aws: Config refactor to use pointer for fields [breaking change]

This change refactors the SDK so that Config fields are pointers instead of
values. Primarily this change fixes the bug preventing setting Config fields
to the field's zero value when creating new service client instances.

```go
svc := s3.New(&aws.Config{
    Region:     aws.String("us-west-2"),
    MaxRetries: aws.Int(10),
})
```

Additionally builder pattern `WithX` methods were added to Config. These
methods can be chained so inline building of Config objects can be done
without using pointers for the fields.

```go
svc := s3.New(aws.NewConfig().WithRegion("us-west-2").WithMaxRetries(10))
```

Fixes #157
Fixes #276
Fixes #124

Author: jasdel

aws/config.go

@@ -32,20 +32,11 @@ const DefaultRetries = -1
 // You may modify this global structure to change all default configuration
 // in the SDK. Note that configuration options are copied by value, so any
 // modifications must happen before constructing a client.
-var DefaultConfig = &Config{
-	Credentials:             DefaultChainCredentials,
-	Endpoint:                "",
-	Region:                  os.Getenv("AWS_REGION"),
-	DisableSSL:              false,
-	HTTPClient:              http.DefaultClient,
-	LogHTTPBody:             false,
-	LogLevel:                0,
-	Logger:                  os.Stdout,
-	MaxRetries:              DefaultRetries,
-	DisableParamValidation:  false,
-	DisableComputeChecksums: false,
-	S3ForcePathStyle:        false,
-}
+var DefaultConfig = NewConfig().
+	WithCredentials(DefaultChainCredentials).
+	WithRegion(os.Getenv("AWS_REGION")).
+	WithHTTPClient(http.DefaultClient).
+	WithMaxRetries(DefaultRetries)
 
 // A Config provides service configuration for service clients. By default,
 // all clients will use the {DefaultConfig} structure.
@@ -60,7 +51,7 @@ type Config struct {
 	//
 	// @note You must still provide a `Region` value when specifying an
 	//   endpoint for a client.
-	Endpoint string
+	Endpoint *string
 
 	// The region to send requests to. This parameter is required and must
 	// be configured globally or on a per-client basis unless otherwise
@@ -69,11 +60,11 @@ type Config struct {
 	//
 	// @see http://docs.aws.amazon.com/general/latest/gr/rande.html
 	//   AWS Regions and Endpoints
-	Region string
+	Region *string
 
 	// Set this to `true` to disable SSL when sending requests. Defaults
 	// to `false`.
-	DisableSSL bool
+	DisableSSL *bool
 
 	// The HTTP client to use when sending requests. Defaults to
 	// `http.DefaultClient`.
@@ -84,12 +75,12 @@ type Config struct {
 	//
 	// @note `LogLevel` must be set to a non-zero value in order to activate
 	//   body logging.
-	LogHTTPBody bool
+	LogHTTPBody *bool
 
 	// An integer value representing the logging level. The default log level
 	// is zero (0), which represents no logging. Set to a non-zero value to
 	// perform logging.
-	LogLevel uint
+	LogLevel *int
 
 	// The logger writer interface to write logging messages to. Defaults to
 	// standard out.
@@ -98,15 +89,15 @@ type Config struct {
 	// The maximum number of times that a request will be retried for failures.
 	// Defaults to -1, which defers the max retry setting to the service specific
 	// configuration.
-	MaxRetries int
+	MaxRetries *int
 
 	// Disables semantic parameter validation, which validates input for missing
 	// required fields and/or other semantic request input errors.
-	DisableParamValidation bool
+	DisableParamValidation *bool
 
 	// Disables the computation of request and response checksums, e.g.,
 	// CRC32 checksums in Amazon DynamoDB.
-	DisableComputeChecksums bool
+	DisableComputeChecksums *bool
 
 	// Set this to `true` to force the request to use path-style addressing,
 	// i.e., `http://s3.amazonaws.com/BUCKET/KEY`. By default, the S3 client will
@@ -116,111 +107,166 @@ type Config struct {
 	// @note This configuration option is specific to the Amazon S3 service.
 	// @see http://docs.aws.amazon.com/AmazonS3/latest/dev/VirtualHosting.html
 	//   Amazon S3: Virtual Hosting of Buckets
-	S3ForcePathStyle bool
+	S3ForcePathStyle *bool
 }
 
