fix: handle _redirects for If-None-Match headers

CHANGELOG.md

@@ -49,6 +49,7 @@ The following emojis are used to highlight certain changes:
 ### Fixed
 
 - Removed mentions of unused ARC algorithm ([#336](https://github.com/ipfs/boxo/issues/366#issuecomment-1597253540))
+- Handle `_redirects` file when `If-None-Match` header is present ([#412](https://github.com/ipfs/boxo/pull/412))
 
 ### Security
 

gateway/gateway_test.go

@@ -523,6 +523,61 @@ func TestRedirects(t *testing.T) {
 		res = mustDoWithoutRedirect(t, req)
 		require.Equal(t, http.StatusNotFound, res.StatusCode)
 	})
+
+	t.Run("_redirects file with If-None-Match header", func(t *testing.T) {
+		t.Parallel()
+
+		backend, root := newMockBackend(t, "redirects-spa.car")
+		backend.namesys["/ipns/example.com"] = path.FromCid(root)
+
+		ts := newTestServerWithConfig(t, backend, Config{
+			Headers:   map[string][]string{},
+			NoDNSLink: false,
+			PublicGateways: map[string]*PublicGateway{
+				"example.com": {
+					UseSubdomains:         true,
+					DeserializedResponses: true,
+				},
+			},
+			DeserializedResponses: true,
+		})
+
+		missingPageURL := ts.URL + "/missing-page"
+
+		do := func(method string) {
+			// Make initial request to non-existing page that should return the contents
+			// of index.html as per the _redirects file.
+			req := mustNewRequest(t, method, missingPageURL, nil)
+			req.Header.Add("Accept", "text/html")
+			req.Host = "example.com"
+
+			res := mustDoWithoutRedirect(t, req)
+			defer res.Body.Close()
+
+			// Check statuses and body.
+			require.Equal(t, http.StatusOK, res.StatusCode)
+			body, err := io.ReadAll(res.Body)
+			require.NoError(t, err)
+			require.Equal(t, "hello world\n", string(body))
+
+			// Check Etag.
+			etag := res.Header.Get("Etag")
+			require.NotEmpty(t, etag)
+
+			// Repeat request with Etag as If-None-Match value. Expect 304 Not Modified.
+			req = mustNewRequest(t, method, missingPageURL, nil)
+			req.Header.Add("Accept", "text/html")
+			req.Host = "example.com"
+			req.Header.Add("If-None-Match", etag)
+
+			res = mustDoWithoutRedirect(t, req)
+			defer res.Body.Close()
+			require.Equal(t, http.StatusNotModified, res.StatusCode)
+		}
+
+		do(http.MethodGet)
+		do(http.MethodHead)
+	})
 }
 
 func TestDeserializedResponses(t *testing.T) {

gateway/handler.go

@@ -703,9 +703,19 @@ func (i *handler) handleIfNoneMatch(w http.ResponseWriter, r *http.Request, rq *
 	if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
 		pathMetadata, err := i.backend.ResolvePath(r.Context(), rq.immutablePath)
 		if err != nil {
-			err = fmt.Errorf("failed to resolve %s: %w", debugStr(rq.contentPath.String()), err)
-			i.webError(w, r, err, http.StatusInternalServerError)
-			return true
+			var forwardedPath ImmutablePath
+			var continueProcessing bool
+			if isWebRequest(rq.responseFormat) {
+				forwardedPath, continueProcessing = i.handleWebRequestErrors(w, r, rq.mostlyResolvedPath(), rq.immutablePath, rq.contentPath, err, rq.logger)
+				if continueProcessing {
+					pathMetadata, err = i.backend.ResolvePath(r.Context(), forwardedPath)
+				}
+			}
+			if !continueProcessing || err != nil {
+				err = fmt.Errorf("failed to resolve %s: %w", debugStr(rq.contentPath.String()), err)
+				i.webError(w, r, err, http.StatusInternalServerError)
+				return true
+			}
 		}
 
 		pathCid := pathMetadata.LastSegment.Cid()

gateway/handler_defaults.go

@@ -33,8 +33,23 @@ func (i *handler) serveDefaults(ctx context.Context, w http.ResponseWriter, r *h
 	case http.MethodHead:
 		var data files.Node
 		pathMetadata, data, err = i.backend.Head(ctx, rq.mostlyResolvedPath())
-		if !i.handleRequestErrors(w, r, rq.contentPath, err) {
-			return false
+		if err != nil {
+			if isWebRequest(rq.responseFormat) {
+				forwardedPath, continueProcessing := i.handleWebRequestErrors(w, r, rq.mostlyResolvedPath(), rq.immutablePath, rq.contentPath, err, rq.logger)
+				if !continueProcessing {
+					return false
+				}
+				pathMetadata, data, err = i.backend.Head(ctx, forwardedPath)
+				if err != nil {
+					err = fmt.Errorf("failed to resolve %s: %w", debugStr(rq.contentPath.String()), err)
+					i.webError(w, r, err, http.StatusInternalServerError)
+					return false
+				}
+			} else {
+				if !i.handleRequestErrors(w, r, rq.contentPath, err) {
+					return false
+				}
+			}
 		}
 		defer data.Close()
 		if _, ok := data.(files.Directory); ok {