top: enable screen contents preservation at end-of-job

The title of this commit is actually quite misleading.

Were it more accurate, it would at least mention a tty
emulator's scrollback buffer, which was the cumulation
of a long pursuit to reduce the SIGWINCH overhead when
a window manager carelessly floods an application with
that signal *while* a user is still resizing a window!

Disabling and enabling that scrollback buffer resulted
in the final top display replaced with original screen
contents, a phenomenon acknowledged at the time but it
also represented a user interface change which has now
produced the first request for return to old behavior.

After the SIGWINCH dust settled, another problem arose
regarding behaviors under the 'screen' window manager.
In response, top was refactored a bit to avoid display
corruption. That was before discovering 'screen' could
duplicate the scrollback buffer behavior top expected.

As it turns out, the 'screen' refactoring had probably
made scrollback buffer manipulation unnecessary. Still
one could argue that a window should not be allowed to
scroll while a constantly updating program was active.

The solution represented in this commit returns former
behavior at program end (retaining top's last screen).
And if we ever wish to disable scrollback buffers, the
associated logic was retained but made conditional. It
is not reflected in configure.ac but might be someday.

Lastly, this commit corrects cursor positioning when a
^C is issued under 'Fields Management' at any terminal
that didn't have a scrollback buffer (i.e. a console).

Reference(s):
https://bugzilla.redhat.com/show_bug.cgi?id=977561
http://www.freelists.org/post/procps/top-library-miscellaneous-tweaks,1
. screen program refactor
commit 0fe393ff27
. scrollback buffer disabled
commit dedaf6e1a8
. sigwinch management defines
commit adca737758
commit 4f33b6b8c5

Signed-off-by: Jim Warner <james.warner@comcast.net>

top/top.c

@@ -72,8 +72,9 @@ static struct termios Tty_original,    // our inherited terminal definition
                       Tty_raw;         // for unsolicited input
 static int Ttychanged = 0;
 
-        /* Last established cursor state/shape */
+        /* Last established cursor state/shape, and is re-position needed */
 static const char *Cursor_state = "";
+static int         Cursor_repos;
 
         /* Program name used in error messages and local 'rc' file name */
 static char *Myname;
@@ -350,12 +351,15 @@ static void at_eoj (void) {
    if (Ttychanged) {
       tcsetattr(STDIN_FILENO, TCSAFLUSH, &Tty_original);
       if (keypad_local) putp(keypad_local);
+      if (Cursor_repos) putp(tg2(0, Screen_rows));
       putp("\n");
+#ifdef OFF_SCROLLBK
       if (exit_ca_mode) {
          // this next will also replace top's most recent screen with the
          // original display contents that were visible at our invocation
          putp(exit_ca_mode);
       }
+#endif
       putp(Cap_curs_norm);
       putp(Cap_clr_eol);
 #ifndef RMAN_IGNORED
@@ -591,17 +595,13 @@ static void sig_endpgm (int dont_care_sig) {
 
         /*
          * Catches:
-         *    SIGTSTP, SIGTTIN and SIGTTOU
-         * note:
-         *    we don't fiddle with with those enter/exit_ca_mode strings
-         *    because we want to retain most of the last screen contents
-         *    as a visual reminder this program is suspended, not ended! */
+         *    SIGTSTP, SIGTTIN and SIGTTOU */
 static void sig_paused (int dont_care_sig) {
 // POSIX.1-2004 async-signal-safe: tcsetattr, tcdrain, raise
    if (-1 == tcsetattr(STDIN_FILENO, TCSAFLUSH, &Tty_original))
       error_exit(fmtmk(N_fmt(FAIL_tty_set_fmt), strerror(errno)));
    if (keypad_local) putp(keypad_local);
-   putp(tg2(0, Screen_rows));
+   if (Cursor_repos) putp(tg2(0, Screen_rows));
    putp(Cap_curs_norm);
 #ifndef RMAN_IGNORED
    putp(Cap_smam);
@@ -2120,6 +2120,7 @@ static void fields_utility (void) {
    int i, key;
    FLG_t f;
 
+   Cursor_repos = 1;
    spewFI
 signify_that:
    putp(Cap_clr_scr);
@@ -2180,6 +2181,7 @@ signify_that:
             break;
       }
    } while (key != 'q' && key != kbd_ESC);
+   Cursor_repos = 0;
  #undef unSCRL
  #undef swapEM
  #undef spewFI
@@ -3768,8 +3770,10 @@ static void whack_terminal (void) {
    // thanks anyway stdio, but we'll manage buffering at the frame level...
    setbuffer(stdout, Stdout_buf, sizeof(Stdout_buf));
 #endif
+#ifdef OFF_SCROLLBK
    // this has the effect of disabling any troublesome scrollback buffer...
    if (enter_ca_mode) putp(enter_ca_mode);
+#endif
    // and don't forget to ask iokey to initialize his tinfo_tab
    iokey(0);
 } // end: whack_terminal

top/top.h

@@ -41,6 +41,7 @@
 //#define INSP_SAVEBUF            /* preserve 'Insp_buf' contents in a file  */
 //#define INSP_SLIDE_1            /* when scrolling left/right don't move 8  */
 //#define OFF_HST_HASH            /* use BOTH qsort+bsrch vs. hashing scheme */
+//#define OFF_SCROLLBK            /* disable tty emulators scrollback buffer */
 //#define OFF_STDIOLBF            /* disable our own stdout _IOFBF override  */
 //#define PRETEND2_5_X            /* pretend we're linux 2.5.x (for IO-wait) */
 //#define PRETEND8CPUS            /* pretend we're smp with 8 ticsers (sic)  */