-// Copy will return a shallow copy of the Config object.
-func (c Config) Copy() Config {
-	dst := Config{}
-	dst.Credentials = c.Credentials
-	dst.Endpoint = c.Endpoint
-	dst.Region = c.Region
-	dst.DisableSSL = c.DisableSSL
-	dst.HTTPClient = c.HTTPClient
-	dst.LogHTTPBody = c.LogHTTPBody
-	dst.LogLevel = c.LogLevel
-	dst.Logger = c.Logger
-	dst.MaxRetries = c.MaxRetries
-	dst.DisableParamValidation = c.DisableParamValidation
-	dst.DisableComputeChecksums = c.DisableComputeChecksums
-	dst.S3ForcePathStyle = c.S3ForcePathStyle
-
-	return dst
-}
-
-// Merge merges the newcfg attribute values into this Config. Each attribute
-// will be merged into this config if the newcfg attribute's value is non-zero.
-// Due to this, newcfg attributes with zero values cannot be merged in. For
-// example bool attributes cannot be cleared using Merge, and must be explicitly
-// set on the Config structure.
-func (c Config) Merge(newcfg *Config) *Config {
-	if newcfg == nil {
+// NewConfig returns a new Config pointer that can be chained with builder methods to
+// set multiple configuration values inline without using pointers.
+//
+//     svc := s3.New(aws.NewConfig().WithRegion("us-west-2").WithMaxRetries(10))
+//
+func NewConfig() *Config {
+	return &Config{}
+}
+
+// WithCredentials sets a config Credentials value returning a Config pointer
+// for chaining.
+func (c *Config) WithCredentials(creds *credentials.Credentials) *Config {
+	c.Credentials = creds
+	return c
+}
+
+// WithEndpoint sets a config Endpoint value returning a Config pointer for
+// chaining.
+func (c *Config) WithEndpoint(endpoint string) *Config {
+	c.Endpoint = &endpoint
+	return c
+}
+
+// WithRegion sets a config Region value returning a Config pointer for
+// chaining.
+func (c *Config) WithRegion(region string) *Config {
+	c.Region = &region
+	return c
+}
+
+// WithDisableSSL sets a config DisableSSL value returning a Config pointer
+// for chaining.
+func (c *Config) WithDisableSSL(disable bool) *Config {
+	c.DisableSSL = &disable
+	return c
+}
+
+// WithHTTPClient sets a config HTTPClient value returning a Config pointer
+// for chaining.
+func (c *Config) WithHTTPClient(client *http.Client) *Config {
+	c.HTTPClient = client
+	return c
+}
+
+// WithMaxRetries sets a config MaxRetries value returning a Config pointer
+// for chaining.
+func (c *Config) WithMaxRetries(max int) *Config {
+	c.MaxRetries = &max
+	return c
+}
+
+// WithDisableParamValidation sets a config DisableParamValidation value
+// returning a Config pointer for chaining.
+func (c *Config) WithDisableParamValidation(disable bool) *Config {
+	c.DisableParamValidation = &disable
+	return c
+}
+
+// WithDisableComputeChecksums sets a config DisableComputeChecksums value
+// returning a Config pointer for chaining.
+func (c *Config) WithDisableComputeChecksums(disable bool) *Config {
+	c.DisableComputeChecksums = &disable
+	return c
+}
+
+// WithLogHTTPBody sets a config LogHTTPBody value returning a Config pointer
+// for chaining.
+func (c *Config) WithLogHTTPBody(logHTTPBody bool) *Config {
+	c.LogHTTPBody = &logHTTPBody
+	return c
+}
+
+// WithLogLevel sets a config LogLevel value returning a Config pointer for
+// chaining.
+func (c *Config) WithLogLevel(level int) *Config {
+	c.LogLevel = &level
+	return c
+}
+
+// WithLogger sets a config Logger value returning a Config pointer for
+// chaining.
+func (c *Config) WithLogger(logger io.Writer) *Config {
+	c.Logger = logger
+	return c
+}
+
+// WithS3ForcePathStyle sets a config S3ForcePathStyle value returning a Config
+// pointer for chaining.
+func (c *Config) WithS3ForcePathStyle(force bool) *Config {
+	c.S3ForcePathStyle = &force
+	return c
+}
+
+// Merge returns a new Config with the other Config's attribute values merged into
+// this Config. If the other Config's attribute is nil it will not be merged into
+// the new Config to be returned.
+func (c Config) Merge(other *Config) *Config {
+	if other == nil {
 		return &c
 	}
 
