Smoke-test what _helpless receives for a long failed command

Nothing exercised _help50 itself: the head/tail cap and the new command-line
argument were only checked by hand under a pty. Drive _help50 directly in the
image instead, with a fabricated typescript and a fake _helpless, and assert
that a command printing 3000 lines then an error yields the command line as
the second argument and, as the first, output that starts at the program's
first line, carries the omission marker, drops the middle, and ends with the
error. Also check that a failed command with no output yields an empty first
argument, which is what the codespace's empty-output check depends on.

The check fails against the previous help50.sh.

Author: rongxin-liu

tests/smoke.sh

@@ -25,4 +25,27 @@ run "$IMAGE" bash --login -c 'valgrind python x.py < /dev/null; test $? -eq 1' 2
 echo "- _fold wraps without a terminal"
 run "$IMAGE" bash --login -c '. /opt/cs50/lib/cli; TERM= _fold "$(printf "a %.0s" {1..100})"' | head -n 1 | grep -qE '^.{1,80}$'
 
+echo "- _helpless gets the end of a failed command's long output, plus its command line"
+run "$IMAGE" bash --login -c '
+    export HELP50=$(mktemp)
+    _helpless() { printf "%s" "$1" > /tmp/output; printf "%s" "$2" > /tmp/cmd; }
+    . /etc/profile.d/help50.sh
+
+    # A command that prints 3000 lines and then fails, as script(1) records it
+    { printf "$ ./slow\r\n"; seq 3000 | sed "s/$/\r/"; printf "Error: boom\r\n"; } > "$HELP50"
+    set -o history; history -s ./slow; set +o history
+    false; _help50
+    test "$(cat /tmp/cmd)" = ./slow &&
+    test "$(head -n 1 /tmp/output)" = 1 &&
+    grep -qx "\[... 1914 lines omitted ...\]" /tmp/output &&
+    ! grep -qx 1000 /tmp/output &&
+    test "$(tail -n 1 /tmp/output)" = "Error: boom" || exit 1
+
+    # A command that fails without output
+    : > "$HELP50"
+    set -o history; history -s ./slow; set +o history
+    false; _help50
+    test "$(cat /tmp/cmd)" = ./slow && test ! -s /tmp/output || exit 1
+'
+
 echo "OK"