[Elastic Agent] Fix composed.Verifier so files are removed when inval…

…id (#30281)

* Fix composed.Verifier.

* Add tests and changelog.

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -94,6 +94,7 @@
 - Allow HTTP metrics to run in bootstrap mode. Add ability to adjust timeouts for Fleet Server. {pull}28260[28260]
 - Fix agent configuration overwritten by default fleet config. {pull}29297[29297]
 - Allow agent containers to use basic auth to create a service token. {pull}29651[29651]
+- Fix issue where a failing artifact verification does not remove the bad artifact. {pull}30281[30281]
 
 ==== New features
 

x-pack/elastic-agent/pkg/artifact/download/composed/verifier.go

@@ -33,9 +33,8 @@ func NewVerifier(verifiers ...download.Verifier) *Verifier {
 func (e *Verifier) Verify(spec program.Spec, version string, removeOnFailure bool) (bool, error) {
 	var err error
 
-	for i, v := range e.vv {
-		isLast := (i + 1) == len(e.vv)
-		b, e := v.Verify(spec, version, isLast && removeOnFailure)
+	for _, v := range e.vv {
+		b, e := v.Verify(spec, version, removeOnFailure)
 		if e == nil {
 			return b, nil
 		}

x-pack/elastic-agent/pkg/artifact/download/composed/verifier_test.go

@@ -0,0 +1,95 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package composed
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/agent/program"
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/artifact/download"
+)
+
+type ErrorVerifier struct {
+	called bool
+}
+
+func (d *ErrorVerifier) Verify(spec program.Spec, version string, removeOnFailure bool) (bool, error) {
+	d.called = true
+	return false, errors.New("failing")
+}
+
+func (d *ErrorVerifier) Called() bool { return d.called }
+
+type FailVerifier struct {
+	called bool
+}
+
+func (d *FailVerifier) Verify(spec program.Spec, version string, removeOnFailure bool) (bool, error) {
+	d.called = true
+	return false, nil
+}
+
+func (d *FailVerifier) Called() bool { return d.called }
+
+type SuccVerifier struct {
+	called          bool
+	removeOnFailure bool
+}
+
+func (d *SuccVerifier) Verify(spec program.Spec, version string, removeOnFailure bool) (bool, error) {
+	d.called = true
+	return true, nil
+}
+
+func (d *SuccVerifier) Called() bool { return d.called }
+
+func TestVerifier(t *testing.T) {
+	testCases := []verifyTestCase{
+		{
+			verifiers:      []CheckableVerifier{&ErrorVerifier{}, &SuccVerifier{}, &FailVerifier{}},
+			checkFunc:      func(d []CheckableVerifier) bool { return d[0].Called() && d[1].Called() && !d[2].Called() },
+			expectedResult: true,
+		}, {
+			verifiers:      []CheckableVerifier{&SuccVerifier{}, &ErrorVerifier{}, &FailVerifier{}},
+			checkFunc:      func(d []CheckableVerifier) bool { return d[0].Called() && !d[1].Called() && !d[2].Called() },
+			expectedResult: true,
+		}, {
+			verifiers:      []CheckableVerifier{&FailVerifier{}, &ErrorVerifier{}, &SuccVerifier{}},
+			checkFunc:      func(d []CheckableVerifier) bool { return d[0].Called() && !d[1].Called() && !d[2].Called() },
+			expectedResult: false,
+		}, {
+			verifiers:      []CheckableVerifier{&ErrorVerifier{}, &FailVerifier{}, &SuccVerifier{}},
+			checkFunc:      func(d []CheckableVerifier) bool { return d[0].Called() && d[1].Called() && !d[2].Called() },
+			expectedResult: false,
+		}, {
+			verifiers:      []CheckableVerifier{&ErrorVerifier{}, &ErrorVerifier{}, &SuccVerifier{}},
+			checkFunc:      func(d []CheckableVerifier) bool { return d[0].Called() && d[1].Called() && d[2].Called() },
+			expectedResult: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		d := NewVerifier(tc.verifiers[0], tc.verifiers[1], tc.verifiers[2])
+		r, _ := d.Verify(program.Spec{Name: "a", Cmd: "a", Artifact: "a/a"}, "b", true)
+
+		assert.Equal(t, tc.expectedResult, r)
+
+		assert.True(t, tc.checkFunc(tc.verifiers))
+	}
+}
+
+type CheckableVerifier interface {
+	download.Verifier
+	Called() bool
+}
+
+type verifyTestCase struct {
+	verifiers      []CheckableVerifier
+	checkFunc      func(verifiers []CheckableVerifier) bool
+	expectedResult bool
+}