MDEV-14448: Ctrl-C should not exit the client

This patch introduces the following behaviour for Linux while
maintaining old behaviour for Windows:

Ctrl + C (sigint) clears the current buffer and redraws the prompt.
Ctrl-C no longer exits the client if no query is running.
Ctrl-C kills the current running query if there is one. If there is an
error communicating with the server while trying to issue a KILL QUERY,
the client exits. This is in line with the past behaviour of Ctrl-C.
On Linux Ctrl-D can be used to close the client.
On Windows Ctrl-C and Ctrl-BREAK still exits the client if no query is running.
Windows can also exit the client via \q<enter> or exit<enter>.

== Implementation details ==
The Linux implementation has two corner cases, based on which library is
used: libreadline or libedit, both are handled in code to achieve the
same user experience.

Additional code is taken from MySQL, ensuring there is identical
behaviour on Windows, to MySQL's mysql client implementation for other
CTRL- related signals.

* The CTRL_CLOSE, CTRL_LOGOFF, CTRL_SHUTDOWN will issue the equivalent
  of CTRL-C and "end" the program. This ensures that the query is killed
  when the client is closed by closing the terminal, logging off the
  user or shutting down the system. The latter two signals are not sent
  for interactive applications, but it handles the case when a user has
  defined a service to use mysql client to issue a command. See
  https://learn.microsoft.com/en-us/windows/console/handlerroutine

This patch is built on top of the initial work done by Anel Husakovic
<anel@mariadb.org>.
Closes #2815

client/mysql.cc

@@ -89,6 +89,7 @@ extern "C" {
 #undef bcmp				// Fix problem with new readline
 #if defined(__WIN__)
 #include <conio.h>
+#include <windows.h>
 #else
 # ifdef __APPLE__
 #  include <editline/readline.h>
@@ -170,6 +171,9 @@ static int connect_flag=CLIENT_INTERACTIVE;
 static my_bool opt_binary_mode= FALSE;
 static my_bool opt_connect_expired_password= FALSE;
 static int interrupted_query= 0;
+#ifdef USE_LIBEDIT_INTERFACE
+static int sigint_received= 0;
+#endif
 static char *current_host,*current_db,*current_user=0,*opt_password=0,
             *current_prompt=0, *delimiter_str= 0,
             *default_charset= (char*) MYSQL_AUTODETECT_CHARSET_NAME,
@@ -1072,7 +1076,32 @@ extern "C" sig_handler handle_sigint(int sig);
 static sig_handler window_resize(int sig);
 #endif
 
+static void end_in_sig_handler(int sig);
+static bool kill_query(const char *reason);
 
+#ifdef _WIN32
+static BOOL WINAPI ctrl_handler_windows(DWORD fdwCtrlType)
+{
+  switch (fdwCtrlType)
+  {
+    // Handle the CTRL-C signal.
+    case CTRL_C_EVENT:
+    case CTRL_BREAK_EVENT:
+      handle_sigint(SIGINT);
+      return TRUE; // this means that the signal is handled
+    case CTRL_CLOSE_EVENT:
+    case CTRL_LOGOFF_EVENT:
+    case CTRL_SHUTDOWN_EVENT:
+      kill_query("Terminate");
+      end_in_sig_handler(SIGINT + 1);
+      aborted= 1;
+  }
+  // This means to pass the signal to the next handler. This allows
+  // my_cgets and the internal ReadConsole call to exit and gracefully
+  // abort the program.
+  return FALSE;
+}
+#endif
 const char DELIMITER_NAME[]= "delimiter";
 const uint DELIMITER_NAME_LEN= sizeof(DELIMITER_NAME) - 1;
 inline bool is_delimiter_command(char *name, ulong len)
@@ -1205,11 +1234,12 @@ int main(int argc,char *argv[])
   if (!status.batch)
     ignore_errors=1;				// Don't abort monitor
 
-  if (opt_sigint_ignore)
-    signal(SIGINT, SIG_IGN);
-  else
-    signal(SIGINT, handle_sigint);              // Catch SIGINT to clean up
-  signal(SIGQUIT, mysql_end);			// Catch SIGQUIT to clean up
+#ifndef _WIN32
+  signal(SIGINT, handle_sigint);   // Catch SIGINT to clean up
+  signal(SIGQUIT, mysql_end);      // Catch SIGQUIT to clean up
+#else
+  SetConsoleCtrlHandler(ctrl_handler_windows, TRUE);
+#endif
 
 #if defined(HAVE_TERMIOS_H) && defined(GWINSZ_IN_SYS_IOCTL)
   /* Readline will call this if it installs a handler */
@@ -1376,30 +1406,35 @@ static bool do_connect(MYSQL *mysql, const char *host, const char *user,
 }
 
 
-/*
-  This function handles sigint calls
-  If query is in process, kill query
-  If 'source' is executed, abort source command
-  no query in process, terminate like previous behavior
- */
+void end_in_sig_handler(int sig)
+{
+#ifdef _WIN32
+  /*
+   When SIGINT is raised on Windows, the OS creates a new thread to handle the
+   interrupt. Once that thread completes, the main thread continues running
+   only to find that it's resources have already been free'd when the sigint
+   handler called mysql_end().
+  */
+  mysql_thread_end();
+#else
+  mysql_end(sig);
+#endif
+}
 
