lib: allow process kill by signal number

This brings the behaviour in line with the documentation.

PR-URL: #16944
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/process.js

@@ -158,8 +158,8 @@ function setupKillAndExit() {
     }
 
     // preserve null signal
-    if (0 === sig) {
-      err = process._kill(pid, 0);
+    if (sig === (sig | 0)) {
+      err = process._kill(pid, sig);
     } else {
       sig = sig || 'SIGTERM';
       if (constants[sig]) {

test/parallel/test-process-kill-pid.js

@@ -64,6 +64,13 @@ common.expectsError(() => process.kill(1, 'test'), {
   message: 'Unknown signal: test'
 });
 
+// Test that kill throws an error for invalid signal numbers
+common.expectsError(() => process.kill(1, 987), {
+  code: 'EINVAL',
+  type: Error,
+  message: 'kill EINVAL'
+});
+
 // Test kill argument processing in valid cases.
 //
 // Monkey patch _kill so that we don't actually send any signals, particularly
@@ -95,6 +102,11 @@ kill(0, undefined, 0, 15);
 kill('0', 'SIGHUP', 0, 1);
 kill('0', undefined, 0, 15);
 
+// Confirm that numeric signal arguments are supported
+
+kill(0, 1, 0, 1);
+kill(0, 15, 0, 15);
+
 // negative numbers are meaningful on unix
 kill(-1, 'SIGHUP', -1, 1);
 kill(-1, undefined, -1, 15);