data-source/aws_lambda_function: Implement standalone Read function a…

…nd refactor testing to check data source state values against the resource state values

References:
* #5812
* hashicorp/terraform#10810 (comment)
* #6966

Previously, the `aws_lambda_function` data source was utilizing the Read function from the `aws_lambda_function` resource. This legacy practice has longterm maintenance issues with missing schema and documentation updates.

Here we implement a fresh new Read function for the data source that includes the following changes:

* Properly error when Lambda Function is not found
* Always return the `arn` attribute as unqualified (e.g. without a qualifier or version suffix)
* Always return the `qualified_arn` attribute as qualified (e.g. with the qualifier or version suffix)
* Always return the `tags` attribute

The acceptance testing changes modernize and simplify the testing:

* Utilize `resource.TestCheckResourceAttrPair()` where possible to ensure data source state values match appropriate resource state values
* Consolidate random naming to single variable
* Only provision VPC resources in VPC specific test

Output from acceptance testing:

```
--- PASS: TestAccDataSourceAWSLambdaFunction_version (20.89s)
--- PASS: TestAccDataSourceAWSLambdaFunction_environment (22.75s)
--- PASS: TestAccDataSourceAWSLambdaFunction_alias (23.68s)
--- PASS: TestAccDataSourceAWSLambdaFunction_basic (23.76s)
--- PASS: TestAccDataSourceAWSLambdaFunction_layers (28.82s)
--- PASS: TestAccDataSourceAWSLambdaFunction_vpc (36.48s)
```

aws/data_source_aws_lambda_function.go

@@ -1,6 +1,12 @@
 package aws
 
 import (
+	"fmt"
+	"log"
+	"strings"
+
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/lambda"
 	"github.com/hashicorp/terraform/helper/schema"
 )
 
@@ -151,11 +157,114 @@ func dataSourceAwsLambdaFunction() *schema.Resource {
 				Type:     schema.TypeString,
 				Computed: true,
 			},
+			"tags": tagsSchemaComputed(),
 		},
 	}
 }
 
 func dataSourceAwsLambdaFunctionRead(d *schema.ResourceData, meta interface{}) error {
-	d.SetId(d.Get("function_name").(string))
-	return resourceAwsLambdaFunctionRead(d, meta)
+	conn := meta.(*AWSClient).lambdaconn
+	functionName := d.Get("function_name").(string)
+
+	input := &lambda.GetFunctionInput{
+		FunctionName: aws.String(functionName),
+	}
+
+	if v, ok := d.GetOk("qualifier"); ok {
+		input.Qualifier = aws.String(v.(string))
+	}
+
+	log.Printf("[DEBUG] Getting Lambda Function: %s", input)
+	output, err := conn.GetFunction(input)
+
+	if err != nil {
+		return fmt.Errorf("error getting Lambda Function (%s): %s", functionName, err)
+	}
+
+	if output == nil {
+		return fmt.Errorf("error getting Lambda Function (%s): empty response", functionName)
+	}
+
+	function := output.Configuration
+
+	functionARN := aws.StringValue(function.FunctionArn)
+	qualifierSuffix := fmt.Sprintf(":%s", d.Get("qualifier").(string))
+	versionSuffix := fmt.Sprintf(":%s", aws.StringValue(function.Version))
+
+	qualifiedARN := functionARN
+	if !strings.HasSuffix(functionARN, qualifierSuffix) && !strings.HasSuffix(functionARN, versionSuffix) {
+		qualifiedARN = functionARN + versionSuffix
+	}
+
+	unqualifiedARN := strings.TrimSuffix(functionARN, qualifierSuffix)
+
+	d.Set("arn", unqualifiedARN)
+
+	deadLetterConfig := []interface{}{}
+	if function.DeadLetterConfig != nil {
+		deadLetterConfig = []interface{}{
+			map[string]interface{}{
+				"target_arn": aws.StringValue(function.DeadLetterConfig.TargetArn),
+			},
+		}
+	}
+	if err := d.Set("dead_letter_config", deadLetterConfig); err != nil {
+		return fmt.Errorf("error setting dead_letter_config: %s", err)
+	}
+
+	d.Set("description", function.Description)
+
+	if err := d.Set("environment", flattenLambdaEnvironment(function.Environment)); err != nil {
+		return fmt.Errorf("error setting environment: %s", err)
+	}
+
+	d.Set("handler", function.Handler)
+	d.Set("invoke_arn", lambdaFunctionInvokeArn(aws.StringValue(function.FunctionArn), meta))
+	d.Set("kms_key_arn", function.KMSKeyArn)
+	d.Set("last_modified", function.LastModified)
+
+	if err := d.Set("layers", flattenLambdaLayers(function.Layers)); err != nil {
+		return fmt.Errorf("Error setting layers for Lambda Function (%s): %s", d.Id(), err)
+	}
+
+	d.Set("memory_size", function.MemorySize)
+	d.Set("qualified_arn", qualifiedARN)
+
+	reservedConcurrentExecutions := int64(-1)
+	if output.Concurrency != nil {
+		reservedConcurrentExecutions = aws.Int64Value(output.Concurrency.ReservedConcurrentExecutions)
+	}
+	d.Set("reserved_concurrent_executions", reservedConcurrentExecutions)
+
+	d.Set("role", function.Role)
+	d.Set("runtime", function.Runtime)
+	d.Set("source_code_hash", function.CodeSha256)
+	d.Set("source_code_size", function.CodeSize)
+
+	if err := d.Set("tags", tagsToMapGeneric(output.Tags)); err != nil {
+		return fmt.Errorf("error setting tags: %s", err)
+	}
+
+	tracingConfig := []map[string]interface{}{
+		{
+			"mode": lambda.TracingModePassThrough,
+		},
+	}
+	if function.TracingConfig != nil {
+		tracingConfig[0]["mode"] = aws.StringValue(function.TracingConfig.Mode)
+	}
+	if err := d.Set("tracing_config", tracingConfig); err != nil {
+		return fmt.Errorf("error setting tracing_config: %s", tracingConfig)
+	}
+
+	d.Set("timeout", function.Timeout)
+	d.Set("version", function.Version)
+
+	if err := d.Set("vpc_config", flattenLambdaVpcConfigResponse(function.VpcConfig)); err != nil {
+		return fmt.Errorf("error setting vpc_config: %s", err)
+	}
+
+	d.SetId(aws.StringValue(function.FunctionName))
+
+	return nil
 }