drive: parse depth 1 PROPFIND results to include children in cache

Clients often perform a PROPFIND for the parent directory before
performing PROPFIND for specific children within that directory.
The PROPFIND for the parent directory is usually done at depth 1,
meaning that we already have information for all of the children.
By immediately adding that to the cache, we save a roundtrip to
the remote peer on the PROPFIND for the specific child.

Updates tailscale/corp#19779

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

drive/driveimpl/compositedav/compositedav.go

@@ -81,6 +81,16 @@ type Handler struct {
 	staticRoot string
 }
 
+var cacheInvalidatingMethods = map[string]bool{
+	"PUT":       true,
+	"POST":      true,
+	"COPY":      true,
+	"MKCOL":     true,
+	"MOVE":      true,
+	"PROPPATCH": true,
+	"DELETE":    true,
+}
+
 // ServeHTTP implements http.Handler.
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if r.Method == "PROPFIND" {
@@ -88,11 +98,12 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if r.Method != "GET" {
-		// If the user is performing a modification (e.g. PUT, MKDIR, etc),
+	_, shouldInvalidate := cacheInvalidatingMethods[r.Method]
+	if shouldInvalidate {
+		// If the user is performing a modification (e.g. PUT, MKDIR, etc.),
 		// we need to invalidate the StatCache to make sure we're not knowingly
 		// showing stale stats.
-		// TODO(oxtoacart): maybe be more selective about invalidating cache
+		// TODO(oxtoacart): maybe only invalidate specific paths
 		h.StatCache.invalidate()
 	}