patch 9.1.0321: Garbled output on serial terminals with XON/XOFF flow control

Problem:  When used terminal with XON/XOFF flow control, vim tries to
          still make CTRL-S mapping available, which results in severe
          screen corruption, especially on large redraws, and even
          spurious inputs (John Tsiombikas)
Solution: Disallow CTRL-S mapping if such terminal is recognized.
          Don't remove IXON from the bitmask inversion.
          (Anton Sharonov)

*** When started like this:

    TERM=vt420 vim

:set termcap

    shows "t_xon=y"

map <C-S> :echo "abc"<CR>

    does nothing (after <C-S> output freezes and subsequent <C-Q>
    unfreezes it)

*** When started like this:

    TERM=xterm vim

:set termcap

    shows "t_xon="

map <C-S> :echo "abc"<CR>

    works (after <C-S> one see "abc" string echo-ed)

fixes: #12674
closes: #14542

Signed-off-by: Anton Sharonov <anton.sharonov@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/optiondefs.h

@@ -3014,6 +3014,7 @@ static struct vimoption options[] =
     p_term("t_8f", T_8F)
     p_term("t_8b", T_8B)
     p_term("t_8u", T_8U)
+    p_term("t_xo", T_XON)
 
 // terminal key codes are not in here
 

src/os_unix.c

@@ -3766,7 +3766,8 @@ mch_settmode(tmode_T tmode)
     {
 	// ~ICRNL enables typing ^V^M
 	// ~IXON disables CTRL-S stopping output, so that it can be mapped.
-	tnew.c_iflag &= ~(ICRNL | IXON);
+	tnew.c_iflag &= ~(ICRNL |
+		(T_XON == NULL || *T_XON == NUL ? IXON : 0));
 	tnew.c_lflag &= ~(ICANON | ECHO | ISIG | ECHOE
 # if defined(IEXTEN)
 		    | IEXTEN	    // IEXTEN enables typing ^V on SOLARIS

src/term.c

@@ -1798,6 +1798,8 @@ get_term_entries(int *height, int *width)
 	T_DA = (char_u *)"y";
     if ((T_UT == NULL || T_UT == empty_option) && tgetflag("ut") > 0)
 	T_UT = (char_u *)"y";
+    if ((T_XON == NULL || T_XON == empty_option) && tgetflag("xo") > 0)
+	T_XON = (char_u *)"y";
 
     /*
      * get key codes

src/termdefs.h

@@ -115,10 +115,11 @@ enum SpecialKey
     KS_SRI,	// restore icon text
     KS_FD,	// disable focus event tracking
     KS_FE,	// enable focus event tracking
-    KS_CF	// set terminal alternate font
+    KS_CF,	// set terminal alternate font
+    KS_XON	// terminal uses xon/xoff handshaking
 };
 
-#define KS_LAST	    KS_CF
+#define KS_LAST	    KS_XON
 
 /*
  * the terminal capabilities are stored in this array
@@ -224,6 +225,7 @@ extern char_u *(term_strings[]);    // current terminal strings
 #define T_SRI	(TERM_STR(KS_SRI))	// restore icon text
 #define T_FD	(TERM_STR(KS_FD))	// disable focus event tracking
 #define T_FE	(TERM_STR(KS_FE))	// enable focus event tracking
+#define T_XON	(TERM_STR(KS_XON))	// terminal uses xon/xoff handshaking
 
 typedef enum {
     TMODE_COOK,	    // terminal mode for external cmds and Ex mode

src/testdir/test_cmdline.vim

@@ -3792,4 +3792,22 @@ func Test_buffer_completion()
   call assert_equal("\"b Xbuf_complete/Foobar.c Xbuf_complete/MyFoobar.c AFoobar.h", @:)
 endfunc
 
+" :set t_??
+func Test_term_option()
+  set wildoptions&
+  let _cpo = &cpo
+  set cpo-=C
+  let expected='"set t_AB t_AF t_AU t_AL t_al t_bc t_BE t_BD t_cd t_ce t_Ce t_CF t_cl t_cm'
+        \ .. ' t_Co t_CS t_Cs t_cs t_CV t_da t_db t_DL t_dl t_ds t_Ds t_EC t_EI t_fs t_fd t_fe'
+        \ .. ' t_GP t_IE t_IS t_ke t_ks t_le t_mb t_md t_me t_mr t_ms t_nd t_op t_RF t_RB t_RC'
+        \ .. ' t_RI t_Ri t_RK t_RS t_RT t_RV t_Sb t_SC t_se t_Sf t_SH t_SI t_Si t_so t_SR t_sr'
+        \ .. ' t_ST t_Te t_te t_TE t_ti t_TI t_Ts t_ts t_u7 t_ue t_us t_Us t_ut t_vb t_ve t_vi'
+        \ .. ' t_VS t_vs t_WP t_WS t_XM t_xn t_xs t_ZH t_ZR t_8f t_8b t_8u t_xo t_#2 t_#4 t_%i'
+        \ .. ' t_*7 t_@7 t_F1 t_F2 t_k1 t_k2 t_k3 t_k4 t_k5 t_k6 t_k7 t_k8 t_k9 t_k; t_kB t_kD'
+        \ .. ' t_kI t_kN t_kP t_kb t_kd t_kh t_kl t_kr t_ku'
+  call feedkeys(":set t_\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal(expected, @:)
+  let &cpo = _cpo
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/testdir/test_terminal3.vim

@@ -931,5 +931,25 @@ func Test_terminal_term_start_error()
   delfunc s:term_start_error
 endfunc
 
+func Test_terminal_vt420()
+  CheckRunVimInTerminal
+  " For Termcap
+  CheckUnix
+  let rows=15
+  call writefile([':set term=vt420'], 'Xterm420', 'D')
+
+  let buf = RunVimInTerminal('-S Xterm420', #{rows: rows})
+  call TermWait(buf, 100)
+  call term_sendkeys(buf, ":set t_xo?\<CR>")
+  call WaitForAssert({-> assert_match('t_xo=y', term_getline(buf, rows))})
+  call StopVimInTerminal(buf)
+
+  call writefile([''], 'Xterm420')
+  let buf = RunVimInTerminal('-S Xterm420', #{rows: rows})
+  call TermWait(buf, 100)
+  call term_sendkeys(buf, ":set t_xo?\<CR>")
+  call WaitForAssert({-> assert_match('t_xo=\s\+', term_getline(buf, rows))})
+  call StopVimInTerminal(buf)
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    321,
 /**/
     320,
 /**/