptors.
+// On older kernels the reported size is zero.
+func currentFDsFast() (n int, ok bool) {
+	fd, err := procSelfFDFd()
+	if err != nil {
+		return 0, false
+	}
+	var st syscall.Stat_t
+	if err := syscall.Fstat(fd, &st); err != nil {
+		return 0, false
+	}
+	return int(st.Size), st.Size > 0
+}
+
+// currentFDsDirwalk returns the number of open file descriptors by
+// walking /proc/self/fd. It works on all kernels but is O(n) in the
+// number of open file descriptors.
+func currentFDsDirwalk() int {
 	c := counterPool.Get().(*counter)
 	defer counterPool.Put(c)
 	c.n = 0
 	dirwalk.WalkShallow(mem.S("/proc/self/fd"), c.cb)
 	return c.n
 }
+
+func currentFDs() int {
+	if n, ok := currentFDsFast(); ok {
+		return n
+	}
+	return currentFDsDirwalk()
+}