fakeexec impementation

Signed-off-by: Paddy Steed <paddy.steed@engineerbetter.com>

internal/fakeexec/fakeexec.go

@@ -0,0 +1,107 @@
+/*
+Package fakeexec is used to mock calls to exec.Command.
+The following function must be included in a *_test.go file.
+	func TestExecCommandHelper(t *testing.T) {
+		if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+			return
+		}
+		fmt.Print(os.Getenv("STDOUT"))
+		i, _ := strconv.Atoi(os.Getenv("EXIT_STATUS"))
+		os.Exit(i)
+	}
+*/
+package fakeexec
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// Expect controls the expected execution of a command
+type Expect struct {
+	f        func(t testing.TB, actualCommand string, actualArgs ...string)
+	exitCode int
+	stdout   string
+}
+
+// E represents a set of expected executions of a command
+type E struct {
+	t  testing.TB
+	es []*Expect
+}
+
+// New create a new E
+func New(t testing.TB) *E {
+	e := new(E)
+	e.t = t
+	return e
+}
+
+// Exits set the exit code of the execution
+func (e *Expect) Exits(code int) {
+	e.exitCode = code
+}
+
+// Outputs sets the executions standard output
+func (e *Expect) Outputs(stdout string) {
+	e.stdout = stdout
+}
+
+// Expect is adds a new expectation
+func (e *E) Expect(command string, args ...string) *Expect {
+	return e.ExpectFunc(func(t testing.TB, actualCommand string, actualArgs ...string) {
+		exp := fmt.Sprintf("%s %s", command, strings.Join(args, " "))
+		act := fmt.Sprintf("%s %s", actualCommand, strings.Join(actualArgs, " "))
+		require.Equal(t, exp, act)
+	})
+}
+
+// ExpectFunc adds a new expectation, command and args can be checked for correctness by the supplied function
+func (e *E) ExpectFunc(f func(t testing.TB, command string, args ...string)) *Expect {
+	ex := &Expect{
+		f: f,
+	}
+	e.es = append(e.es, ex)
+	return ex
+}
+
+// Cmd returns a function with a signiture that matches (os/exec).Command
+func (e *E) Cmd() func(command string, args ...string) *exec.Cmd {
+	return func(command string, args ...string) *exec.Cmd {
+		e.t.Helper()
+		if len(e.es) == 0 {
+			e.t.Fatalf("unexpected call to %s %v", command, args)
+		}
+		thisProg, err := os.Executable()
+		if err != nil {
+			panic(err)
+		}
+		var expectation *Expect
+		expectation, e.es = e.es[0], e.es[1:]
+		expectation.f(e.t, command, args...)
+		cs := []string{"-test.run=TestExecCommandHelper", "--", command}
+		cs = append(cs, args...)
+		cmd := exec.Command(thisProg, cs...)
+		es := strconv.Itoa(expectation.exitCode)
+		cmd.Env = []string{
+			"GO_WANT_HELPER_PROCESS=1",
+			"EXIT_STATUS=" + es,
+			"STDOUT=" + expectation.stdout,
+		}
+		return cmd
+	}
+}
+
+// Finish checks all expectations have been met
+func (e *E) Finish() {
+	e.t.Helper()
+	if len(e.es) != 0 {
+		e.t.Fatalf("%d missing calls to os.Exec", len(e.es))
+	}
+}