-sig_handler handle_sigint(int sig)
+
+/*
+  Kill a running query. Returns true if we were unable to connect to the server.
+*/
+bool kill_query(const char *reason)
 {
   char kill_buffer[40];
   MYSQL *kill_mysql= NULL;
 
-  /* terminate if no query being executed, or we already tried interrupting */
-  if (!executing_query || (interrupted_query == 2))
-  {
-    tee_fprintf(stdout, "Ctrl-C -- exit!\n");
-    goto err;
-  }
-
   kill_mysql= mysql_init(kill_mysql);
   if (!do_connect(kill_mysql,current_host, current_user, opt_password, "", 0))
   {
-    tee_fprintf(stdout, "Ctrl-C -- sorry, cannot connect to server to kill query, giving up ...\n");
-    goto err;
+    tee_fprintf(stdout, "%s -- sorry, cannot connect to server to kill query, giving up ...\n", reason);
+    return true;
   }
 
   /* First time try to kill the query, second time the connection */
@@ -1414,27 +1449,65 @@ sig_handler handle_sigint(int sig)
           (interrupted_query == 1) ? "QUERY " : "",
           mysql_thread_id(&mysql));
   if (verbose)
-    tee_fprintf(stdout, "Ctrl-C -- sending \"%s\" to server ...\n",
+    tee_fprintf(stdout, "%s -- sending \"%s\" to server ...\n", reason,
                 kill_buffer);
   mysql_real_query(kill_mysql, kill_buffer, (uint) strlen(kill_buffer));
   mysql_close(kill_mysql);
-  tee_fprintf(stdout, "Ctrl-C -- query killed. Continuing normally.\n");
+  if (interrupted_query == 1)
+    tee_fprintf(stdout, "%s -- query killed.\n", reason);
+  else
+    tee_fprintf(stdout, "%s -- connection killed.\n", reason);
+
   if (in_com_source)
     aborted= 1;                                 // Abort source command
-  return;
+  return false;
+}
+
+/*
+  This function handles sigint calls
+  If query is in process, kill query
+  If 'source' is executed, abort source command
+  no query in process, regenerate prompt.
+*/
+sig_handler handle_sigint(int sig)
+{
+  if (opt_sigint_ignore)
+    return;
 
-err:
-#ifdef _WIN32
   /*
-   When SIGINT is raised on Windows, the OS creates a new thread to handle the
-   interrupt. Once that thread completes, the main thread continues running 
-   only to find that it's resources have already been free'd when the sigint 
-   handler called mysql_end(). 
+     On Unix only, if no query is being executed just clear the prompt,
+     don't exit. On Windows we exit.
   */
-  mysql_thread_end();
+  if (!executing_query)
+  {
+#ifndef _WIN32
+    tee_fprintf(stdout, "^C\n");
+#ifdef USE_LIBEDIT_INTERFACE
+    /* Libedit will regenerate it outside of the signal handler. */
+    sigint_received= 1;
 #else
-  mysql_end(sig);
-#endif  
+    rl_on_new_line();           // Regenerate the prompt on a newline
+    rl_replace_line("", 0);     // Clear the previous text
+    rl_redisplay();
+#endif
+#else // WIN32
+    tee_fprintf(stdout, "Ctrl-C -- exit!\n");
+    end_in_sig_handler(sig);
+#endif
+    return;
+  }
+
+  /*
+    When executing a query, this newline makes the prompt look like so:
+    ^C
+    Ctrl-C -- query killed.
+  */
+  tee_fprintf(stdout, "\n");
+  if (kill_query("Ctrl-C"))
+  {
+    aborted= 1;
+    end_in_sig_handler(sig);
+  }
 }
 
 
@@ -1973,6 +2046,12 @@ static int get_options(int argc, char **argv)
   return(0);
 }
 
+static inline void reset_prompt(char *in_string, bool *ml_comment) {
+  glob_buffer.length(0);
+  *ml_comment = false;
+  *in_string = 0;
+}
+
 static int read_and_execute(bool interactive)
 {
 #if defined(__WIN__)
@@ -2063,7 +2142,7 @@ static int read_and_execute(bool interactive)
       size_t clen;
       do
       {
-	line= my_cgets((char*)tmpbuf.ptr(), tmpbuf.alloced_length()-1, &clen);
+        line= my_cgets((char*)tmpbuf.ptr(), tmpbuf.alloced_length()-1, &clen);
         buffer.append(line, clen);
         /* 
            if we got buffer fully filled than there is a chance that
@@ -2085,8 +2164,34 @@ static int read_and_execute(bool interactive)
         the readline/libedit library.
       */
       if (line)
+      {
         free(line);
+        glob_buffer.length(0);
+      }
       line= readline(prompt);
+#ifdef USE_LIBEDIT_INTERFACE
+      /*
+        libedit handles interrupts different than libreadline.
+        libreadline has its own signal handlers, thus a sigint during readline
+        doesn't force readline to return null string.
+
+        However libedit returns null if the interrupt signal is raised.
+        We can also get an empty string when ctrl+d is pressed (EoF).
+
+        We need this sigint_received flag, to differentiate between the two
+        cases. This flag is only set during our handle_sigint function when
+        LIBEDIT_INTERFACE is used.
+      */
+      if (!line && sigint_received)
+      {
+        // User asked to clear the input.
+        sigint_received= 0;
+        reset_prompt(&in_string, &ml_comment);
+        continue;
+      }
+      // For safety, we always mark this as cleared.
+      sigint_received= 0;
+#endif
 #endif /* defined(__WIN__) */
 
       /*