fileio: add WRITE_DATA_FILE_MODE_0400 mode

src/basic/fileio.c

@@ -1706,7 +1706,7 @@ int write_data_file_atomic_at(
                         return r;
         }
 
-        r = fchmod_umask(fd, 0644);
+        r = fchmod_umask(fd, FLAGS_SET(flags, WRITE_DATA_FILE_MODE_0400) ? 0400 : 0644);
         if (r < 0)
                 return r;
 

src/basic/fileio.h

@@ -166,6 +166,7 @@ int fopen_mode_to_flags(const char *mode);
 
 typedef enum WriteDataFileFlags {
         WRITE_DATA_FILE_MKDIR_0755 = 1 << 0,
+        WRITE_DATA_FILE_MODE_0400  = 1 << 1,
 } WriteDataFileFlags;
 
 int write_data_file_atomic_at(int dir_fd, const char *path, const struct iovec *iovec, WriteDataFileFlags flags);