drive: actually cache results on statcache

Updates #11967

Signed-off-by: Percy Wegmann <percy@tailscale.com>

drive/driveimpl/compositedav/propfind.go

@@ -24,33 +24,26 @@ func (h *Handler) handlePROPFIND(w http.ResponseWriter, r *http.Request) {
 		// Delegate to a Child.
 		depth := getDepth(r)
 
-		cached := h.StatCache.get(r.URL.Path, depth)
-		if cached != nil {
-			w.Header().Del("Content-Length")
-			w.WriteHeader(http.StatusMultiStatus)
-			w.Write(cached)
-			return
-		}
+		status, result := h.StatCache.getOr(r.URL.Path, depth, func() (int, []byte) {
+			// Use a buffering ResponseWriter so that we can manipulate the result.
+			// The only thing we use from the original ResponseWriter is Header().
+			bw := &bufferingResponseWriter{ResponseWriter: w}
 
-		// Use a buffering ResponseWriter so that we can manipulate the result.
-		// The only thing we use from the original ResponseWriter is Header().
-		bw := &bufferingResponseWriter{ResponseWriter: w}
+			mpl := h.maxPathLength(r)
+			h.delegate(mpl, pathComponents[mpl-1:], bw, r)
 
-		mpl := h.maxPathLength(r)
-		h.delegate(mpl, pathComponents[mpl-1:], bw, r)
+			// Fixup paths to add the requested path as a prefix.
+			pathPrefix := shared.Join(pathComponents[0:mpl]...)
+			b := hrefRegex.ReplaceAll(bw.buf.Bytes(), []byte(fmt.Sprintf("<D:href>%s/$1</D:href>", pathPrefix)))
 
-		// Fixup paths to add the requested path as a prefix.
-		pathPrefix := shared.Join(pathComponents[0:mpl]...)
-		b := hrefRegex.ReplaceAll(bw.buf.Bytes(), []byte(fmt.Sprintf("<D:href>%s/$1</D:href>", pathPrefix)))
-
-		if h.StatCache != nil && bw.status == http.StatusMultiStatus && b != nil {
-			h.StatCache.set(r.URL.Path, depth, b)
-		}
+			return bw.status, b
+		})
 
 		w.Header().Del("Content-Length")
-		w.WriteHeader(bw.status)
-		w.Write(b)
-
+		w.WriteHeader(status)
+		if result != nil {
+			w.Write(result)
+		}
 		return
 	}
 

drive/driveimpl/compositedav/stat_cache.go

@@ -4,6 +4,7 @@
 package compositedav
 
 import (
+	"net/http"
 	"sync"
 	"time"
 
@@ -25,6 +26,23 @@ type StatCache struct {
 	cachesByDepthAndPath map[int]*ttlcache.Cache[string, []byte]
 }
 
+// getOr checks the cache for the named value at the given depth. If a cached
+// value was found, it returns http.StatusMultiStatus along with the cached
+// value. Otherwise, it executes the given function and returns the resulting
+// status and value. If the function returned http.StatusMultiStatus, getOr
+// caches the resulting value at the given name and depth before returning.
+func (c *StatCache) getOr(name string, depth int, or func() (int, []byte)) (int, []byte) {
+	cached := c.get(name, depth)
+	if cached != nil {
+		return http.StatusMultiStatus, cached
+	}
+	status, next := or()
+	if c != nil && status == http.StatusMultiStatus && next != nil {
+		c.set(name, depth, next)
+	}
+	return status, next
+}
+
 func (c *StatCache) get(name string, depth int) []byte {
 	if c == nil {
 		return nil