Add exec.Command stub mechanism that matches by arguments

Our previous command stub mechanism matches stubs sequentially, which leads to brittle tests when the exec calls get reordered or removed in the implementation.

internal/run/stub.go

@@ -0,0 +1,91 @@
+package run
+
+import (
+	"fmt"
+	"os/exec"
+	"regexp"
+	"strings"
+)
+
+type T interface {
+	Helper()
+	Errorf(string, ...interface{})
+}
+
+func Stub() (*CommandStubber, func(T)) {
+	cs := &CommandStubber{}
+	teardown := SetPrepareCmd(func(cmd *exec.Cmd) Runnable {
+		s := cs.find(cmd.Args)
+		if s == nil {
+			panic(fmt.Sprintf("no exec stub for `%s`", strings.Join(cmd.Args, " ")))
+		}
+		for _, c := range s.callbacks {
+			c(cmd.Args)
+		}
+		s.matched = true
+		return s
+	})
+
+	return cs, func(t T) {
+		defer teardown()
+		var unmatched []string
+		for _, s := range cs.stubs {
+			if s.matched {
+				continue
+			}
+			unmatched = append(unmatched, s.pattern.String())
+		}
+		if len(unmatched) == 0 {
+			return
+		}
+		t.Helper()
+		t.Errorf("umatched stubs (%d): %s", len(unmatched), strings.Join(unmatched, ", "))
+	}
+}
+
+type CommandStubber struct {
+	stubs []*commandStub
+}
+
+func (cs *CommandStubber) Register(p string, exitStatus int, output string, callbacks ...CommandCallback) {
+	cs.stubs = append(cs.stubs, &commandStub{
+		pattern:    regexp.MustCompile(p),
+		exitStatus: exitStatus,
+		stdout:     output,
+		callbacks:  callbacks,
+	})
+}
+
+func (cs *CommandStubber) find(args []string) *commandStub {
+	line := strings.Join(args, " ")
+	for _, s := range cs.stubs {
+		if !s.matched && s.pattern.MatchString(line) {
+			return s
+		}
+	}
+	return nil
+}
+
+type CommandCallback func([]string)
+
+type commandStub struct {
+	pattern    *regexp.Regexp
+	matched    bool
+	exitStatus int
+	stdout     string
+	callbacks  []CommandCallback
+}
+
+func (s *commandStub) Run() error {
+	if s.exitStatus != 0 {
+		return fmt.Errorf("%s exited with status %d", s.pattern, s.exitStatus)
+	}
+	return nil
+}
+
+func (s *commandStub) Output() ([]byte, error) {
+	if s.exitStatus != 0 {
+		return []byte(nil), fmt.Errorf("%s exited with status %d", s.pattern, s.exitStatus)
+	}
+	return []byte(s.stdout), nil
+}