switch to storing sessions in the context

This patch allows associating a "session key" with a context by calling
`NewSession(ctx)`. When the session is used in a request, the exchange can call
`GetSession(ctx)` to make the request in the appropriate session, or create a
new session with the given session key.

Implements ipfs/kubo#7198

go.mod

@@ -4,3 +4,5 @@ require (
 	github.com/ipfs/go-block-format v0.0.2
 	github.com/ipfs/go-cid v0.0.5
 )
+
+go 1.13

interface.go

@@ -35,3 +35,49 @@ type SessionExchange interface {
 	Interface
 	NewSession(context.Context) Fetcher
 }
+
+// SessionKey is an opaque type uniquely identifying a session. Each SessionKey
+// should be allocated with new(SessionKey).
+type SessionKey struct {
+	// Needs to be at least 1 byte wide to guarantee a unique memory address
+	// We could consider putting, e.g., a description/name here in the
+	// future.
+	_ byte
+}
+
+type sessionContextKey struct{}
+type sessionContextValue struct {
+	sesKey *SessionKey
+	sesCtx context.Context
+}
+
+// NewSession registers a new session with the context. The session will be
+// closed when the passed-in context is canceled.
+//
+// If there's already a session associated with the context, the existing
+// session will be used.
+//
+// This function does not initialize any state, it just creates a new
+// *SessionKey and associates it with the context.
+func NewSession(ctx context.Context) context.Context {
+	if _, ok := ctx.Value(sessionContextKey{}).(*sessionContextValue); ok {
+		// Leave the session alone.
+		return ctx
+	}
+	return context.WithValue(ctx, sessionContextKey{}, &sessionContextValue{
+		sesKey: new(SessionKey),
+		sesCtx: ctx, // save this so we can construct the session with the _session_ context.
+	})
+}
+
+// GetSession loads the fetcher from the context. The returned context is the
+// context associated with the session itself.
+//
+// If there is no session associated with this context, this function returns
+// a new session key and the passed-in context.
+func GetSession(ctx context.Context) (*SessionKey, context.Context) {
+	if s, ok := ctx.Value(sessionContextKey{}).(*sessionContextValue); ok {
+		return s.sesKey, s.sesCtx
+	}
+	return new(SessionKey), ctx
+}