Add a package to test calls to os.Exit

Add a small internal package to ease testing of calls to `os.Exit(1)`. This is a
small, well-encapsulated concern that's a good fit for an internal package.

internal/exit/exit.go

@@ -0,0 +1,64 @@
+// Copyright (c) 2016 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+// Package exit provides stubs so that unit tests can exercise code that calls
+// os.Exit(1).
+package exit
+
+import "os"
+
+var real = func() { os.Exit(1) }
+
+// Exit normally terminates the process by calling os.Exit(1). If the package
+// is stubbed, it instead records a call in the testing spy.
+func Exit() {
+	real()
+}
+
+// A StubbedExit is a testing fake for os.Exit.
+type StubbedExit struct {
+	Exited bool
+	prev   func()
+}
+
+// Stub substitutes a fake for the call to os.Exit(1).
+func Stub() *StubbedExit {
+	s := &StubbedExit{prev: real}
+	real = s.exit
+	return s
+}
+
+// WithStub runs the supplied function with Exit stubbed. It returns the stub
+// used, so that users can test whether the process would have crashed.
+func WithStub(f func()) *StubbedExit {
+	s := Stub()
+	defer s.Unstub()
+	f()
+	return s
+}
+
+// Unstub restores the previous exit function.
+func (se *StubbedExit) Unstub() {
+	real = se.prev
+}
+
+func (se *StubbedExit) exit() {
+	se.Exited = true
+}