Some Possible Options for fixing aws.Config.Merge

[jasdel]

I just pushed two branches for some possible ways we could resolve the aws.Config.Merge problem of not being able to clear fields which were set. For example DisableSSL can never be set to false with Merge() e.g. <service>.New(&aws.Config{...}). This issue applies to all non pointer types in aws.Config. Issue #157

Both of the two solutions I am proposing are breaking changes, because they make modifications to the Config struct, and the aws.String() helper functions. The goal of these changes are to ensure configurations can be merged correctly without oddities such as not being able to set a field to its zero value because the zero value is used to determine if the incoming configuration parameter is set.

Both proposed solutions renamed aws.String to aws.StringPtr to clarify what aws.String function does. This applied to the other pointer helper functions as well. An IntPtr was added for *int. In addition helper functions were added for each type to return the value that a pointer points to or the type's zero value if the pointer is nil. This saves some of the hassle of checking if a field is nil before dereferencing it in circumstances where nil should be treated as zero. (addresses #124)

We are looking for feedback from the community on what you think of these two methods. Both methods have their strengths and weaknesses, but we would like to fix this pain point.

The best method to view the code of these changes is via the branch. Viewing the commit on Github probably will not be a pleasant experience due to the number of changes.

ConfigMergeViaPointer: Config use pointers instead of values

Switches all Config field primitive types to be pointers. This change is the simplest idea to implement and understand, but will create pain points when using the fields because of the need to nil check before each use. Some of the pain points can be mitigated by using the PtrValue helper method to access the pointer's value or zero value if nil.

Pros:

• Config.Merge works as expected.
• Fields can used with vanilla Go syntax without extra levels of indirection.

Cons:

• Need to deal with the possibility fields will be nil when accessing.
• Need to use extra syntax to set field value.

Example:

    // Code prior to update:
    svc := s3.New(aws.Config{
        Region:           "us-west-2",
        MaxRetries:       2,
        S3ForcePathStyle: true,
    }
    // Prior code read Config.Region value
    if svc.Config.Region == "us-west-2" {...}

    // After Updating to this change:
    svc := s3.New(aws.Config{
        Region:           aws.StringPtr("us-west-2"),
        MaxRetries:       aws.IntPtr(2),
        S3ForcePathStyle: aws.BoolPtr(true),
    }
    // After this change reading Config.Region value
    if aws.StringPtrValue(svc.Config.Region) == "us-west-2" {...}

ConfigMergeViaStruct: Config use custom types instead of primitives

Switches all Config field primitive types to custom wrapper types. The custom types wrap the value and the state if the field was set or not. Constructor helper methods were created for each type wrapper to make it simpler to set field values without multiple lines of code.

Pros:

• Config.Merge works as expected.
• Fields are always values so no nil pointers to deal with.
• Fields can be checked if they are set separately from their value.
• Fields default to zero value for type without the need for additional helper methods.
• It may be possible for the struct wrappers to also be used to address the pain point of pointer primitives in service API input/output structs. (addresses part of StringValue used instead of String type #114)

Cons:

• Struct wrapping the primitive fields
• Extra method to get primitive value .Get()
• Need to use extra syntax to set field value.

Example:

    // Code Prior to this update
    svc := s3.New(&aws.Config{
        Region:           "us-west-2",
        MaxRetries:       2,
        S3ForcePathStyle: true,
    })
    // Prior code read Config.Region value
    if svc.Config.Region == "us-west-2" {...}

    // After Updating to this change:
    svc := s3.New(&aws.Config{
        Region:           aws.String("us-west-2"),
        MaxRetries:       aws.Int(2),
        S3ForcePathStyle: aws.Bool(true),
    })
    // After this change reading Config.Region value
    if svc.Config.Region.Get() == "us-west-2" {...}