Only use rl_extend_line_buffer if available

The BSD editline library (libedit, version 3.1-20221030) doesn't
implement the rl_extend_line_buffer function. Alternative approaches to
replace the line buffer using the readline interface don't give the
desired behavior. So simply don't use history expansion in the
completion function if the rl_extend_line_buffer function is not
available.

config.h.in

@@ -9,6 +9,9 @@
 /* Define the name of the executing macro variable in libreadline. */
 #undef EXECUTING_MACRO_NAME
 
+/* Define if rl_extend_line_buffer is resolved in libreadline. */
+#undef EXTEND_LINE_BUFFER
+
 /* Define to 1 if you have the <dlfcn.h> header file. */
 #undef HAVE_DLFCN_H
 

configure

@@ -12689,6 +12689,37 @@ $as_echo "yes" >&6; };
 
 $as_echo "#define CLEANUP_AFER_SIGNAL 1" >>confdefs.h
 
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+
+# check for readline's rl_extend_line_buffer
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for rl_extend_line_buffer() in -lreadline" >&5
+$as_echo_n "checking for rl_extend_line_buffer() in -lreadline... " >&6; }
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+    extern void rl_extend_line_buffer(int len);
+    rl_extend_line_buffer(1);
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; };
+
+$as_echo "#define EXTEND_LINE_BUFFER 1" >>confdefs.h
+
 else
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }

configure.ac

@@ -241,6 +241,18 @@ AC_TRY_LINK(,[
     [ Define if rl_cleanup_after_signal is resolved in libreadline. ]),
     AC_MSG_RESULT(no))
 
+# check for readline's rl_extend_line_buffer
+
+AC_MSG_CHECKING([for rl_extend_line_buffer() in -lreadline])
+AC_TRY_LINK(,[
+    extern void rl_extend_line_buffer(int len);
+    rl_extend_line_buffer(1);
+],
+    AC_MSG_RESULT(yes);
+    AC_DEFINE(EXTEND_LINE_BUFFER, 1,
+    [ Define if rl_extend_line_buffer is resolved in libreadline. ]),
+    AC_MSG_RESULT(no))
+
 
 AC_MSG_CHECKING([for the readline version number])
 AC_TRY_RUN([

tclreadline.c

@@ -26,11 +26,13 @@
 #endif
 
 
+#ifdef EXTEND_LINE_BUFFER
 /*
  * this prototype may be missing
  * in readline.h
  */
 void rl_extend_line_buffer(int len);
+#endif
 
 #ifdef EXECUTING_MACRO_HACK
 /**
@@ -700,6 +702,10 @@ TclReadlineCompletion(char* text, int start, int end)
     int status;
     rl_completion_append_character = ' '; /* reset, just in case ... */
 
+    /* Only enable history expansion like '!!<TAB>' if the rl_extend_line_buffer
+     * function is available; e.g. libedit doesn't provide it, and alternative
+     * approaches to replace the line buffer don't give the desired behavior */
+#ifdef EXTEND_LINE_BUFFER
     if (tclrl_use_history_expansion && text && ('!' == text[0]
             || (start && rl_line_buffer[start - 1] == '!' /* for '$' */))) {
         char* expansion = (char*) NULL;
@@ -721,6 +727,7 @@ TclReadlineCompletion(char* text, int start, int end)
         }
         FREE(expansion);
     }
+#endif
 
     if (tclrl_custom_completer) {
         char start_s[BUFSIZ], end_s[BUFSIZ];