internal/keyvaluetags: Create {SERVICE}GetTag generator, support EC2 …

…list/get, use in aws_ec2_tag implementation

Reference: #9061

The GetTag generator simplifies the creation of the new individual service tag resources into a consistent implementation. This consistent implementation can be used to automatically generate the service tag resources themselves in the future.

Output from acceptance testing:

```
--- PASS: TestAccAWSEc2Tag_basic (485.52s)
--- PASS: TestAccAWSEc2Tag_disappears (529.67s)
--- PASS: TestAccAWSEc2Tag_Value (588.49s)
```

aws/internal/keyvaluetags/README.md

@@ -19,6 +19,8 @@ Any tagging functions that cannot be generated should be hand implemented in a s
 ```text
 aws/internal/keyvaluetags
 ├── generators
+│   ├── createtags (generates create_tags_gen.go)
+│   ├── gettag (generates get_tag_gen.go)
 │   ├── listtags (generates list_tags_gen.go)
 │   ├── servicetags (generates service_tags_gen.go)
 │   └── updatetags (generates update_tags_gen.go)

aws/internal/keyvaluetags/generators/gettag/main.go

@@ -0,0 +1,133 @@
+// +build ignore
+
+package main
+
+import (
+	"bytes"
+	"go/format"
+	"log"
+	"os"
+	"sort"
+	"strings"
+	"text/template"
+
+	"github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags"
+)
+
+const filename = `get_tag_gen.go`
+
+var serviceNames = []string{
+	"dynamodb",
+	"ec2",
+	"ecs",
+	"route53resolver",
+}
+
+type TemplateData struct {
+	ServiceNames []string
+}
+
+func main() {
+	// Always sort to reduce any potential generation churn
+	sort.Strings(serviceNames)
+
+	templateData := TemplateData{
+		ServiceNames: serviceNames,
+	}
+	templateFuncMap := template.FuncMap{
+		"ClientType":                        keyvaluetags.ServiceClientType,
+		"ListTagsFunction":                  keyvaluetags.ServiceListTagsFunction,
+		"ListTagsInputFilterIdentifierName": keyvaluetags.ServiceListTagsInputFilterIdentifierName,
+		"ListTagsInputResourceTypeField":    keyvaluetags.ServiceListTagsInputResourceTypeField,
+		"ListTagsOutputTagsField":           keyvaluetags.ServiceListTagsOutputTagsField,
+		"TagPackage":                        keyvaluetags.ServiceTagPackage,
+		"Title":                             strings.Title,
+	}
+
+	tmpl, err := template.New("gettag").Funcs(templateFuncMap).Parse(templateBody)
+
+	if err != nil {
+		log.Fatalf("error parsing template: %s", err)
+	}
+
+	var buffer bytes.Buffer
+	err = tmpl.Execute(&buffer, templateData)
+
+	if err != nil {
+		log.Fatalf("error executing template: %s", err)
+	}
+
+	generatedFileContents, err := format.Source(buffer.Bytes())
+
+	if err != nil {
+		log.Fatalf("error formatting generated file: %s", err)
+	}
+
+	f, err := os.Create(filename)
+
+	if err != nil {
+		log.Fatalf("error creating file (%s): %s", filename, err)
+	}
+
+	defer f.Close()
+
+	_, err = f.Write(generatedFileContents)
+
+	if err != nil {
+		log.Fatalf("error writing to file (%s): %s", filename, err)
+	}
+}
+
+var templateBody = `
+// Code generated by generators/gettag/main.go; DO NOT EDIT.
+
+package keyvaluetags
+
+import (
+	"github.com/aws/aws-sdk-go/aws"
+{{- range .ServiceNames }}
+	"github.com/aws/aws-sdk-go/service/{{ . }}"
+{{- end }}
+)
+
+{{- range .ServiceNames }}
+
+// {{ . | Title }}GetTag fetches an individual {{ . }} service tag for a resource.
+// Returns whether the key exists, the key value, and any errors.
+// This function will optimise the handling over {{ . | Title }}ListTags, if possible.
+// The identifier is typically the Amazon Resource Name (ARN), although
+// it may also be a different identifier depending on the service.
+func {{ . | Title }}GetTag(conn {{ . | ClientType }}, identifier string{{ if . | ListTagsInputResourceTypeField }}, resourceType string{{ end }}, key string) (bool, *string, error) {
+	{{- if . | ListTagsInputFilterIdentifierName }}
+	input := &{{ . | TagPackage  }}.{{ . | ListTagsFunction }}Input{
+		Filters: []*{{ . | TagPackage  }}.Filter{
+			{
+				Name:   aws.String("{{ . | ListTagsInputFilterIdentifierName }}"),
+				Values: []*string{aws.String(identifier)},
+			},
+			{
+				Name:   aws.String("key"),
+				Values: []*string{aws.String(key)},
+			},
+		},
+	}
+
+	output, err := conn.{{ . | ListTagsFunction }}(input)
+
+	if err != nil {
+		return false, nil, err
+	}
+
+	listTags := {{ . | Title }}KeyValueTags(output.{{ . | ListTagsOutputTagsField }})
+	{{- else }}
+	listTags, err := {{ . | Title }}ListTags(conn, identifier{{ if . | ListTagsInputResourceTypeField }}, resourceType{{ end }})
+
+	if err != nil {
+		return false, nil, err
+	}
+	{{- end }}
+
+	return listTags.KeyExists(key), listTags.KeyValue(key), nil
+}
+{{- end }}
+`

