[Add] Implemented Copy-on-Write and scriptable rm.

OK, this commit finally implements the Copy-on-Write architecure I spoke
about in previous commits.. It also refactors command execution to be
safer and more extensible.

To enable CoW and centralise state-changing, command scripts no longer
modify the VFS directly. Instead, they return a table describing their
intended action '{ action = "rm", target = "file.txt" }'. The C++
CommandProcessor is then responsible for interpreting this and executing
it safely.

common/src/command_processor.cpp

@@ -1,7 +1,9 @@
 #include "command_processor.h"
+#include <optional>
 #include <sstream>
 #include "vfs.h"
 #include "lua_processor.h"
+#include "vfs_manager.h"
 
 CommandProcessor::CommandProcessor(vfs_node* starting_dir) {
   _current_dir = starting_dir;
@@ -42,7 +44,13 @@ std::string CommandProcessor::process_command(const std::string& command) {
   }
   if(root->children.count("bin") && root->children["bin"]->children.count(script_filename)) {
     vfs_node* script_node = root->children["bin"]->children[script_filename];
-    return _lua->execute(script_node->content, _current_dir, args);
+    sol::object result = _lua->execute(script_node->content, _current_dir, args);
+    if(result.is<std::string>()) {
+      return result.as<std::string>();
+    } else if(result.is<sol::table>()) {
+      return _handle_vfs_action(result.as<sol::table>());
+    }
+    return "[Script returned an unexpected type]";
   }
 
 
@@ -68,3 +76,59 @@ std::string CommandProcessor::process_command(const std::string& command) {
 
   return "Unknown command: " + command + "\n";
 }
+
+std::string CommandProcessor::_handle_vfs_action(sol::table action) {
+  std::string action_name = action["action"].get_or<std::string>("");
+  if(action_name == "rm") {
+    std::string path = action["target"].get_or<std::string>("");
+    if(path.empty()) return "rm: missing operand";
+
+    /*
+     * std::map::count was failing weirdly..
+     * Instead, iterate manually to find the node.
+     */
+    vfs_node* target_node = nullptr;
+    for(auto const& [key, node] : _current_dir->children) {
+      if(key == path) {
+        target_node = node;
+        break;
+      }
+    }
+
+    if(target_node != nullptr) {
+      if(_current_dir->read_only) {
+        /* === Copy on Write === */
+        fprintf(stderr, "CoW: Write attempt on read-only dir '%s'. "
+                "Creating private copy.\n",
+                _current_dir->name.c_str());
+
+        vfs_node* grand_parent = _current_dir->parent;
+        if(!grand_parent) {
+          return "rm: Permission denied. Cannot modify the root filesystem.";
+        }
+
+        /* Copy the directory. */
+        vfs_node* private_copy = copy_vfs_node(_current_dir, grand_parent);
+
+        /* Link the new private copy into the VFS tree. */
+        grand_parent->children[_current_dir->name] = private_copy;
+
+        /* Update our own state to point to the new writable dir. */
+        _current_dir = private_copy;
+
+        /* Find the target node again within our new private copy. */
+        target_node = _current_dir->children[path];
+      }
+      /* Go ahead and delete, you have a writable target now :) */
+      if(target_node->type == DIR_NODE) {
+        return std::string("rm: cannot remove '") + path + "': Is a directory.";
+      }
+      _current_dir->children.erase(path);
+      delete target_node;
+      return ""; /* Success. */
+    }
+      return std::string("rm: cannot remove '") + path + "': No such file or directory.";
+  }
+  return "Error: Unknown VFS action '" + action_name + "'";
+}
+

common/src/command_processor.h

@@ -14,6 +14,7 @@ public:
   vfs_node* get_current_dir(void);
 
 private:
+  std::string _handle_vfs_action(sol::table action);
   vfs_node* _current_dir;
   LuaProcessor* _lua;
 };

common/src/lua_processor.cpp

@@ -1,4 +1,5 @@
 #include "lua_processor.h"
+#include <sol/forward.hpp>
 #include "vfs.h"
 
