src: make start of signal_wrap reenterable

Author: theanarkh

src/signal_wrap.cc

@@ -118,6 +118,8 @@ class SignalWrap : public HandleWrap {
       }
     }
 #endif
+    // Save the old signum
+    int old_signum = wrap->handle_.signum;
     int err = uv_signal_start(
         &wrap->handle_,
         [](uv_signal_t* handle, int signum) {
@@ -131,7 +133,10 @@ class SignalWrap : public HandleWrap {
         signum);
 
     if (err == 0) {
-      CHECK(!wrap->active_);
+      // Subtract 1 if previously registered
+      if (wrap->active_) {
+        DecreaseSignalHandlerCount(old_signum);
+      }
       wrap->active_ = true;
       Mutex::ScopedLock lock(handled_signals_mutex);
       handled_signals[signum]++;

test/parallel/test-signal-reuse.js

@@ -0,0 +1,21 @@
+// Flags: --expose-internals
+'use strict';
+const common = require('../common');
+const { internalBinding } = require('internal/test/binding');
+const { signals } = internalBinding('constants').os;
+const Signal = internalBinding('signal_wrap').Signal;
+
+common.skipIfWorker();
+
+const signal = new Signal();
+signal.start(signals.SIGUSR2);
+signal.onsignal = common.mustCall(() => {
+  process.nextTick(common.mustCall(() => {
+    signal.start(signals.SIGINT);
+    signal.onsignal = common.mustCall(() => {
+      signal.close();
+    });
+    process.kill(0, signals.SIGINT);
+  }));
+});
+process.kill(0, signals.SIGUSR2);