Mount only onto non-existent paths, existent parent

If mountpoint already exists, then we're silently making its children
inaccessible which may not be what we expected/intended.

Create a new forwarder inode upon mounting.

lib/9p.js

@@ -18,6 +18,7 @@ const VIRTIO_9P_MAX_TAGLEN = 254;
 
 var EPERM = 1;       /* Operation not permitted */
 var ENOENT = 2;      /* No such file or directory */
+var EEXIST = 17;      /* File exists */
 var EINVAL = 22;     /* Invalid argument */
 var EOPNOTSUPP = 95;  /* Operation is not supported */
 var ENOTEMPTY = 39;  /* Directory not empty */

lib/filesystem.js

@@ -1489,8 +1489,6 @@ FS.prototype.set_forwarder = function(idx, mount_id, foreign_id)
  */
 FS.prototype.create_forwarder = function(mount_id, foreign_id)
 {
-    dbg_assert(foreign_id !== 0, "Filesystem: root forwarder should not be created.");
-
     const inode = this.CreateInode();
 
     const idx = this.inodes.length;
@@ -1578,9 +1576,10 @@ FS.prototype.follow_fs = function(inode)
 };
 
 /**
- * Mount another filesystem onto an existing directory.
+ * Mount another filesystem to given path.
- * @param {string} path Path to existing directrory relative to this filesystem.
+ * @param {string} path
  * @param {FS} fs
+ * @return {number} inode id of mount point if successful, or -errno if mounting failed.
  */
 FS.prototype.Mount = function(path, fs)
 {
@@ -1589,23 +1588,30 @@ FS.prototype.Mount = function(path, fs)
 
     const path_infos = this.SearchPath(path);
 
-    if(path_infos.id === -1)
+    if(path_infos.parentid === -1)
     {
-        dbg_log("Mount failed: path not found: " + path, LOG_9P);
+        dbg_log("Mount failed: parent for path not found: " + path, LOG_9P);
-        return -1;
+        return -ENOENT;
+    }
+    if(path_infos.id !== -1)
+    {
+        dbg_log("Mount failed: file already exists at path: " + path, LOG_9P);
+        return -EEXIST;
     }
     if(path_infos.forward_path)
     {
         const parent = this.inodes[path_infos.parentid];
-        this.follow_fs(parent).Mount(path_infos.forward_path, fs);
+        const ret = this.follow_fs(parent).Mount(path_infos.forward_path, fs);
-        return;
+        if(ret < 0) return ret;
+        return this.get_forwarder(parent.mount_id, ret);
     }
 
     const mount_id = this.mounts.length;
     this.mounts.push(new FSMountInfo(fs));
 
-    // Existing inode is already linked. Just set forwarding information.
+    const idx = this.create_forwarder(mount_id, 0);
-    this.set_forwarder(path_infos.id, mount_id, 0);
+    this.inodes[idx].name = path_infos.name;
+    this.link_under_dir(idx, path_infos.parentid);
 
-    return path_infos.id;
+    return idx;
 };

src/browser/starter.js

@@ -1089,10 +1089,18 @@ V86Starter.prototype.mount_fs = function(path, baseurl, basefs, callback)
         {
             return;
         }
-        if(idx === -1)
+        if(idx === -ENOENT)
         {
             callback(new FileNotFoundError());
         }
+        if(idx === -EEXIST)
+        {
+            callback(new FileExistsError());
+        }
+        if(idx < 0)
+        {
+            callback(new Error("Failed to mount. Error number: " + (-idx)));
+        }
         else
         {
             callback(null);
@@ -1204,6 +1212,18 @@ V86Starter.prototype.read_file = function(file, callback)
     }
 };
 
+/**
+ * @ignore
+ * @constructor
+ *
+ * @param {string=} message
+ */
+function FileExistsError(message)
+{
+    this.message = message || "File already exists";
+}
+FileNotFoundError.prototype = Error.prototype;
+
 /**
  * @ignore
  * @constructor

tests/devices/virtio_9p.js

@@ -1337,6 +1337,7 @@ function do_mounts()
         {
             const path = tests[test_num].mounts[mount_num].path;
             emulator.serial0_send("mkdir -p /mnt" +  path + "\n");
+            emulator.serial0_send("rmdir /mnt" +  path + "\n");
             emulator.serial0_send("echo done-premount\n");
             next_trigger = "done-premount";
             next_trigger_handler = () => mount(mount_num);