Don't enqueue a repaint in the middle of one

This can easily lead to an infinite loop, if a variable handler
triggers a repaint and the variable is set in the prompt, e.g. some of
the git variables.

A simple way to reproduce:

    function fish_mode_prompt
        commandline -f repaint
    end

Repainting executes the mode prompt, which triggers a repaint, which
triggers the mode prompt, ....

So we just set a flag and check it.

Fixes #7324.

src/builtin_commandline.cpp

@@ -294,8 +294,14 @@ maybe_t<int> builtin_commandline(parser_t &parser, io_streams_t &streams, wchar_
             return STATUS_INVALID_ARGS;
         }
 
+        using rl = readline_cmd_t;
         for (i = w.woptind; i < argc; i++) {
             if (auto mc = input_function_get_code(argv[i])) {
+                // Don't enqueue a repaint if we're currently in the middle of one,
+                // because that's an infinite loop.
+                if (mc == rl::repaint_mode || mc == rl::force_repaint || mc == rl::repaint) {
+                    if (ld.is_repaint) continue;
+                }
                 // Inserts the readline function at the back of the queue.
                 reader_queue_ch(*mc);
             } else {

src/parser.h

@@ -151,6 +151,10 @@ struct library_data_t {
     /// Number of recursive calls to builtin_complete().
     uint32_t builtin_complete_recursion_level{0};
 
+    /// If we're currently repainting the commandline.
+    /// Useful to stop infinite loops.
+    bool is_repaint{false};
+
     /// Whether we called builtin_complete -C without parameter.
     bool builtin_complete_current_commandline{false};
 

src/reader.cpp

@@ -2781,10 +2781,17 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             //
             // Because some users set `fish_mode_prompt` to an empty function and display the mode
             // elsewhere, we detect if the mode output is empty.
+
+            // Don't go into an infinite loop of repainting.
+            // This can happen e.g. if a variable triggers a repaint,
+            // and the variable is set inside the prompt (#7324).
+            // builtin commandline will refuse to enqueue these.
+            parser().libdata().is_repaint = true;
             exec_mode_prompt();
             if (!mode_prompt_buff.empty()) {
                 s_reset_line(&screen, true /* redraw prompt */);
                 if (this->is_repaint_needed()) this->layout_and_repaint(L"mode");
+                parser().libdata().is_repaint = false;
                 break;
             }
             // Else we repaint as normal.
@@ -2792,10 +2799,12 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
         }
         case rl::force_repaint:
         case rl::repaint: {
+            parser().libdata().is_repaint = true;
             exec_prompt();
             s_reset_line(&screen, true /* redraw prompt */);
             this->layout_and_repaint(L"readline");
             force_exec_prompt_and_repaint = false;
+            parser().libdata().is_repaint = false;
             break;
         }
         case rl::complete:

tests/pexpects/commandline.py

@@ -23,3 +23,10 @@ sendline(
 expect_prompt()
 sendline("echo one \"two three\" four'five six'{7} 'eight~")
 expect_prompt("\r\n@GUARD:2@\r\n(.*)\r\n@/GUARD:2@\r\n")
+
+# Check that we don't infinitely loop here.
+sendline("function fish_mode_prompt; commandline -f repaint; end")
+expect_prompt()
+
+sendline("echo foo")
+expect_prompt("foo")