Add proper way of storing value for status current-command

There should be no functional changes in this commit.

The global variable `$_` set in the parser variables by `reader.cpp` and
read by the `status` builtin was deprecated in fish 2.0 but kept around
internally because there's no good way to store/share/forward parser
variables.

A new enum is added that identifies the status variable and they are
stored in a private array in the parser. There is no need for
synchronization because they are only set during job init and never
thereafter. This is currently asserted via ASSERT_IS_MAIN_THREAD() but
that assert can be dropped in the interest of making the parser possible
to clone and use from worker threads.

The old `$_` global variable is still kept for backwards compatibility,
though it will be dropped in a future release.

src/builtins/fg.cpp

@@ -108,8 +108,12 @@ maybe_t<int> builtin_fg(parser_t &parser, io_streams_t &streams, const wchar_t *
     }
 
     wcstring ft = tok_command(job->command());
-    // For compatibility with fish 2.0's $_, now replaced with `status current-command`
-    if (!ft.empty()) parser.set_var_and_fire(L"_", ENV_EXPORT, std::move(ft));
+    if (!ft.empty()) {
+        // Provide value for `status current-command`
+        parser.set_status_var(parser_status_var_t::current_command, ft);
+        // Also provide a value for the deprecated fish 2.0 $_ variable
+        parser.set_var_and_fire(L"_", ENV_EXPORT, std::move(ft));
+    }
     reader_write_title(job->command(), parser);
 
     // Note if tty transfer fails, we still try running the job.

src/builtins/status.cpp

@@ -449,10 +449,9 @@ maybe_t<int> builtin_status(parser_t &parser, io_streams_t &streams, const wchar
         }
         case STATUS_CURRENT_CMD: {
             CHECK_FOR_UNEXPECTED_STATUS_ARGS(opts.status_cmd)
-            // HACK: Go via the deprecated variable to get the command.
-            const auto var = parser.vars().get(L"_");
-            if (!var.missing_or_empty()) {
-                streams.out.append(var->as_string());
+            const auto &var = parser.get_status_var(parser_status_var_t::current_command);
+            if (!var.empty()) {
+                streams.out.append(var);
                 streams.out.push_back(L'\n');
             } else {
                 streams.out.append(program_name);

src/parser.h

@@ -244,6 +244,11 @@ struct eval_res_t {
         : status(status), break_expand(break_expand), was_empty(was_empty), no_status(no_status) {}
 };
 
+enum class parser_status_var_t : uint8_t {
+    current_command,
+    count_,
+};
+
 class parser_t : public std::enable_shared_from_this<parser_t> {
     friend class parse_execution_context_t;
 
@@ -264,6 +269,9 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     /// top down using range-based for loops.
     std::deque<block_t> block_list;
 
+    /// Variables set by the single-threaded parser or reader and queryable by any consumer.
+    std::array<wcstring, (int) parser_status_var_t::count_> status_vars_;
+
     /// The 'depth' of the fish call stack.
     int eval_level = -1;
 
@@ -389,6 +397,17 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     statuses_t get_last_statuses() const { return vars().get_last_statuses(); }
     void set_last_statuses(statuses_t s) { vars().set_last_statuses(std::move(s)); }
 
+    /// Get a parser status variable
+    const wcstring& get_status_var(parser_status_var_t var) const {
+        return status_vars_.at((int) var);
+    }
+
+    /// Set a parser status variable
+    void set_status_var(parser_status_var_t var, wcstring val) {
+        ASSERT_IS_MAIN_THREAD();
+        status_vars_[(int) var] = std::move(val);
+    }
+
     /// Cover of vars().set(), which also fires any returned event handlers.
     /// \return a value like ENV_OK.
     int set_var_and_fire(const wcstring &key, env_mode_flags_t mode, wcstring val);

src/reader.cpp

@@ -2494,7 +2494,9 @@ static void reader_interactive_init(parser_t &parser) {
 
     termsize_container_t::shared().invalidate_tty();
 
-    // For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    // Provide value for `status current-command`
+    parser.set_status_var(parser_status_var_t::current_command, L"fish");
+    // Also provide a value for the deprecated fish 2.0 $_ variable
     parser.vars().set_one(L"_", ENV_GLOBAL, L"fish");
 }
 
@@ -2628,8 +2630,12 @@ void reader_data_t::set_buffer_maintaining_pager(const wcstring &b, size_t pos,
 static eval_res_t reader_run_command(parser_t &parser, const wcstring &cmd) {
     wcstring ft = tok_command(cmd);
 
-    // For compatibility with fish 2.0's $_, now replaced with `status current-command`
-    if (!ft.empty()) parser.vars().set_one(L"_", ENV_GLOBAL, ft);
+    // Provide values for `status current-command` and `status current-commandline`
+    if (!ft.empty()) {
+        parser.set_status_var(parser_status_var_t::current_command, ft);
+        // Also provide a value for the deprecated fish 2.0 $_ variable
+        parser.vars().set_one(L"_", ENV_GLOBAL, ft);
+    }
 
     outputter_t &outp = outputter_t::stdoutput();
     reader_write_title(cmd, parser);
@@ -2651,7 +2657,9 @@ static eval_res_t reader_run_command(parser_t &parser, const wcstring &cmd) {
 
     term_steal();
 
-    // For compatibility with fish 2.0's $_, now replaced with `status current-command`
+    // Provide value for `status current-command`
+    parser.set_status_var(parser_status_var_t::current_command, program_name);
+    // Also provide a value for the deprecated fish 2.0 $_ variable
     parser.vars().set_one(L"_", ENV_GLOBAL, program_name);
 
     if (have_proc_stat()) {