syscall: treat ENFILE as a temporary error

ENFILE is returned from accept when the whole system has run out of
file descriptors. Mark the error as temporary, so that accept loops
continue working.

Fixes #35131
Updates #1891

Change-Id: Idf44c084731898ff4c720d06c250d3b8a42de312
Reviewed-on: https://go-review.googlesource.com/c/go/+/203117
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_unix.go

@@ -138,7 +138,7 @@ func (e Errno) Is(target error) bool {
 }
 
 func (e Errno) Temporary() bool {
-	return e == EINTR || e == EMFILE || e.Timeout()
+	return e == EINTR || e == EMFILE || e == ENFILE || e.Timeout()
 }
 
 func (e Errno) Timeout() bool {

src/syscall/syscall_unix_test.go

@@ -384,3 +384,9 @@ func TestSetsockoptString(t *testing.T) {
 		t.Fatalf("SetsockoptString: did not fail")
 	}
 }
+
+func TestENFILETemporary(t *testing.T) {
+	if !syscall.ENFILE.Temporary() {
+		t.Error("ENFILE is not treated as a temporary error")
+	}
+}