 LuaProcessor::LuaProcessor(void) {
@@ -9,51 +10,27 @@ LuaProcessor::LuaProcessor(void) {
                               "name",       &vfs_node::name,
                               "type",       &vfs_node::type,
                               "children",   &vfs_node::children,
-                              "read_only",  &vfs_node::read_only);
-
-  /* Create the 'vfs' table for our API. */
-  sol::table vfs_api = _lua.create_table("vfs");
-  vfs_api.set_function("rm", [](std::string path, vfs_node* current_dir) {
-    /* for now, we'll just support deleting from the current directory. */
-    if(current_dir->children.count(path)) {
-      vfs_node* target_node = current_dir->children[path];
-      if(target_node->read_only || current_dir->read_only) {
-        return std::string("rm: Permission denied. File or directory is read-only.");
-      }
-      if(target_node->type == DIR_NODE) {
-        return std::string("rm: cannot remove '" + path + "': Is a directory.");
-      }
-
-      current_dir->children.erase(path);
-      delete target_node; /* Clean up memry. */
-      return std::string(""); /* Success. */
-    }
-    return std::string("rm: cannot remove '" + path + "': No such file or directory.");
-  });
-}
+                              "read_only",  &vfs_node::read_only); }
 
 LuaProcessor::~LuaProcessor(void) {}
 
-std::string LuaProcessor::execute(const std::string& script, vfs_node* current_dir,
+sol::object LuaProcessor::execute(const std::string& script, vfs_node* current_dir,
                                   const std::vector<std::string>& args) {
   try {
     /* Pass C++ objects/points into the Lua env. */
     _lua["current_dir"] = current_dir;
 
     /* Create and populate the 'arg' table for the script. */
-    sol::table arg_table = _lua.create_table();;;;
+    sol::table arg_table = _lua.create_table();
     for(size_t i = 0; i < args.size(); ++i) {
-      arg_table[i+1] = args[i]; /* Lua array are 1-indexed. Not that Skuzzy would know. */
+      arg_table[i+1] = args[i]; /* Lua arrays 1-indexed. */
     }
     _lua["arg"] = arg_table;
 
-    sol::object result = _lua.script(script);
-    if(result.is<std::string>()) {
-      return result.as<std::string>();
-    }
+    return _lua.script(script);
+
   } catch(const sol::error& e) {
-    return e.what();
+    /* Return the error message as a string in a sol::object. */
+    return sol::make_object(_lua, e.what());
   }
-  /* Shouldn't reach this, just shutting the compiler up. */
-  return "[Unknown error in LuaProcessor::execute]";
 }

common/src/lua_processor.h

@@ -11,7 +11,7 @@ public:
   ~LuaProcessor(void);
 
   /* Executes a string of lua code and returns result as a string. */
-  std::string execute(const std::string& script, vfs_node* current_dir,
+  sol::object execute(const std::string& script, vfs_node* current_dir,
                       const std::vector<std::string>& args);
 private:
   sol::state _lua;

common/src/vfs_manager.cpp

@@ -11,6 +11,24 @@ vfs_node* new_node(std::string name, vfs_node_type type, vfs_node* parent) {
   return node;
 }
 
+vfs_node* copy_vfs_node(vfs_node* original, vfs_node* new_parent) {
+  if(!original) {
+    return nullptr;
+  }
+
+  /* Create the new node and copy its properties. */
+  vfs_node* new_copy = new_node(original->name, original->type, new_parent);
+  new_copy->read_only = false; /* The new copy is *always* writeable. */
+  new_copy->content = original->content;
+
+  /* Recursively copy all children. */
+  for(auto const& [key, child_node] : original->children) {
+    new_copy->children[key] = copy_vfs_node(child_node, new_copy);
+  }
+
+  return new_copy;
+}
+
 VFSManager::VFSManager(void) {
   /* Create template VFS that holds shared, read-only directories. */
   _vfs_root = new_node("/", DIR_NODE, nullptr);
@@ -46,7 +64,7 @@ VFSManager::VFSManager(void) {
   rm_script->content = R"lua(-- /bin/rm.lua - Removes a file.
                              local file_to_remove = arg[1]
                              if not file_to_remove then return "rm: missing operand" end
-                             return vfs.rm(file_to_remove, current_dir)
+                             return { action = "rm", target = file_to_remove }
                              )lua";
   bin->children["rm.lua"] = rm_script;
 }

common/src/vfs_manager.h

@@ -4,6 +4,9 @@
 
 #include "vfs.h"
 
+/* Recursive copy function for our Copy-on-Write behaviour. */
+vfs_node* copy_vfs_node(vfs_node* original, vfs_node* new_parent);
+
 class VFSManager {
 public:
   VFSManager(void);