sys_readdir and memfs_readdir closer to targeted look and feel.

We now have a single dirbuf of size PAGE_SIZE kept on the vnode. This is
to be used for directory contents only. The reason it's kept on FS0 is
because the contents are modified by calls such as mkdir or create, and otherwise
these would have been handled by mm0 on the page cache buffers, which wouldn't work.

tasks/fs0/src/syscalls.c

@@ -90,21 +90,39 @@ int sys_read(l4id_t sender, int fd, void *buf, int count)
  */
 int sys_readdir(l4id_t sender, int fd, void *buf, int count)
 {
+	unsigned long vnum;
 	struct vnode *v;
 	struct tcb *t;
+	struct dirbuf *db;
+	unsigned long nbytes;
+	int err;
 
 	/* Get the task */
 	BUG_ON(!(t = find_task(sender)));
 
-	/* Convert fd to vnode. */
-	BUG_ON(!(v = t->fd[fd]));
+	/* Convert fd to vnum. */
+	BUG_ON(!(vnum = t->fd[fd]));
+
+	/* Lookup vnode */
+	if (!(v = vfs_lookup_byvnum(vnum)))
+		return -EINVAL; /* No such vnode */
 
 	/* Ensure vnode is a directory */
 	if (!vfs_isdir(v))
 		return -ENOTDIR;
 
-	/* Simply read its contents */
-	v->ops.readdir(v, buf, count);
+	/* Read the whole content from fs, if haven't done so yet */
+	if ((err = v->ops.readdir(v)) < 0)
+		return err;
+
+	/* Bytes to read, minimum of vnode size and count requested */
+	nbytes = (v->size <= count) ? v->size : count;
+
+	/* Do we have those bytes at hand? */
+	if (v->dirbuf->buffer && (v->dirbuf->npages * PAGE_SIZE) >= nbytes) {
+		memcpy(buf, v->dirbuf->buffer, nbytes); /* Finish the job */
+		return nbytes;
+	}
 
 	return 0;
 }