watch: Fix buggy line-deletion behaviour with --no-linewrap

This change is largely based upon Justin's patch, I just moved the
reset_ansi() parts out otherwise you get strange colour reset
behaviours.

Original patch message:
I used the --no-linewrap (-w) option for the first time today, watching
some wide output that didn't quite fit in my tmux pane. Quickly I
noticed a problem: while --no-linewrap did indeed eliminate the
spillover of lines too long for the terminal "window" width, it *also*
resulted in a bunch of lines from the program output being hidden
entirely.

After some fiddling around, the exact problematic behavior appears to be
as follows:
 1. Lines which would have wrapped (more than $COLUMNS chars long) are
    handled correctly.
 2. Lines which would *not* have wrapped (shorter than $COLUMNS) are
    printed; but then the next line is *not* printed! For long sequences
    of non-wrap-length lines, you get an every-other-line-is-visible
    sort of effect.

The logic underlying the problem seems to be this: in the run_command
loop, if the x loop goes all the way to completion (meaning we've
reached the right-side edge of the window area), there's a small block
of code for --no-linewrap whose main purpose is to call find_eol, which
eats input until it hits a newline (or EOF). Clearly this is intended to
be done for lines that are too long, so that the excess characters are
discarded and the input pointer is ready to go for the subsequent line.

However, this code isn't in any way conditional on the value of eolseen!
Short/wouldn't-wrap lines will have encountered a newline character
before exhausting the entire x loop, and therefore eolseen will be true.
Long/would-wrap lines will not have encountered a newline when the x
loop is exhausted, and so eolseen will be false.

Nevertheless, find_eol is called in *both* cases. For long lines, it
does what it's meant to do. For short lines, *the newline has already
been encountered and dealt with*, and so the actual effect of find_eol
is to eat the entirety of the next line, all the way through to its
newline, such that it isn't printed at all.

References:
 procps-ng/procps!157

Signed-off-by: Craig Small <csmall@dropbear.xyz>

2 files changed, 5 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index c319ad5..0c473c7 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,7 @@ procps-ng-NEXT
   * w: Add --pids option                                   merge #159
   * watch: Pass through beep                               issue #104
   * watch: -r option to not re-exec on SIGWINCH            merge #125
+  * watch: find eol with --no-linewrap                     merge #157
 
 procps-ng-4.0.2
 ---------------
diff --git a/src/watch.c b/src/watch.c
index b7ad005..f9a152d 100644
--- a/src/watch.c
+++ b/src/watch.c
@@ -774,8 +774,11 @@ static int run_command(char *restrict command, char **restrict command_argv)
 		    reset_ansi();
 		    if (flags & WATCH_COLOR)
 			attrset(A_NORMAL);
-		    find_eol(p);
 		}
+                if (!line_wrap && !eolseen)
+                {
+                    find_eol(p);
+                }
 	}
 
 	fclose(p);