Update "Accept" header parsing for list values

In Go's header parsing, the same header multiple times results in multiple entries in the `r.Header[...]` slice, but Go does no further parsing beyond that (and in https://golang.org/cl/4528086 it was determined that until/unless the stdlib itself needs it, Go will not do so).

The consequence here for parsing of `Accept:` headers is that we support the way Go outputs headers, but not all language HTTP libraries have a facility to output multiple headers instead of a single list header.

This change ensures that the following (valid) header blocks all parse to the same result for the purposes of what is being tested here:

```
Accept: a/b
Accept: b/c
Accept: d/e
```

```
Accept: a/b; q=0.5, b/c
Accept: d/e
```

```
Accept: a/b; q=0.1, b/c; q=0.2, d/e; q=0.8
```

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

docs/handlers/api_test.go

@@ -1586,8 +1586,8 @@ func testManifestAPIManifestList(t *testing.T, env *testEnv, args manifestArgs)
 	if err != nil {
 		t.Fatalf("Error constructing request: %s", err)
 	}
-	req.Header.Set("Accept", manifestlist.MediaTypeManifestList)
+	// multiple headers in mixed list format to ensure we parse correctly server-side
-	req.Header.Add("Accept", schema1.MediaTypeSignedManifest)
+	req.Header.Set("Accept", fmt.Sprintf(` %s ; q=0.8 , %s ; q=0.5 `, manifestlist.MediaTypeManifestList, schema1.MediaTypeSignedManifest))
 	req.Header.Add("Accept", schema2.MediaTypeManifest)
 	resp, err = http.DefaultClient.Do(req)
 	if err != nil {

docs/handlers/images.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/docker/distribution"
 	ctxu "github.com/docker/distribution/context"
@@ -98,8 +99,23 @@ func (imh *imageManifestHandler) GetImageManifest(w http.ResponseWriter, r *http
 
 	supportsSchema2 := false
 	supportsManifestList := false
-	if acceptHeaders, ok := r.Header["Accept"]; ok {
+	// this parsing of Accept headers is not quite as full-featured as godoc.org's parser, but we don't care about "q=" values
-		for _, mediaType := range acceptHeaders {
+	// https://github.com/golang/gddo/blob/e91d4165076d7474d20abda83f92d15c7ebc3e81/httputil/header/header.go#L165-L202
+	for _, acceptHeader := range r.Header["Accept"] {
+		// r.Header[...] is a slice in case the request contains the same header more than once
+		// if the header isn't set, we'll get the zero value, which "range" will handle gracefully
+
+		// we need to split each header value on "," to get the full list of "Accept" values (per RFC 2616)
+		// https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
+		for _, mediaType := range strings.Split(acceptHeader, ",") {
+			// remove "; q=..." if present
+			if i := strings.Index(mediaType, ";"); i >= 0 {
+				mediaType = mediaType[:i]
+			}
+
+			// it's common (but not required) for Accept values to be space separated ("a/b, c/d, e/f")
+			mediaType = strings.TrimSpace(mediaType)
+
 			if mediaType == schema2.MediaTypeManifest {
 				supportsSchema2 = true
 			}