go.blog/context: don't use the address of an empty struct as a context

key, as this is not guaranteed to be unique.

Fixes golang/go#8443.

LGTM=campoy
R=adg, campoy
CC=dsymonds, golang-codereviews
https://golang.org/cl/121890043

3 files changed, 21 insertions, 13 deletions

diff --git a/content/context.article b/content/context.article
index 73de06d..054a911 100644
--- a/content/context.article
+++ b/content/context.article
@@ -40,9 +40,9 @@ The `Err` method returns an error indicating why the `Context` was canceled.
 The [[/pipelines][Pipelines and Cancelation]] article discusses the `Done`
 channel idiom in more detail.
 
-A `Context` does _not_ have a `Cancel` method for same reason the `Done` channel
-is receive-only: the function receiving a cancelation signal is usually not the
-one that sends the signal.
+A `Context` does _not_ have a `Cancel` method for the same reason the `Done`
+channel is receive-only: the function receiving a cancelation signal is usually
+not the one that sends the signal.
 In particular, when a parent operation starts goroutines for sub-operations,
 those sub-operations should not be able to cancel the parent.
 Instead, the `WithCancel` function (described below) provides a way to cancel a
@@ -140,10 +140,10 @@ multiple goroutines.
 Packages like `userip` hide the details of this mapping and provide
 strongly-typed access to a specific `Context` value.
 
-To avoid key collisions, `userip` defines an unexported variable `key` and uses
-its address as the context key:
+To avoid key collisions, `userip` defines an unexported type `key` and uses
+a value of this type as the context key:
 
-.code context/userip/userip.go /var key/
+.code context/userip/userip.go /The key type/,/const userIPKey/
 
 `FromRequest` extracts a `userIP` value from an `http.Request`:
 
diff --git a/content/context/gorilla/gorilla.go b/content/context/gorilla/gorilla.go
index 08267c3..c9b6474 100644
--- a/content/context/gorilla/gorilla.go
+++ b/content/context/gorilla/gorilla.go
@@ -22,12 +22,14 @@ type wrapper struct {
 	req *http.Request
 }
 
-var reqKey struct{}
+type key int
+
+const reqKey key = 0
 
 // Value returns Gorilla's context package's value for this Context's request
 // and key. It delegates to the parent Context if there is no such value.
 func (ctx *wrapper) Value(key interface{}) interface{} {
-	if key == &reqKey {
+	if key == reqKey {
 		return ctx.req
 	}
 	if val, ok := gcontext.GetOk(ctx.req, key); ok {
@@ -42,6 +44,6 @@ func HTTPRequest(ctx context.Context) (*http.Request, bool) {
 	// We cannot use ctx.(*wrapper).req to get the request because ctx may
 	// be a Context derived from a *wrapper. Instead, we use Value to
 	// access the request if it is anywhere up the Context tree.
-	req, ok := ctx.Value(&reqKey).(*http.Request)
+	req, ok := ctx.Value(reqKey).(*http.Request)
 	return req, ok
 }
diff --git a/content/context/userip/userip.go b/content/context/userip/userip.go
index 3001ade..ba3851e 100644
--- a/content/context/userip/userip.go
+++ b/content/context/userip/userip.go
@@ -21,18 +21,24 @@ func FromRequest(req *http.Request) (net.IP, error) {
 	return userIP, nil
 }
 
-// The address &key is the context key.
-var key struct{}
+// The key type is unexported to prevent collisions with context keys defined in
+// other packages.
+type key int
+
+// userIPkey is the context key for the user IP address.  Its value of zero is
+// arbitrary.  If this package defined other context keys, they would have
+// different integer values.
+const userIPKey key = 0
 
 // NewContext returns a new Context carrying userIP.
 func NewContext(ctx context.Context, userIP net.IP) context.Context {
-	return context.WithValue(ctx, &key, userIP)
+	return context.WithValue(ctx, userIPKey, userIP)
 }
 
 // FromContext extracts the user IP address from ctx, if present.
 func FromContext(ctx context.Context) (net.IP, bool) {
 	// ctx.Value returns nil if ctx has no value for the key;
 	// the net.IP type assertion returns ok=false for nil.
-	userIP, ok := ctx.Value(&key).(net.IP)
+	userIP, ok := ctx.Value(userIPKey).(net.IP)
 	return userIP, ok
 }