feat: check if there are other handlers

Error if there are other handlers for the signals, but allow to override
this behavior for testing.

index.js

@@ -47,7 +47,8 @@ function plugin (fastify, opts, next) {
     onClose: onClose.bind(fastify),
     onTimeout: onTimeout.bind(fastify),
     onError: onError.bind(fastify),
-    timeout: 2000
+    timeout: 2000,
+    strict: true
   }
   const config = Object.assign({}, DEFAULTS, opts)
 
@@ -63,8 +64,14 @@ function plugin (fastify, opts, next) {
   if (config.timeout < 1) {
     return next(new RangeError(`timeout must be greather than 0, received ${config.timeout}`))
   }
+  if (typeof config.strict !== 'boolean') {
+    return next(new TypeError(`strict must be a boolean, received ${typeof config.strict}`))
+  }
 
   for (const signal of ['SIGINT', 'SIGTERM']) {
+    if (config.strict && process.listenerCount(signal) > 0) {
+      return next(new Error(`A ${signal} handler is already registered`))
+    }
     process.once(signal, onCloseSignal.bind(fastify, config))
   }
   next()

test/plugin.test.js

@@ -41,12 +41,19 @@ test('invalid options', async t => {
         timeout: 0
       },
       error: new RangeError('timeout must be greather than 0, received 0')
+    },
+    {
+      opts: {
+        strict: 'true'
+      },
+      error: new TypeError('strict must be a boolean, received string')
     }
   ]
 
   for (const [index, item] of list.entries()) {
     const fastify = Fastify()
-    await t.rejects(() => fastify.register(plugin, item.opts), item.error, `item ${index}`)
+    const opts = Object.assign({}, item.opts, item.opts.strict ? {} : { strict: false })
+    await t.rejects(() => fastify.register(plugin, opts), item.error, `item ${index}`)
   }
   t.end()
 })
@@ -82,7 +89,8 @@ test('valid options', async t => {
 
   for (const [index, item] of list.entries()) {
     const fastify = Fastify()
-    await t.resolves(() => fastify.register(plugin, item.opts), `item ${index}`)
+    const opts = Object.assign({}, item.opts, { strict: false })
+    await t.resolves(() => fastify.register(plugin, opts), `item ${index}`)
   }
   t.end()
 })