[6s] Allow 6s to know about filesystems

Added a j6_proto_vfs_tag/_get_tag pair of messages to the VFS protocol
to allow filesystems to label themselves, and gave 6s the concept of
current fs and current working directory.

src/libraries/j6/include/j6/protocols/vfs.h

@@ -8,4 +8,6 @@ enum j6_proto_vfs_tag
 {
     j6_proto_vfs_load = j6_proto_base_first_proto_id,
     j6_proto_vfs_file,
-};
+    j6_proto_vfs_get_tag,
+    j6_proto_vfs_tag,
+};

src/libraries/j6/include/j6/protocols/vfs.hh

@@ -13,7 +13,13 @@ class API client
 public:
     /// Constructor.
     /// \arg vfs_mb  Handle to the VFS service's mailbox
-    client(j6_handle_t vfs_mb);
+    client(j6_handle_t vfs_mb = 0);
+
+    /// Copy constructor
+    client(const client& c);
+
+    /// Check if this client's handle is valid
+    inline bool valid() const { return m_service != j6_handle_invalid; }
 
     /// Load a file into a VMA
     /// \arg path  Path of the file to load
@@ -21,8 +27,13 @@ public:
     /// \arg size  [out] Size of the file
     j6_status_t load_file(char *path, j6_handle_t &vma, size_t &size);
 
+    /// Get fs tag
+    /// \arg tag   [out] The filesystem's tag
+    /// \arg size  [inout] Size of the input buffer, length of the returned string
+    j6_status_t get_tag(char *tag, size_t &len);
+
 private:
     j6_handle_t m_service;
 };
 
-} // namespace j6::proto::vfs
+} // namespace j6::proto::vfs

src/libraries/j6/protocols/vfs.cpp

@@ -1,3 +1,4 @@
+#include "j6/types.h"
 #include <j6/errors.h>
 #include <j6/protocols/vfs.hh>
 #include <j6/syscalls.h>
@@ -12,6 +13,11 @@ client::client(j6_handle_t vfs_mb) :
 {
 }
 
+client::client(const client& c) :
+    m_service {c.m_service}
+{
+}
+
 inline size_t simple_strlen(const char *s) { size_t n = 0; while (s && *s) s++, n++; return n; }
 
 j6_status_t
@@ -61,5 +67,31 @@ client::load_file(char *path, j6_handle_t &vma, size_t &size)
 }
 
 
+j6_status_t
+client::get_tag(char *tag, size_t &len)
+{
+    if (len < sizeof(j6_status_t))
+        return j6_err_insufficient;
+
+    uint64_t message_tag = j6_proto_vfs_get_tag;
+    size_t handle_count = 0;
+
+    size_t in_len = 0;
+    j6_status_t s = j6_mailbox_call(m_service, &message_tag,
+        tag, &in_len, len, nullptr, &handle_count, 0);
+
+    if (s != j6_status_ok)
+        return s;
+
+    if (message_tag == j6_proto_base_status)
+        return *reinterpret_cast<j6_status_t*>(tag); // contains a status
+
+    if (message_tag != j6_proto_vfs_tag)
+        return j6_err_unexpected;
+
+    len = in_len;
+    return j6_status_ok; // data is now in `tag` and `len`
+}
+
 } // namespace j6::proto::vfs
 #endif // __j6kernel