[Heartbeat] Produce error rather than panic on missing source

Fixes elastic#24403.

With the changes to the heartbeat config syntax in 7.12 the `source`
field is now required. Our config validation code didn't actually check
for this field's presence, which caused an NPE.

This PR adds a validation checking for that config's presence. It also
adds tests for the validation code for config sub-fields. There were no
defects found in the validations for source.inline, or source.browser,
but a few tests were missing.

Instead of the panic seen in elastic#24403 users will now get the error seen
below.

```
2021-03-05T15:41:40.146-0600	ERROR	instance/beat.go:952	Exiting: could not create monitor: job err could not parse suite config: config 'source' must be specified for this monitor, if upgrading from a previous experimental version please see our new config docs accessing 'heartbeat.monitors.0' (source:'sample-synthetics-config/heartbeat.yml')
Exiting: could not create monitor: job err could not parse suite config: config 'source' must be specified for this monitor, if upgrading from a previous experimental version please see our new config docs accessing 'heartbeat.monitors.0' (source:'sample-synthetics-config/heartbeat.yml')
```

x-pack/heartbeat/monitors/browser/config.go

@@ -31,6 +31,7 @@ type Config struct {
 
 var ErrNameRequired = fmt.Errorf("config 'name' must be specified for this monitor")
 var ErrIdRequired = fmt.Errorf("config 'id' must be specified for this monitor")
+var ErrSourceRequired = fmt.Errorf("config 'source' must be specified for this monitor, if upgrading from a previous experimental version please see our new config docs")
 
 func (c *Config) Validate() error {
 	if c.Name == "" {
@@ -40,5 +41,9 @@ func (c *Config) Validate() error {
 		return ErrIdRequired
 	}
 
+	if c.Source == nil {
+		return ErrSourceRequired
+	}
+
 	return nil
 }

x-pack/heartbeat/monitors/browser/config_test.go

@@ -0,0 +1,55 @@
+// 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 browser
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/source"
+)
+
+func TestConfig_Validate(t *testing.T) {
+	testSource := source.Source{Inline: &source.InlineSource{Script: "//something"}}
+
+	tests := []struct {
+		name    string
+		cfg     *Config
+		wantErr error
+	}{
+		{
+			"no error",
+			&Config{Id: "myid", Name: "myname", Source: &testSource},
+			nil,
+		},
+		{
+			"no id",
+			&Config{Name: "myname", Source: &testSource},
+			ErrIdRequired,
+		},
+		{
+			"no name",
+			&Config{Id: "myid", Source: &testSource},
+			ErrNameRequired,
+		},
+		{
+			"no source",
+			&Config{Id: "myid", Name: "myname"},
+			ErrSourceRequired,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.cfg.Validate()
+
+			if tt.wantErr != nil {
+				require.Equal(t, tt.wantErr, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}

x-pack/heartbeat/monitors/browser/source/inline.go

@@ -14,9 +14,11 @@ type InlineSource struct {
 	BaseSource
 }
 
+var ErrNoInlineScript = fmt.Errorf("no 'script' value specified for inline source")
+
 func (s *InlineSource) Validate() error {
 	if !regexp.MustCompile("\\S").MatchString(s.Script) {
-		return fmt.Errorf("no 'script' value specified for inline source")
+		return ErrNoInlineScript
 	}
 
 	return nil

x-pack/heartbeat/monitors/browser/source/inline_test.go

@@ -0,0 +1,42 @@
+// 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 source
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInlineSourceValidation(t *testing.T) {
+	type testCase struct {
+		name    string
+		source  *InlineSource
+		wantErr error
+	}
+	testCases := []testCase{
+		{
+			"no error",
+			&InlineSource{Script: "a script"},
+			nil,
+		},
+		{
+			"no script",
+			&InlineSource{},
+			ErrNoInlineScript,
+		},
+	}
+
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.source.Validate()
+			if tt.wantErr != nil {
+				require.Equal(t, tt.wantErr, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}

x-pack/heartbeat/monitors/browser/source/source.go

@@ -28,7 +28,7 @@ func (s *Source) Active() ISource {
 	return s.ActiveMemo
 }
 
-var ErrInvalidSource = fmt.Errorf("no or unknown source type specified for synthetic monitor")
+var ErrInvalidSource = fmt.Errorf("no or unknown source type specified for synthetic monitor.")
 
 func (s *Source) Validate() error {
 	if s.Active() == nil {

x-pack/heartbeat/monitors/browser/source/source_test.go

@@ -0,0 +1,42 @@
+// 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 source
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSourceValidation(t *testing.T) {
+	type testCase struct {
+		name    string
+		source  Source
+		wantErr error
+	}
+	testCases := []testCase{
+		{
+			"no error",
+			Source{Inline: &InlineSource{}},
+			nil,
+		},
+		{
+			"no concrete source",
+			Source{},
+			ErrInvalidSource,
+		},
+	}
+
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.source.Validate()
+			if tt.wantErr != nil {
+				require.Equal(t, tt.wantErr, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}