Some further steps towards removing R_NULL

Introduce char_event_type_t::check_exit to represent "check for exit"
instead of R_NULL.

src/input.cpp

@@ -348,12 +348,12 @@ static void input_mapping_execute(const input_mapping_t &m, bool allow_commands)
     }
 
     if (has_commands && !allow_commands) {
-        // We don't want to run commands yet. Put the characters back and return R_NULL.
+        // We don't want to run commands yet. Put the characters back and return check_exit.
         for (wcstring::const_reverse_iterator it = m.seq.rbegin(), end = m.seq.rend(); it != end;
              ++it) {
             input_common_next_ch(*it);
         }
-        input_common_next_ch(R_NULL);
+        input_common_next_ch(char_event_type_t::check_exit);
         return;  // skip the input_set_bind_mode
     } else if (has_functions && !has_commands) {
         // Functions are added at the head of the input queue.
@@ -374,11 +374,11 @@ static void input_mapping_execute(const input_mapping_t &m, bool allow_commands)
             parser_t::principal_parser().eval(cmd, io_chain_t(), TOP);
         }
         proc_set_last_statuses(std::move(last_statuses));
-        input_common_next_ch(R_NULL);
+        input_common_next_ch(char_event_type_t::check_exit);
     } else {
         // Invalid binding, mixed commands and functions.  We would need to execute these one by
         // one.
-        input_common_next_ch(R_NULL);
+        input_common_next_ch(char_event_type_t::check_exit);
     }
 
     // Empty bind mode indicates to not reset the mode (#2871)
@@ -504,9 +504,9 @@ char_event_t input_readch(bool allow_commands) {
         } else {
             input_common_next_ch(evt);
             input_mapping_execute_matching_or_generic(allow_commands);
-            // Regarding allow_commands, we're in a loop, but if a fish command
-            // is executed, R_NULL is unread, so the next pass through the loop
-            // we'll break out and return it.
+            // Regarding allow_commands, we're in a loop, but if a fish command is executed,
+            // check_exit is unread, so the next pass through the loop we'll break out and return
+            // it.
         }
     }
 }

src/input.h

@@ -31,8 +31,8 @@ void init_input();
 /// key press, and is returned as such.
 ///
 /// The argument determines whether fish commands are allowed to be run as bindings. If false, when
-/// a character is encountered that would invoke a fish command, it is unread and R_NULL is
-/// returned.
+/// a character is encountered that would invoke a fish command, it is unread and
+/// char_event_type_t::check_exit is returned.
 char_event_t input_readch(bool allow_commands = true);
 
 /// Enqueue a character or a readline function to the queue of unread characters that input_readch

src/input_common.cpp

@@ -50,10 +50,12 @@ static char_event_t lookahead_pop() {
 }
 
 /// \return the next lookahead char, or none if none. Discards timeouts.
-static maybe_t<wchar_t> lookahead_pop_char() {
+static maybe_t<char_event_t> lookahead_pop_evt() {
     while (has_lookahead()) {
         auto evt = lookahead_pop();
-        if (evt.is_char()) return evt.get_char();
+        if (! evt.is_timeout()) {
+            return evt;
+        }
     }
     return none();
 }
@@ -116,7 +118,7 @@ static char_event_t readb() {
                 if (interrupt_handler) {
                     if (auto interrupt_evt = interrupt_handler()) {
                         return *interrupt_evt;
-                    } else if (auto mc = lookahead_pop_char()) {
+                    } else if (auto mc = lookahead_pop_evt()) {
                         return *mc;
                     }
                 }
@@ -142,7 +144,7 @@ static char_event_t readb() {
 
             if (ioport > 0 && FD_ISSET(ioport, &fdset)) {
                 iothread_service_completion();
-                if (auto mc = lookahead_pop_char()) {
+                if (auto mc = lookahead_pop_evt()) {
                     return *mc;
                 }
             }
@@ -183,7 +185,7 @@ void update_wait_on_escape_ms(const environment_t &vars) {
 }
 
 char_event_t input_common_readch() {
-    if (auto mc = lookahead_pop_char()) {
+    if (auto mc = lookahead_pop_evt()) {
         return *mc;
     }
     wchar_t res;
@@ -208,7 +210,7 @@ char_event_t input_common_readch() {
             case (size_t)(-1): {
                 std::memset(&state, '\0', sizeof(state));
                 debug(2, L"Illegal input");
-                return R_NULL;
+                return char_event_type_t::check_exit;
             }
             case (size_t)(-2): {
                 break;

src/input_common.h

@@ -11,8 +11,6 @@
 
 enum {
     R_MIN = INPUT_COMMON_BASE,
-    // R_NULL is sometimes returned by the input when a character was requested but none could be
-    // delivered, or when an exception happened.
     R_NULL = R_MIN,
 
     R_BEGINNING_OF_LINE,
@@ -89,7 +87,11 @@ enum class char_event_type_t {
     timeout,
 
     /// end-of-file was reached.
-    eof
+    eof,
+
+    /// An event was handled internally, or an interrupt was received. Check to see if the reader
+    /// loop should exit.
+    check_exit,
 };
 
 class char_event_t {
@@ -105,6 +107,8 @@ class char_event_t {
 
     bool is_eof() const { return type == char_event_type_t::eof; }
 
+    bool is_check_exit() const { return type == char_event_type_t::check_exit; }
+
     bool is_readline() const {
         return is_char() && c_ >= R_BEGIN_INPUT_FUNCTIONS && c_ < R_END_INPUT_FUNCTIONS;
     }

src/reader.cpp

@@ -2501,7 +2501,7 @@ maybe_t<wcstring> reader_data_t::readline(int nchars) {
             }
         }
 
-        if (!event_needing_handling) {
+        if (!event_needing_handling || event_needing_handling->is_check_exit()) {
             event_needing_handling = R_NULL;
         } else if (event_needing_handling->is_eof()) {
             reader_force_exit();