Keep minimal available fd to accelerate fd allocation

Use fd.next to store the iteration start position, which can be used to accelerate allocating new FDs.
And adding the corresponding gtest benchmark to measure performance.
@tanjianfeng

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/gvisor/pull/758 from DarcySail:master 96685ec7886dfe1a64988406831d3bc002b438cc
PiperOrigin-RevId: 276351250

1 files changed, 36 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/fd_table_test.go b/pkg/sentry/kernel/fd_table_test.go
index 2413788e7..2bcb6216a 100644
--- a/pkg/sentry/kernel/fd_table_test.go
+++ b/pkg/sentry/kernel/fd_table_test.go
@@ -70,6 +70,42 @@ func TestFDTableMany(t *testing.T) {
 		if err := fdTable.NewFDAt(ctx, 1, file, FDFlags{}); err != nil {
 			t.Fatalf("fdTable.NewFDAt(1, r, FDFlags{}): got %v, wanted nil", err)
 		}
+
+		i := int32(2)
+		fdTable.Remove(i)
+		if fds, err := fdTable.NewFDs(ctx, 0, []*fs.File{file}, FDFlags{}); err != nil || fds[0] != i {
+			t.Fatalf("Allocated %v FDs but wanted to allocate %v: %v", i, maxFD, err)
+		}
+	})
+}
+
+func TestFDTableOverLimit(t *testing.T) {
+	runTest(t, func(ctx context.Context, fdTable *FDTable, file *fs.File, _ *limits.LimitSet) {
+		if _, err := fdTable.NewFDs(ctx, maxFD, []*fs.File{file}, FDFlags{}); err == nil {
+			t.Fatalf("fdTable.NewFDs(maxFD, f): got nil, wanted error")
+		}
+
+		if _, err := fdTable.NewFDs(ctx, maxFD-2, []*fs.File{file, file, file}, FDFlags{}); err == nil {
+			t.Fatalf("fdTable.NewFDs(maxFD-2, {f,f,f}): got nil, wanted error")
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, maxFD-3, []*fs.File{file, file, file}, FDFlags{}); err != nil {
+			t.Fatalf("fdTable.NewFDs(maxFD-3, {f,f,f}): got %v, wanted nil", err)
+		} else {
+			for _, fd := range fds {
+				fdTable.Remove(fd)
+			}
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, maxFD-1, []*fs.File{file}, FDFlags{}); err != nil || fds[0] != maxFD-1 {
+			t.Fatalf("fdTable.NewFDAt(1, r, FDFlags{}): got %v, wanted nil", err)
+		}
+
+		if fds, err := fdTable.NewFDs(ctx, 0, []*fs.File{file}, FDFlags{}); err != nil {
+			t.Fatalf("Adding an FD to a resized map: got %v, want nil", err)
+		} else if len(fds) != 1 || fds[0] != 0 {
+			t.Fatalf("Added an FD to a resized map: got %v, want {1}", fds)
+		}
 	})
 }