Implement a readonly mode

This commit introduces the `readonly` variable as well as the `-ro`
command line option which prevent buffers from being overwritten on
disk when the `write` command is used without arguments. Some buffers
can selectively be put in readonly mode by setting the `readonly`
variable on the `buffer` scope, the `global` mode will affect all
buffers (even those who will be open subsequently), using the `window`
scope will have no effect.

Closes #685

src/buffer.cc

@@ -591,6 +591,13 @@ void Buffer::set_fs_timestamp(timespec ts)
 
 void Buffer::on_option_changed(const Option& option)
 {
+    if (option.name() == "readonly")
+    {
+        if (option.get<bool>())
+            m_flags |= Flags::ReadOnly;
+        else
+            m_flags &= ~Flags::ReadOnly;
+    }
     run_hook_in_own_context("BufSetOption",
                             format("{}={}", option.name(), option.get_as_string()));
 }

src/buffer.hh

@@ -106,7 +106,8 @@ public:
         New      = 1 << 1,
         Fifo     = 1 << 2,
         NoUndo   = 1 << 3,
-        Debug  = 1 << 4
+        Debug    = 1 << 4,
+        ReadOnly = 1 << 5,
     };
 
     Buffer(String name, Flags flags, StringView data = {},

src/buffer_manager.cc

@@ -78,7 +78,8 @@ void BufferManager::backup_modified_buffers()
 {
     for (auto& buf : m_buffers)
     {
-        if ((buf->flags() & Buffer::Flags::File) and buf->is_modified())
+        if ((buf->flags() & Buffer::Flags::File) and buf->is_modified()
+            and !(buf->flags() & Buffer::Flags::ReadOnly))
             write_buffer_to_backup_file(*buf);
     }
 }

src/commands.cc

@@ -239,6 +239,12 @@ void write_buffer(const ParametersParser& parser, Context& context, const ShellC
     if (parser.positional_count() == 0 and !(buffer.flags() & Buffer::Flags::File))
         throw runtime_error("cannot write a non file buffer without a filename");
 
+    // if the buffer is in read-only mode and we try to save it directly
+    // or we try to write to it indirectly using e.g. a symlink, throw an error
+    if ((context.buffer().flags() & Buffer::Flags::ReadOnly)
+        && (parser.positional_count() == 0 || real_path(parser[0]) == buffer.name()))
+        throw runtime_error("cannot overwrite the buffer when in readonly mode");
+
     auto filename = parser.positional_count() == 0 ?
                         buffer.name() : parse_filename(parser[0]);
     write_buffer_to_file(buffer, filename);
@@ -260,7 +266,8 @@ void write_all_buffers()
 {
     for (auto& buffer : BufferManager::instance())
     {
-        if ((buffer->flags() & Buffer::Flags::File) and buffer->is_modified())
+        if ((buffer->flags() & Buffer::Flags::File) and buffer->is_modified()
+            and !(buffer->flags() & Buffer::Flags::ReadOnly))
             write_buffer_to_file(*buffer, buffer->name());
     }
 }

src/main.cc

@@ -265,6 +265,7 @@ void register_options()
     reg.declare_option("modelinefmt", "format string used to generate the modeline",
                        "%val{bufname} %val{cursor_line}:%val{cursor_char_column} "_str);
     reg.declare_option("debug", "various debug flags", DebugFlags::None);
+    reg.declare_option("readonly", "prevent buffers from being modified", false);
 }
 
 struct convert_to_client_mode
@@ -460,7 +461,7 @@ int run_client(StringView session, StringView init_command, UIType ui_type)
 }
 
 int run_server(StringView session, StringView init_command,
-               bool ignore_kakrc, bool daemon, UIType ui_type,
+               bool ignore_kakrc, bool daemon, bool readonly, UIType ui_type,
                ConstArrayView<StringView> files, ByteCoord target_coord)
 {
     static bool terminate = false;
@@ -503,6 +504,8 @@ int run_server(StringView session, StringView init_command,
 
     write_to_debug_buffer("*** This is the debug buffer, where debug info will be written ***");
 
+    GlobalScope::instance().options().get_local_option("readonly").set(readonly);
+
     Server server(session.empty() ? to_string(getpid()) : session.str());
 
     bool startup_error = false;
@@ -536,7 +539,9 @@ int run_server(StringView session, StringView init_command,
         {
             try
             {
-                open_or_create_file_buffer(file);
+                Buffer *buffer = open_or_create_file_buffer(file);
+                if (readonly)
+                    buffer->flags() |= Buffer::Flags::ReadOnly;
             }
             catch (Kakoune::runtime_error& error)
             {
@@ -741,7 +746,8 @@ int main(int argc, char* argv[])
                    { "q", { false, "in filter mode, be quiet about errors applying keys" } },
                    { "ui", { true, "set the type of user interface to use (ncurses, dummy, or json)" } },
                    { "l", { false, "list existing sessions" } },
-                   { "clear", { false, "clear dead sessions" } } }
+                   { "clear", { false, "clear dead sessions" } },
+                   { "ro", { false, "readonly mode" } } }
     };
     try
     {
@@ -793,7 +799,8 @@ int main(int argc, char* argv[])
             for (size_t i = 0; i < parser.positional_count(); ++i)
                 files.emplace_back(parser[i]);
 
-            return run_filter(*keys, init_command, files, (bool)parser.get_switch("q"));
+            return run_filter(*keys, init_command, files,
+                              (bool)parser.get_switch("q"));
         }
 
         if (auto server_session = parser.get_switch("c"))
@@ -840,6 +847,7 @@ int main(int argc, char* argv[])
                 return run_server(session, init_command,
                                   (bool)parser.get_switch("n"),
                                   (bool)parser.get_switch("d"),
+                                  (bool)parser.get_switch("ro"),
                                   ui_type, files, target_coord);
             }
             catch (convert_to_client_mode& convert)