Make Ctrl-C clear line and re-prompt in REPL instead of exiting

Use sigsetjmp/siglongjmp to catch SIGINT during readline/fgets input
and jump back to the REPL loop with a fresh prompt. Running Lua code
still gets interrupted via the existing laction/lstop mechanism.

Fixes #11.

lua.c

@@ -37,6 +37,11 @@ static lua_State *globalL = NULL;
 
 static const char *progname = LUA_PROGNAME;
 
+#include <setjmp.h>
+
+static volatile sig_atomic_t sigint_in_repl = 0;
+static sigjmp_buf repl_jmp;
+
 
 #if defined(LUA_USE_POSIX)   /* { */
 
@@ -81,6 +86,16 @@ static void laction (int i) {
 }
 
 
+static void repl_sigint_handler (int i) {
+  if (sigint_in_repl) {
+    sigint_in_repl = 0;
+    siglongjmp(repl_jmp, 1);
+  }
+  signal(i, SIG_DFL);
+  raise(i);
+}
+
+
 static void print_usage (const char *badoption) {
   lua_writestringerror("%s: ", progname);
   if (badoption[1] == 'e' || badoption[1] == 'l')
@@ -566,8 +581,11 @@ static int incomplete (lua_State *L, int status) {
 static int pushline (lua_State *L, int firstline) {
   char buffer[LUA_MAXINPUT];
   size_t l;
+  char *b;
   const char *prmt = get_prompt(L, firstline);
-  char *b = lua_readline(buffer, prmt);
+  sigint_in_repl = 1;
+  b = lua_readline(buffer, prmt);
+  sigint_in_repl = 0;
   lua_pop(L, 1);  /* remove prompt */
   if (b == NULL)
     return 0;  /* no input */
@@ -678,11 +696,19 @@ static void doREPL (lua_State *L) {
   const char *oldprogname = progname;
   progname = NULL;  /* no 'progname' on errors in interactive mode */
   lua_initreadline(L);
+  setsignal(SIGINT, repl_sigint_handler);
+  if (sigsetjmp(repl_jmp, 1)) {
+    /* Ctrl-C during input — print newline, clear stack, re-prompt */
+    lua_writestringerror("%s", "\n");
+    lua_settop(L, 0);
+    setsignal(SIGINT, repl_sigint_handler);
+  }
   while ((status = loadline(L)) != -1) {
     if (status == LUA_OK)
       status = docall(L, 0, LUA_MULTRET);
     if (status == LUA_OK) l_print(L);
     else report(L, status);
+    setsignal(SIGINT, repl_sigint_handler);  /* re-install after docall */
   }
   lua_settop(L, 0);  /* clear stack */
   lua_writeline();