Ensure that process.on('exit') handler is run

This MAY fix #345, but without knowing the root cause of that problem,
it's hard to say for sure.

What IS definitely fixed by this is the behavior when a test program
does this:

```js
process.on('exit', function () {
  process.exit()
})
```

prior to TAP loading and adding its exit event handler.

This also prevents TAP from hooking up to the process.on('exit') event
if it's not piping anywhere, so just doing this won't change your exit
code:

```js
var Test = require('tap').Test
```

lib/tap.js

@@ -17,16 +17,54 @@ TAP.prototype.pipe = function () {
   this.setTimeout(this.options.timeout)
   this.pipe = Test.prototype.pipe
   this.push = Test.prototype.push
-  return this.pipe.apply(this, arguments)
+  var ret = this.pipe.apply(this, arguments)
+  this.process()
+  return ret
+}
+
+function monkeypatchEpipe () {
+  process.stdout.emit = function (emit) {
+    return function (ev, er) {
+      if (ev === 'error' && er.code === 'EPIPE')
+        return this.emit = emit
+      return emit.apply(this, arguments)
+    }
+  }(process.stdout.emit)
+}
+
+function monkeypatchExit () {
+  // ensure that we always get run, even if a user does
+  // process.on('exit', process.exit)
+  process.reallyExit = function (original) {
+    return function reallyExit (code) {
+      code = onExitEvent(code)
+      return original.call(this, code)
+    }
+  }(process.reallyExit)
+}
+
+var didOnExitEvent = false
+function onExitEvent (code) {
+  if (didOnExitEvent)
+    return process.exitCode || code
+
+  didOnExitEvent = true
+
+  if (!tap.results)
+    tap.endAll()
+
+  if (tap.results && !tap.results.ok && code === 0)
+    process.exitCode = 1
+
+  return process.exitCode || code || 0
 }
 
 TAP.prototype.push = function push () {
   this.pipe(process.stdout)
-  process.stdout.emit = function (emit) { return function (ev, er) {
-    if (ev === 'error' && er.code === 'EPIPE')
-      return this.emit = emit
-    return emit.apply(this, arguments)
-  }}(process.stdout.emit)
+
+  monkeypatchEpipe()
+  monkeypatchExit()
+  process.on('exit', onExitEvent)
   process.on('uncaughtException', tap.threw)
   return this.push.apply(tap, arguments)
 }
@@ -58,14 +96,6 @@ module.exports = tap
 tap.mocha = require('./mocha.js')
 tap.mochaGlobals = tap.mocha.global
 
-process.on('exit', function (code) {
-  if (!tap.results && didPipe)
-    tap.endAll()
-
-  if (tap.results && !tap.results.ok && code === 0)
-    process.exit(1)
-})
-
 tap.Test = Test
 tap.Spawn = Spawn
 tap.Stdin = Stdin

test/test/exit-on-exit--bail--buffer.tap

@@ -0,0 +1,48 @@
+TAP version 13
+ok 1 - ___/.*(node|iojs)(.exe)?.*/~~~exit-on-exit.js subtest ___/# time=[0-9.]+(ms)?/~~~ {
+    ok 1 - memoizes identical registry requests ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 2 - tag requests memoize versions ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - (unnamed test)
+        ok 2 - (unnamed test)
+    }
+
+    ok 3 - tag requests memoize tags ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 4 - memoization is scoped to a given cache # TODO
+    ok 5 - inflights concurrent requests ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 6 - supports fetching from an optional cache ___/# time=[0-9.]+(ms)?/~~~ {
+        ok 1 - should be equivalent
+        1..1
+    }
+
+    ok 7 - expires stale request data # TODO
+    ok 8 - falls back to registry if cache entry is invalid JSON # TODO
+    ok 9 - does not insert plain manifests into the cache # TODO
+    ok 10 - falls back to registry if cache entry missing ___/# time=[0-9.]+(ms)?/~~~ {
+        1..0
+    }
+
+    ok 11 - allows forcing use of cache when data stale # TODO
+    1..11
+    # todo: 5
+    ___/# time=[0-9.]+(ms)?/~~~
+}
+
+1..1
+___/# time=[0-9.]+(ms)?/~~~
+

test/test/exit-on-exit--bail.tap

@@ -0,0 +1,48 @@
+TAP version 13
+ok 1 - ___/.*(node|iojs)(.exe)?.*/~~~exit-on-exit.js subtest ___/# time=[0-9.]+(ms)?/~~~ {
+    # Subtest: memoizes identical registry requests
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 1 - memoizes identical registry requests ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: tag requests memoize versions
+        1..2
+        ok 1 - (unnamed test)
+        ok 2 - (unnamed test)
+    ok 2 - tag requests memoize versions ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: tag requests memoize tags
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 3 - tag requests memoize tags ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 4 - memoization is scoped to a given cache # TODO
+    # Subtest: inflights concurrent requests
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 5 - inflights concurrent requests ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: supports fetching from an optional cache
+        ok 1 - should be equivalent
+        1..1
+    ok 6 - supports fetching from an optional cache ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 7 - expires stale request data # TODO
+    ok 8 - falls back to registry if cache entry is invalid JSON # TODO
+    ok 9 - does not insert plain manifests into the cache # TODO
+    # Subtest: falls back to registry if cache entry missing
+        1..0
+    ok 10 - falls back to registry if cache entry missing ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 11 - allows forcing use of cache when data stale # TODO
+    1..11
+    # todo: 5
+    ___/# time=[0-9.]+(ms)?/~~~
+}
+
+1..1
+___/# time=[0-9.]+(ms)?/~~~
+

