http: http-utils added after extraction

Found common http utils that were being multiply duplicated across
many libraries and since am moving things in basecoin/unstable to
add for more functionality, it's better to put them in one
place.

Utilities and tests added:
- [X] FparseJSON
- [X] FparseAndValidateJSON
- [X] ParseRequestJSON
- [X] ParseAndValidateRequestJSON
- [X] WriteCode
- [X] WriteError
- [X] WriteSuccess
- [X] ErrorResponse

During review from @ethanfrey, made updates:
* Removed tt.want since it was a distraction/artifact that made
the reviewer think the tests weren't testing for both failed
and passed results.
* Added ErrorWithCode as WithCode is a common options pattern
in Go that could cause confusion:
  ErrorWithCode(error, int) ErrorResponse
* Using json.NewDecoder(io.Reader) error instead of
ioutil.ReadAll(io.Reader) to slurp all the bytes.
* Added more test scenarios to achieve 100% coverage of http.go

common/http.go

@@ -0,0 +1,153 @@
+package common
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"gopkg.in/go-playground/validator.v9"
+
+	"github.com/pkg/errors"
+)
+
+type ErrorResponse struct {
+	Success bool `json:"success,omitempty"`
+
+	// Err is the error message if Success is false
+	Err string `json:"error,omitempty"`
+
+	// Code is set if Success is false
+	Code int `json:"code,omitempty"`
+}
+
+// ErrorWithCode makes an ErrorResponse with the
+// provided err's Error() content, and status code.
+// It panics if err is nil.
+func ErrorWithCode(err error, code int) *ErrorResponse {
+	return &ErrorResponse{
+		Err:  err.Error(),
+		Code: code,
+	}
+}
+
+// Ensure that ErrorResponse implements error
+var _ error = (*ErrorResponse)(nil)
+
+func (er *ErrorResponse) Error() string {
+	return er.Err
+}
+
+// Ensure that ErrorResponse implements httpCoder
+var _ httpCoder = (*ErrorResponse)(nil)
+
+func (er *ErrorResponse) HTTPCode() int {
+	return er.Code
+}
+
+var errNilBody = errors.Errorf("expecting a non-nil body")
+
+// FparseJSON unmarshals into save, the body of the provided reader.
+// Since it uses json.Unmarshal, save must be of a pointer type
+// or compatible with json.Unmarshal.
+func FparseJSON(r io.Reader, save interface{}) error {
+	if r == nil {
+		return errors.Wrap(errNilBody, "Reader")
+	}
+
+	dec := json.NewDecoder(r)
+	if err := dec.Decode(save); err != nil {
+		return errors.Wrap(err, "Decode/Unmarshal")
+	}
+	return nil
+}
+
+// ParseRequestJSON unmarshals into save, the body of the
+// request. It closes the body of the request after parsing.
+// Since it uses json.Unmarshal, save must be of a pointer type
+// or compatible with json.Unmarshal.
+func ParseRequestJSON(r *http.Request, save interface{}) error {
+	if r == nil || r.Body == nil {
+		return errNilBody
+	}
+	defer r.Body.Close()
+
+	return FparseJSON(r.Body, save)
+}
+
+// ParseRequestAndValidateJSON unmarshals into save, the body of the
+// request and invokes a validator on the saved content. To ensure
+// validation, make sure to set tags "validate" on your struct as
+// per https://godoc.org/gopkg.in/go-playground/validator.v9.
+// It closes the body of the request after parsing.
+// Since it uses json.Unmarshal, save must be of a pointer type
+// or compatible with json.Unmarshal.
+func ParseRequestAndValidateJSON(r *http.Request, save interface{}) error {
+	if r == nil || r.Body == nil {
+		return errNilBody
+	}
+	defer r.Body.Close()
+
+	return FparseAndValidateJSON(r.Body, save)
+}
+
+// FparseAndValidateJSON like FparseJSON unmarshals into save,
+// the body of the provided reader. However, it invokes the validator
+// to check the set validators on your struct fields as per
+// per https://godoc.org/gopkg.in/go-playground/validator.v9.
+// Since it uses json.Unmarshal, save must be of a pointer type
+// or compatible with json.Unmarshal.
+func FparseAndValidateJSON(r io.Reader, save interface{}) error {
+	if err := FparseJSON(r, save); err != nil {
+		return err
+	}
+	return validate(save)
+}
+
+var theValidator = validator.New()
+
+func validate(obj interface{}) error {
+	return errors.Wrap(theValidator.Struct(obj), "Validate")
+}
+
+// WriteSuccess JSON marshals the content provided, to an HTTP
+// response, setting the provided status code and setting header
+// "Content-Type" to "application/json".
+func WriteSuccess(w http.ResponseWriter, data interface{}) {
+	WriteCode(w, data, 200)
+}
+
+// WriteCode JSON marshals content, to an HTTP response,
+// setting the provided status code, and setting header
+// "Content-Type" to "application/json". If JSON marshalling fails
+// with an error, WriteCode instead writes out the error invoking
+// WriteError.
+func WriteCode(w http.ResponseWriter, out interface{}, code int) {
+	blob, err := json.MarshalIndent(out, "", "  ")
+	if err != nil {
+		WriteError(w, err)
+	} else {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(code)
+		w.Write(blob)
+	}
+}
+
+type httpCoder interface {
+	HTTPCode() int
+}
+
+// WriteError is a convenience function to write out an
+// error to an http.ResponseWriter, to send out an error
+// that's structured as JSON i.e the form
+//    {"error": sss, "code": ddd}
+// If err implements the interface HTTPCode() int,
+// it will use that status code otherwise, it will
+// set code to be http.StatusBadRequest
+func WriteError(w http.ResponseWriter, err error) {
+	code := http.StatusBadRequest
+	if httpC, ok := err.(httpCoder); ok {
+		code = httpC.HTTPCode()
+	}
+
+	WriteCode(w, ErrorWithCode(err, code), code)
+}