api: nvim_get_mode()

Asynchronous API functions are served immediately, which means pending
input could change the state of Nvim shortly after an async API function
result is returned.

nvim_get_mode() is different:
  - If RPCs are known to be blocked, it responds immediately (without
    flushing the input/event queue)
  - else it is handled just-in-time before waiting for input, after
    pending input was processed. This makes the result more reliable
    (but not perfect).

Internally this is handled as a special case, but _semantically_ nothing
has changed: API users never know when input flushes, so this internal
special-case doesn't violate that. As far as API users are concerned,
nvim_get_mode() is just another asynchronous API function.

In all cases nvim_get_mode() never blocks for more than the time it
takes to flush the input/event queue (~µs).

Note: This doesn't address #6166; nvim_get_mode() will provoke #6166 if
e.g. `d` is operator-pending.

Closes #6159

src/nvim/os/input.c

@@ -23,6 +23,7 @@
 #include "nvim/main.h"
 #include "nvim/misc1.h"
 #include "nvim/state.h"
+#include "nvim/log.h"
 
 #define READ_BUFFER_SIZE 0xfff
 #define INPUT_BUFFER_SIZE (READ_BUFFER_SIZE * 4)
@@ -38,6 +39,7 @@ static RBuffer *input_buffer = NULL;
 static bool input_eof = false;
 static int global_fd = 0;
 static int events_enabled = 0;
+static bool blocking = false;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "os/input.c.generated.h"
@@ -327,13 +329,27 @@ static unsigned int handle_mouse_event(char **ptr, uint8_t *buf,
   return bufsize;
 }
 
+/// @return true if the main loop is blocked and waiting for input.
+bool input_blocking(void)
+{
+  return blocking;
+}
+
 static bool input_poll(int ms)
 {
   if (do_profiling == PROF_YES && ms) {
     prof_inchar_enter();
   }
 
+  if ((ms == - 1 || ms > 0)
+      && !(events_enabled || input_ready() || input_eof)
+      ) {
+    blocking = true;
+    multiqueue_process_debug(main_loop.events);
+    multiqueue_process_events(main_loop.events);
+  }
   LOOP_PROCESS_EVENTS_UNTIL(&main_loop, NULL, ms, input_ready() || input_eof);
+  blocking = false;
 
   if (do_profiling == PROF_YES && ms) {
     prof_inchar_exit();