test/test/exit-on-exit--buffer.tap

@@ -0,0 +1,48 @@
+TAP version 13
+ok 1 - ___/.*(node|iojs)(.exe)?.*/~~~exit-on-exit.js subtest ___/# time=[0-9.]+(ms)?/~~~ {
+    ok 1 - memoizes identical registry requests ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 2 - tag requests memoize versions ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - (unnamed test)
+        ok 2 - (unnamed test)
+    }
+
+    ok 3 - tag requests memoize tags ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 4 - memoization is scoped to a given cache # TODO
+    ok 5 - inflights concurrent requests ___/# time=[0-9.]+(ms)?/~~~ {
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    }
+
+    ok 6 - supports fetching from an optional cache ___/# time=[0-9.]+(ms)?/~~~ {
+        ok 1 - should be equivalent
+        1..1
+    }
+
+    ok 7 - expires stale request data # TODO
+    ok 8 - falls back to registry if cache entry is invalid JSON # TODO
+    ok 9 - does not insert plain manifests into the cache # TODO
+    ok 10 - falls back to registry if cache entry missing ___/# time=[0-9.]+(ms)?/~~~ {
+        1..0
+    }
+
+    ok 11 - allows forcing use of cache when data stale # TODO
+    1..11
+    # todo: 5
+    ___/# time=[0-9.]+(ms)?/~~~
+}
+
+1..1
+___/# time=[0-9.]+(ms)?/~~~
+

test/test/exit-on-exit.js

@@ -0,0 +1,61 @@
+process.on('exit', function (code) {
+  process.exit()
+})
+
+var t = require('../..')
+
+if (process.argv[2] !== 'subtest')
+  t.spawn(process.execPath, [__filename, 'subtest'], { buffered: true })
+else {
+  var BIG = 10
+  t.test('memoizes identical registry requests', function (t) {
+    t.tearDown(function () {})
+    t.plan(2)
+    setTimeout(function () {
+      t.pass('got a manifest')
+      t.pass('got a manifest')
+    })
+  })
+
+  t.test('tag requests memoize versions', function (t) {
+    t.plan(2)
+    setTimeout(t.pass.bind('got a manifest'))
+    setTimeout(t.pass.bind('got a manifest'))
+  })
+
+  t.test('tag requests memoize tags', function (t) {
+    t.plan(2)
+    t.tearDown(function () {})
+    setTimeout(function () {
+      t.pass('got a manifest')
+      t.pass('got a manifest')
+    })
+  })
+
+  t.test('memoization is scoped to a given cache')
+  t.test('inflights concurrent requests', function (t) {
+    t.plan(2)
+    t.tearDown(function () {})
+    setTimeout(function () {
+      t.pass('got a manifest')
+      t.pass('got a manifest')
+    })
+  })
+
+  t.test('supports fetching from an optional cache', function (t) {
+    t.same(1, '1')
+    t.tearDown(function () {})
+    setTimeout(t.end)
+  })
+
+  t.test('expires stale request data')
+  t.test('falls back to registry if cache entry is invalid JSON')
+  t.test('does not insert plain manifests into the cache')
+  t.test('falls back to registry if cache entry missing', function (t) {
+    t.tearDown(function () {})
+    setTimeout(function () {
+      t.end()
+    })
+  })
+  t.test('allows forcing use of cache when data stale')
+}

test/test/exit-on-exit.tap

@@ -0,0 +1,48 @@
+TAP version 13
+ok 1 - ___/.*(node|iojs)(.exe)?.*/~~~exit-on-exit.js subtest ___/# time=[0-9.]+(ms)?/~~~ {
+    # Subtest: memoizes identical registry requests
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 1 - memoizes identical registry requests ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: tag requests memoize versions
+        1..2
+        ok 1 - (unnamed test)
+        ok 2 - (unnamed test)
+    ok 2 - tag requests memoize versions ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: tag requests memoize tags
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 3 - tag requests memoize tags ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 4 - memoization is scoped to a given cache # TODO
+    # Subtest: inflights concurrent requests
+        1..2
+        ok 1 - got a manifest
+        ok 2 - got a manifest
+    ok 5 - inflights concurrent requests ___/# time=[0-9.]+(ms)?/~~~
+
+    # Subtest: supports fetching from an optional cache
+        ok 1 - should be equivalent
+        1..1
+    ok 6 - supports fetching from an optional cache ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 7 - expires stale request data # TODO
+    ok 8 - falls back to registry if cache entry is invalid JSON # TODO
+    ok 9 - does not insert plain manifests into the cache # TODO
+    # Subtest: falls back to registry if cache entry missing
+        1..0
+    ok 10 - falls back to registry if cache entry missing ___/# time=[0-9.]+(ms)?/~~~
+
+    ok 11 - allows forcing use of cache when data stale # TODO
+    1..11
+    # todo: 5
+    ___/# time=[0-9.]+(ms)?/~~~
+}
+
+1..1
+___/# time=[0-9.]+(ms)?/~~~
+