-	cfg := Config{}
+	dst := c
 
-	if newcfg.Credentials != nil {
-		cfg.Credentials = newcfg.Credentials
-	} else {
-		cfg.Credentials = c.Credentials
+	if other.Credentials != nil {
+		dst.Credentials = other.Credentials
 	}
 
-	if newcfg.Endpoint != "" {
-		cfg.Endpoint = newcfg.Endpoint
-	} else {
-		cfg.Endpoint = c.Endpoint
+	if other.Endpoint != nil {
+		dst.Endpoint = other.Endpoint
 	}
 
-	if newcfg.Region != "" {
-		cfg.Region = newcfg.Region
-	} else {
-		cfg.Region = c.Region
+	if other.Region != nil {
+		dst.Region = other.Region
 	}
 
-	if newcfg.DisableSSL {
-		cfg.DisableSSL = newcfg.DisableSSL
-	} else {
-		cfg.DisableSSL = c.DisableSSL
+	if other.DisableSSL != nil {
+		dst.DisableSSL = other.DisableSSL
 	}
 
-	if newcfg.HTTPClient != nil {
-		cfg.HTTPClient = newcfg.HTTPClient
-	} else {
-		cfg.HTTPClient = c.HTTPClient
+
+	if other.HTTPClient != nil {
+		dst.HTTPClient = other.HTTPClient
 	}
 
-	if newcfg.LogHTTPBody {
-		cfg.LogHTTPBody = newcfg.LogHTTPBody
-	} else {
-		cfg.LogHTTPBody = c.LogHTTPBody
+	if other.LogHTTPBody != nil {
+		dst.LogHTTPBody = other.LogHTTPBody
 	}
 
-	if newcfg.LogLevel != 0 {
-		cfg.LogLevel = newcfg.LogLevel
-	} else {
-		cfg.LogLevel = c.LogLevel
+	if other.LogLevel != nil {
+		dst.LogLevel = other.LogLevel
 	}
 
-	if newcfg.Logger != nil {
-		cfg.Logger = newcfg.Logger
-	} else {
-		cfg.Logger = c.Logger
+	if other.Logger != nil {
+		dst.Logger = other.Logger
 	}
 
-	if newcfg.MaxRetries != DefaultRetries {
-		cfg.MaxRetries = newcfg.MaxRetries
-	} else {
-		cfg.MaxRetries = c.MaxRetries
+	if other.MaxRetries != nil {
+		dst.MaxRetries = other.MaxRetries
 	}
 
-	if newcfg.DisableParamValidation {
-		cfg.DisableParamValidation = newcfg.DisableParamValidation
-	} else {
-		cfg.DisableParamValidation = c.DisableParamValidation
+	if other.DisableParamValidation != nil {
+		dst.DisableParamValidation = other.DisableParamValidation
 	}
 
-	if newcfg.DisableComputeChecksums {
-		cfg.DisableComputeChecksums = newcfg.DisableComputeChecksums
-	} else {
-		cfg.DisableComputeChecksums = c.DisableComputeChecksums
+	if other.DisableComputeChecksums != nil {
+		dst.DisableComputeChecksums = other.DisableComputeChecksums
 	}
 
-	if newcfg.S3ForcePathStyle {
-		cfg.S3ForcePathStyle = newcfg.S3ForcePathStyle
-	} else {
-		cfg.S3ForcePathStyle = c.S3ForcePathStyle
+	if other.S3ForcePathStyle != nil {
+		dst.S3ForcePathStyle = other.S3ForcePathStyle
 	}
 
-	return &cfg
+	return &dst
+}
+
+// Copy will return a shallow copy of the Config object.
+func (c Config) Copy() *Config {
+	dst := c
+	return &dst
 }