context: remove definition of Context

Back in the before time, the best practices surrounding usage of Context
weren't quite worked out. We defined our own type to make usage easier.
As this packaged was used elsewhere, it make it more and more
challenging to integrate with the forked `Context` type. Now that it is
available in the standard library, we can just use that one directly.

To make usage more consistent, we now use `dcontext` when referring to
the distribution context package.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

context/http.go

@@ -1,6 +1,7 @@
 package context
 
 import (
+	"context"
 	"errors"
 	"net"
 	"net/http"
@@ -68,7 +69,7 @@ func RemoteIP(r *http.Request) string {
 // is available at "http.request". Other common attributes are available under
 // the prefix "http.request.". If a request is already present on the context,
 // this method will panic.
-func WithRequest(ctx Context, r *http.Request) Context {
+func WithRequest(ctx context.Context, r *http.Request) context.Context {
 	if ctx.Value("http.request") != nil {
 		// NOTE(stevvooe): This needs to be considered a programming error. It
 		// is unlikely that we'd want to have more than one request in
@@ -87,7 +88,7 @@ func WithRequest(ctx Context, r *http.Request) Context {
 // GetRequest returns the http request in the given context. Returns
 // ErrNoRequestContext if the context does not have an http request associated
 // with it.
-func GetRequest(ctx Context) (*http.Request, error) {
+func GetRequest(ctx context.Context) (*http.Request, error) {
 	if r, ok := ctx.Value("http.request").(*http.Request); r != nil && ok {
 		return r, nil
 	}
@@ -96,13 +97,13 @@ func GetRequest(ctx Context) (*http.Request, error) {
 
 // GetRequestID attempts to resolve the current request id, if possible. An
 // error is return if it is not available on the context.
-func GetRequestID(ctx Context) string {
+func GetRequestID(ctx context.Context) string {
 	return GetStringValue(ctx, "http.request.id")
 }
 
 // WithResponseWriter returns a new context and response writer that makes
 // interesting response statistics available within the context.
-func WithResponseWriter(ctx Context, w http.ResponseWriter) (Context, http.ResponseWriter) {
+func WithResponseWriter(ctx context.Context, w http.ResponseWriter) (context.Context, http.ResponseWriter) {
 	if closeNotifier, ok := w.(http.CloseNotifier); ok {
 		irwCN := &instrumentedResponseWriterCN{
 			instrumentedResponseWriter: instrumentedResponseWriter{
@@ -125,7 +126,7 @@ func WithResponseWriter(ctx Context, w http.ResponseWriter) (Context, http.Respo
 // GetResponseWriter returns the http.ResponseWriter from the provided
 // context. If not present, ErrNoResponseWriterContext is returned. The
 // returned instance provides instrumentation in the context.
-func GetResponseWriter(ctx Context) (http.ResponseWriter, error) {
+func GetResponseWriter(ctx context.Context) (http.ResponseWriter, error) {
 	v := ctx.Value("http.response")
 
 	rw, ok := v.(http.ResponseWriter)
@@ -145,7 +146,7 @@ var getVarsFromRequest = mux.Vars
 // example, if looking for the variable "name", it can be accessed as
 // "vars.name". Implementations that are accessing values need not know that
 // the underlying context is implemented with gorilla/mux vars.
-func WithVars(ctx Context, r *http.Request) Context {
+func WithVars(ctx context.Context, r *http.Request) context.Context {
 	return &muxVarsContext{
 		Context: ctx,
 		vars:    getVarsFromRequest(r),
@@ -155,7 +156,7 @@ func WithVars(ctx Context, r *http.Request) Context {
 // GetRequestLogger returns a logger that contains fields from the request in
 // the current context. If the request is not available in the context, no
 // fields will display. Request loggers can safely be pushed onto the context.
-func GetRequestLogger(ctx Context) Logger {
+func GetRequestLogger(ctx context.Context) Logger {
 	return GetLogger(ctx,
 		"http.request.id",
 		"http.request.method",
@@ -171,7 +172,7 @@ func GetRequestLogger(ctx Context) Logger {
 // Because the values are read at call time, pushing a logger returned from
 // this function on the context will lead to missing or invalid data. Only
 // call this at the end of a request, after the response has been written.
-func GetResponseLogger(ctx Context) Logger {
+func GetResponseLogger(ctx context.Context) Logger {
 	l := getLogrusLogger(ctx,
 		"http.response.written",
 		"http.response.status",
@@ -188,7 +189,7 @@ func GetResponseLogger(ctx Context) Logger {
 
 // httpRequestContext makes information about a request available to context.
 type httpRequestContext struct {
-	Context
+	context.Context
 
 	startedAt time.Time
 	id        string
@@ -247,7 +248,7 @@ fallback:
 }
 
 type muxVarsContext struct {
-	Context
+	context.Context
 	vars map[string]string
 }
 
@@ -282,7 +283,7 @@ type instrumentedResponseWriterCN struct {
 // implemented by the parent ResponseWriter.
 type instrumentedResponseWriter struct {
 	http.ResponseWriter
-	Context
+	context.Context
 
 	mu      sync.Mutex
 	status  int