[24.0 backport] pkg/fileutils: GetTotalUsedFds: reduce allocations

pkg/fileutils/fileutils_linux.go

@@ -0,0 +1,52 @@
+package fileutils
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/containerd/containerd/log"
+	"golang.org/x/sys/unix"
+)
+
+// GetTotalUsedFds Returns the number of used File Descriptors by
+// reading it via /proc filesystem.
+func GetTotalUsedFds() int {
+	name := fmt.Sprintf("/proc/%d/fd", os.Getpid())
+
+	// Fast-path for Linux 6.2 (since [f1f1f2569901ec5b9d425f2e91c09a0e320768f3]).
+	// From the [Linux docs]:
+	//
+	// "The number of open files for the process is stored in 'size' member of
+	// stat() output for /proc/<pid>/fd for fast access."
+	//
+	// [Linux docs]: https://docs.kernel.org/filesystems/proc.html#proc-pid-fd-list-of-symlinks-to-open-files:
+	// [f1f1f2569901ec5b9d425f2e91c09a0e320768f3]: https://github.com/torvalds/linux/commit/f1f1f2569901ec5b9d425f2e91c09a0e320768f3
+	var stat unix.Stat_t
+	if err := unix.Stat(name, &stat); err == nil && stat.Size > 0 {
+		return int(stat.Size)
+	}
+
+	f, err := os.Open(name)
+	if err != nil {
+		log.G(context.TODO()).WithError(err).Error("Error listing file descriptors")
+		return -1
+	}
+	defer f.Close()
+
+	var fdCount int
+	for {
+		names, err := f.Readdirnames(100)
+		fdCount += len(names)
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			log.G(context.TODO()).WithError(err).Error("Error listing file descriptors")
+			return -1
+		}
+	}
+	// Note that the slow path has 1 more file-descriptor, due to the open
+	// file-handle for /proc/<pid>/fd during the calculation.
+	return fdCount
+}

pkg/fileutils/fileutils_test.go

@@ -240,3 +240,10 @@ func TestCreateIfNotExistsFile(t *testing.T) {
 		t.Errorf("Should have been a file, seems it's not")
 	}
 }
+
+func BenchmarkGetTotalUsedFds(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = GetTotalUsedFds()
+	}
+}