pkg/mock: add a mock of os.Exit for testing purposes

pkg/mock/osexit/osexit.go

@@ -0,0 +1,85 @@
+// Public Domain (-) 2018-present, The Core Authors.
+// See the Core UNLICENSE file for details.
+
+// Package osexit mocks the os.Exit function.
+//
+// To use, first set a package-specific osExit function, e.g.
+//
+//     var osExit = os.Exit
+//
+// Then use that when you want to call os.Exit, e.g.
+//
+//     if somethingFatal {
+//         osExit(1)
+//         return
+//     }
+//
+// Make sure to return immediately after the call to osExit, so that testing
+// code will match real code as closely as possible.
+//
+// You can now use the utility functions provided by this package to override
+// osExit for testing purposes, e.g.
+//
+//     osExit = osexit.Set()
+//     invokeCodeCallingExit()
+//     if !osexit.Called() {
+//         t.Fatalf("os.Exit was not called as expected")
+//     }
+//
+package osexit
+
+import (
+	"sync"
+)
+
+var (
+	called bool
+	mu     sync.RWMutex // protects called, status
+	status int
+)
+
+// Called returns whether the mock os.Exit function was called.
+func Called() bool {
+	mu.RLock()
+	c := called
+	mu.RUnlock()
+	return c
+}
+
+// Func provides a mock for the os.Exit function. Special care must be taken
+// when testing os.Exit to make sure no code runs after the call to Exit. It's
+// recommended to put a return statement after Exit calls so that the behaviour
+// of the mock matches that of the real function as much as possible.
+func Func(code int) {
+	mu.Lock()
+	if called {
+		mu.Unlock()
+		return
+	}
+	called = true
+	status = code
+	mu.Unlock()
+}
+
+// Reset resets the state of the mock function.
+func Reset() {
+	mu.Lock()
+	called = false
+	status = 0
+	mu.Unlock()
+}
+
+// Set returns the mock os.Exit function after calling Reset.
+func Set() func(int) {
+	Reset()
+	return Func
+}
+
+// Status returns the status code that the mock os.Exit function was called
+// with.
+func Status() int {
+	mu.RLock()
+	s := status
+	mu.RUnlock()
+	return s
+}

pkg/mock/osexit/osexit_test.go

@@ -0,0 +1,38 @@
+// Public Domain (-) 2018-present, The Core Authors.
+// See the Core UNLICENSE file for details.
+
+package osexit_test
+
+import (
+	"os"
+	"testing"
+
+	"espra.com/pkg/mock/osexit"
+)
+
+var osExit = os.Exit
+
+func TestOsExit(t *testing.T) {
+	osExit = osexit.Set()
+	osExit(2)
+	if !osexit.Called() {
+		t.Fatalf("mock exit function was not called")
+	}
+	status := osexit.Status()
+	if status != 2 {
+		t.Fatalf("mock exit function did not set the right status code: got %d, want 2", status)
+	}
+	osExit(3)
+	status = osexit.Status()
+	if status != 2 {
+		t.Fatalf("mock exit function overrode the status set by a previous call: got %d, want 2", status)
+	}
+	osexit.Reset()
+	if osexit.Called() {
+		t.Fatalf("the reset mock exit function claims to have been called")
+	}
+	status = osexit.Status()
+	if status != 0 {
+		t.Fatalf("the reset mock exit function returned a non-zero status code: got %d, want 0", status)
+	}
+}