Use `struct{}` as underlying type for context keys (#6965)

This is an alternative to #6963.

Signed-off-by: beorn7 <beorn@grafana.com>

promql/engine.go

@@ -152,9 +152,7 @@ type query struct {
 	ng *Engine
 }
 
-type queryCtx int
-
-var queryOrigin queryCtx
+type queryOrigin struct{}
 
 // Statement implements the Query interface.
 func (q *query) Statement() parser.Statement {
@@ -441,7 +439,7 @@ func (ng *Engine) exec(ctx context.Context, q *query) (v parser.Value, w storage
 				f = append(f, "error", err)
 			}
 			f = append(f, "stats", stats.NewQueryStats(q.Stats()))
-			if origin := ctx.Value(queryOrigin); origin != nil {
+			if origin := ctx.Value(queryOrigin{}); origin != nil {
 				for k, v := range origin.(map[string]interface{}) {
 					f = append(f, k, v)
 				}
@@ -2097,7 +2095,7 @@ func shouldDropMetricName(op parser.ItemType) bool {
 
 // NewOriginContext returns a new context with data about the origin attached.
 func NewOriginContext(ctx context.Context, data map[string]interface{}) context.Context {
-	return context.WithValue(ctx, queryOrigin, data)
+	return context.WithValue(ctx, queryOrigin{}, data)
 }
 
 func formatDate(t time.Time) string {

util/httputil/context.go

@@ -21,14 +21,12 @@ import (
 	"github.com/prometheus/prometheus/promql"
 )
 
-type ctxParam int
-
-var pathParam ctxParam
+type pathParam struct{}
 
 // ContextWithPath returns a new context with the given path to be used later
 // when logging the query.
 func ContextWithPath(ctx context.Context, path string) context.Context {
-	return context.WithValue(ctx, pathParam, path)
+	return context.WithValue(ctx, pathParam{}, path)
 }
 
 // ContextFromRequest returns a new context with identifiers of
@@ -40,7 +38,7 @@ func ContextFromRequest(ctx context.Context, r *http.Request) context.Context {
 		ip, _, _ = net.SplitHostPort(r.RemoteAddr)
 	}
 	var path string
-	if v := ctx.Value(pathParam); v != nil {
+	if v := ctx.Value(pathParam{}); v != nil {
 		path = v.(string)
 	}
 	return promql.NewOriginContext(ctx, map[string]interface{}{