aws/internal/keyvaluetags/generators/listtags/main.go

@@ -51,6 +51,7 @@ var serviceNames = []string{
 	"dlm",
 	"docdb",
 	"dynamodb",
+	"ec2",
 	"ecr",
 	"ecs",
 	"efs",
@@ -129,6 +130,7 @@ func main() {
 	templateFuncMap := template.FuncMap{
 		"ClientType":                           keyvaluetags.ServiceClientType,
 		"ListTagsFunction":                     keyvaluetags.ServiceListTagsFunction,
+		"ListTagsInputFilterIdentifierName":    keyvaluetags.ServiceListTagsInputFilterIdentifierName,
 		"ListTagsInputIdentifierField":         keyvaluetags.ServiceListTagsInputIdentifierField,
 		"ListTagsInputIdentifierRequiresSlice": keyvaluetags.ServiceListTagsInputIdentifierRequiresSlice,
 		"ListTagsInputResourceTypeField":       keyvaluetags.ServiceListTagsInputResourceTypeField,
@@ -189,6 +191,14 @@ import (
 // it may also be a different identifier depending on the service.
 func {{ . | Title }}ListTags(conn {{ . | ClientType }}, identifier string{{ if . | ListTagsInputResourceTypeField }}, resourceType string{{ end }}) (KeyValueTags, error) {
 	input := &{{ . | TagPackage  }}.{{ . | ListTagsFunction }}Input{
+		{{- if . | ListTagsInputFilterIdentifierName }}
+		Filters: []*{{ . | TagPackage  }}.Filter{
+			{
+				Name:   aws.String("{{ . | ListTagsInputFilterIdentifierName }}"),
+				Values: []*string{aws.String(identifier)},
+			},
+		},
+		{{- else }}
 		{{- if . | ListTagsInputIdentifierRequiresSlice }}
 		{{ . | ListTagsInputIdentifierField }}:   aws.StringSlice([]string{identifier}),
 		{{- else }}
@@ -197,6 +207,7 @@ func {{ . | Title }}ListTags(conn {{ . | ClientType }}, identifier string{{ if .
 		{{- if . | ListTagsInputResourceTypeField }}
 		{{ . | ListTagsInputResourceTypeField }}: aws.String(resourceType),
 		{{- end }}
+		{{- end }}
 	}
 
 	output, err := conn.{{ . | ListTagsFunction }}(input)

aws/internal/keyvaluetags/generators/servicetags/main.go

@@ -155,6 +155,7 @@ func main() {
 		"TagKeyType":        keyvaluetags.ServiceTagKeyType,
 		"TagPackage":        keyvaluetags.ServiceTagPackage,
 		"TagType":           keyvaluetags.ServiceTagType,
+		"TagType2":          keyvaluetags.ServiceTagType2,
 		"TagTypeKeyField":   keyvaluetags.ServiceTagTypeKeyField,
 		"TagTypeValueField": keyvaluetags.ServiceTagTypeValueField,
 		"Title":             strings.Title,
@@ -259,6 +260,31 @@ func (tags KeyValueTags) {{ . | Title }}Tags() []*{{ . | TagPackage }}.{{ . | Ta
 }
 
 // {{ . | Title }}KeyValueTags creates KeyValueTags from {{ . }} service tags.
+{{- if . | TagType2 }}
+// Accepts []*{{ . | TagPackage }}.{{ . | TagType }} and []*{{ . | TagPackage }}.{{ . | TagType2 }}.
+func {{ . | Title }}KeyValueTags(tags interface{}) KeyValueTags {
+	switch tags := tags.(type) {
+	case []*{{ . | TagPackage }}.{{ . | TagType }}:
+		m := make(map[string]*string, len(tags))
+
+		for _, tag := range tags {
+			m[aws.StringValue(tag.{{ . | TagTypeKeyField }})] = tag.{{ . | TagTypeValueField }}
+		}
+
+		return New(m)
+	case []*{{ . | TagPackage }}.{{ . | TagType2 }}:
+		m := make(map[string]*string, len(tags))
+
+		for _, tag := range tags {
+			m[aws.StringValue(tag.{{ . | TagTypeKeyField }})] = tag.{{ . | TagTypeValueField }}
+		}
+
+		return New(m)
+	default:
+		return New(nil)
+	}
+}
+{{- else }}
 func {{ . | Title }}KeyValueTags(tags []*{{ . | TagPackage }}.{{ . | TagType }}) KeyValueTags {
 	m := make(map[string]*string, len(tags))
 
@@ -269,4 +295,5 @@ func {{ . | Title }}KeyValueTags(tags []*{{ . | TagPackage }}.{{ . | TagType }})
 	return New(m)
 }
 {{- end }}
+{{- end }}
 `

aws/internal/keyvaluetags/key_value_tags.go

@@ -1,5 +1,6 @@
 //go:generate go run -tags generate generators/servicetags/main.go
 //go:generate go run -tags generate generators/listtags/main.go
+//go:generate go run -tags generate generators/gettag/main.go
 //go:generate go run -tags generate generators/createtags/main.go
 //go:generate go run -tags generate generators/updatetags/main.go
 
@@ -138,6 +139,28 @@ func (tags KeyValueTags) Ignore(ignoreTags KeyValueTags) KeyValueTags {
 	return result
 }
 
+// KeyExists returns true if a tag key exists.
+// If the key is not found, returns nil.
+// Use KeyExists to determine if key is present.
+func (tags KeyValueTags) KeyExists(key string) bool {
+	if _, ok := tags[key]; ok {
+		return true
+	}
+
+	return false
+}
+
+// KeyValue returns a tag key value.
+// If the key is not found, returns nil.
+// Use KeyExists to determine if key is present.
+func (tags KeyValueTags) KeyValue(key string) *string {
+	if v, ok := tags[key]; ok {
+		return v
+	}
+
+	return nil
+}
+
 // Keys returns tag keys.
 func (tags KeyValueTags) Keys() []string {
 	result := make([]string, 0, len(tags))