Stop using real headers for side channels (#1367)

I thought it was clever to use If-None-Match because it was pretty close
to what I wanted, but the way we're wrapping transports means that this
actually breaks if the Etag matches. Whoops!

Just change this to an invalid header so that this stops breaking, we
can fix it better later by making the cache not look like a transport.

Signed-off-by: Jon Johnson <jon.johnson@chainguard.dev>

pkg/apk/apk/cache.go

@@ -209,7 +209,7 @@ func (t *cacheTransport) get(ctx context.Context, request *http.Request, cacheFi
 }
 
 func (t *cacheTransport) fetchAndCache(ctx context.Context, request *http.Request, cacheFile string) (*http.Response, error) {
-	initialEtag := request.Header.Get("If-None-Match")
+	initialEtag := request.Header.Get("I-Cant-Believe-Its-Not-If-None-Match")
 	if initialEtag == "" {
 		resp, err := t.head(request, cacheFile)
 		if err != nil {
@@ -232,7 +232,7 @@ func (t *cacheTransport) fetchAndCache(ctx context.Context, request *http.Reques
 	// Since we already send a HEAD request to get that etag, we want to avoid sending a redundant HEAD request above if we can.
 	// However, we don't actually want to pass along this header because it's using the "parsed" version of the Etag from
 	// etagFromResponse and doesn't actually attempt to follow HTTP semantics, so we remove it here to avoid any confusion.
-	request.Header.Del("If-None-Match")
+	request.Header.Del("I-Cant-Believe-Its-Not-If-None-Match")
 
 	etagFile, err := t.get(ctx, request, cacheFile, initialEtag)
 	if err != nil {

pkg/apk/apk/index.go

@@ -289,7 +289,7 @@ func fetchRepositoryIndex(ctx context.Context, u string, etag string, opts *inde
 		// also really wants to do a HEAD request in order to do etag-based caching itself. To avoid the double HEAD,
 		// I'm stuffing the etag into the If-None-Match header, which isn't exactly correct semantics but it rhymes.
 		// The alternative is to rewrite everything, which I don't have time to do right now, so it is what it is.
-		req.Header.Set("If-None-Match", etag)
+		req.Header.Set("I-Cant-Believe-Its-Not-If-None-Match", etag)
 	}
 
 	if opts.auth == nil {