Fall back to sync() if syncfs() is unavailable

It seems better to err on the side of safety (vs. performance).

(cherry picked from commit 90a1e6abbd)

qrexec-lib/pack.c

@@ -89,7 +89,7 @@ void wait_for_result(void)
     if (hdr.error_code != 0) {
         switch (hdr.error_code) {
             case EEXIST:
-                call_error_handler("File copy: not overwriting existing file. Clean QubesIncoming dir, and retry copy%s%s", last_filename_prefix, last_filename);
+                call_error_handler("A file named %s already exists in QubesIncoming dir", last_filename);
                 break;
             case EINVAL:
                 call_error_handler("File copy: Corrupted data from packer%s%s", last_filename_prefix, last_filename);

qrexec-lib/unpack.c

@@ -237,6 +237,8 @@ int do_unpack(void)
     cwd_fd = open(".", O_RDONLY);
     if (cwd_fd >= 0 && syncfs(cwd_fd) == 0 && close(cwd_fd) == 0)
         errno = saved_errno;
+#else
+    sync();
 #endif
 
     send_status_and_crc(errno, untrusted_namebuf);

rpm_spec/qubes-utils.spec

@@ -19,6 +19,8 @@ Requires:	%{name}-libs
 Requires:	ImageMagick
 Requires:	pycairo
 BuildRequires:  qubes-libvchan-devel
+# for meminfo-writer
+BuildRequires:  xen-devel
 
 %description
 Common Linux files for Qubes Dom0 and VM