Overhaul _eprint() and _eend() so as to better leverage smart terminals

This commit introduces a function named _update_tty_level(), whose
reponsibility is to grade the capability of the terminal, if any, on a
scale of 0 to 2. A value of 0 indicates that no terminal has been
detected, 1 that a dumb terminal has been detected, while 2 indicates
that a smart terminal has been detected. For the terminal to be
considered as smart, all of the following conditions must hold true.

- the TERM variable must not contain "dumb" as a substring
- the stty(1) utility must be able to report the terminal's dimensions
- the cursor's position must be reported for the ECMA-48 CPR sequence

Both the _eprint() and _eend() functions have been adjusted, so as to
evaluate the terminal's capabilites on each occasion that they are
called, with the overall intention of better exploiting the capabilities
of modern terminals, thus providing a better experience for a majority
of Gentoo users. These adjustments are described herewith.

Common to both functions is that the mechanism of tracking whether a
<newline> character is pending for the last printed message is no longer
employed for smart terminals.

As concerns _eprint(), it now begins by not only determining whether a
terminal is attached to STDOUT, but whether it is a smart terminal. In
the case that it is smart, it shall be determined whether the cursor is
situated at the first column. Should it not be, a <newline> character
shall be emitted, on the basis that the last call may have been to a
function that printed a message without a trailing <newline>.

Next, the message shall be checked, so as to determine whether it
contains a trailing <newline> character. Should it not, the message
shall be printed, along with a trailing DECSC sequence. The effect of
this sequence is to instruct the terminal emulator to save the current
position of the cursor.

If, on the other hand, the message is found to contain a trailing
<newline> character, it shall be stripped prior to printing the message.
The terminal shall then be asked to report the new position of the
cursor. Should the cursor turn out to be situated at the very last row
of the terminal, it shall be assumed that vertical scrolling is about to
occur and the cursor shall be moved up by one row, prior to its position
being saved by way of the DECSC sequence. Thereafter, two <newline>
characters shall be printed, so as to advance to the next line (relative
to the message). Otherwise, should the cursor be found to be situated at
a row that precedes the last row of the terminal, the DECSC sequence
shall be emitted, followed by a single <newline> character.

Note that the overall intention is to stash the exact position of the
cursor at the point that it is situated at the end of the printed
message, but just before a <newline> is printed (if any).

As concerns _eend(), and as with _eprint(), it now begins by not only
determining whether a terminal is attached to STDOUT, but whether it is
a smart terminal, though not before saving the last-known dimensions
of the terminal. Next, in the case of a dumb terminal - or no terminal
at all - the message shall be printed with a single preceding space.
That is, no attempt shall be made to right-align the success/failure
indicator. If, on the other hand, the terminal is found to be smart, the
behaviour shall be as described by the next paragraph.

The current dimensions of the terminal shall be compared against the
last-known dimensions of the terminal, as they were prior to the
terminal grading procedure. The purpose of doing so is to determine
whether the terminal has been recently resized. For the terminal not to
have been resized implies that it may yet be possible to write the
success/failure indicator on the very same row as the last printed
message, even if it were <newline>-terminated. With that in mind, the
current position of the cursor shall be saved. Next, the DECRC sequence
shall be emitted to the terminal. The effect of this sequence is to
instruct the terminal emulator to restore the position of the cursor,
as was saved by the DECSC sequence. In other words, the cursor position
shall be moved to wherever it was just after the last message was
printed, albeit before the trailing <newline> character (if any). Next,
the terminal shall be asked to report the new position of the cursor. In
the event that the cursor is found to have moved to any other row than
the immediately preceding one, it shall be assumed that scrolling has
occurred since printing the last message, in which case the cursor shall
be moved back to its prior position. Otherwise, the cursor shall be left
at its present position.

With that out of the way, the degree of indentation required to
right-align the indicator shall be trivially calculated by deducting
the present column number from the total number of columns spanned by
the terminal, with a further 6 deducted to account for the width of the
indicator. Should the resulting value be greater than 0, the cursor
shall be shifted that many cells to the right. Should the resulting
value be negative, it shall be recognised that there is insufficient
room to print the indicator on the same line and, instead, the indicator
shall be printed on the next line. Should the resulting value be 0, it
shall be recognised that the cursor is already situated at the correct
column.

Finally, the indicator shall be printed, along with a trailing <newline>
character.

As a consequence of these changes, the genfun_endcol variable has been
eliminated, as it is no longer required.

It should be noted that both DECSC and DECRC are private mode sequences,
and are, thus, not defined by ECMA-48. However, they were introduced by
DEC for the VT100 terminal, which was released in 1978. As such, they
can be considered as a de-facto standard.

Signed-off-by: Kerin Millar <kfm@plushkava.net>
Signed-off-by: Sam James <sam@gentoo.org>

1 files changed, 165 insertions, 44 deletions

diff --git a/functions.sh b/functions.sh
index 34bb266..7838c90 100644
--- a/functions.sh
+++ b/functions.sh
@@ -9,6 +9,11 @@
 # intended for internal use shall be prefixed with "genfun_" to indicate so,
 # and to reduce the probability of name space conflicts.
 
+# FIXME. There is just one invocation of ./ecma48-cpr, which can be found in
+# the _update_cursor_coords function. If you think it should be named sometime
+# else, I'm open to suggestions. Its behaviour now resembles that of stty size,
+# except that it reports the cursor coordinates (obviously).
+
 #
 #    Called by ebegin, eerrorn, einfon, and ewarnn.
 #
@@ -17,17 +22,72 @@ _eprint() {
 	color=$1
 	shift
 
-	msg=$*
-	if [ -t 1 ]; then
-		printf ' %s*%s %s%s' "${color}" "${NORMAL}" "${genfun_indent}" "${msg}"
-	else
-		printf ' * %s%s' "${genfun_indent}" "${msg}"
+	# Check whether STDOUT is a terminal, and how capable it is.
+	_update_tty_level <&1
+
+	if [ "${genfun_tty}" -eq 2 ]; then
+		# If the cursor is not situated on column 1, print a LF
+		# character. The assumption here is that the last call may have
+		# been via ebegin, or any of the other printing functions that
+		# pass a message without a trailing LF.
+		if [ "${genfun_x}" -ne 1 ]; then
+			printf '\n'
+		fi
+	elif [ "${genfun_is_pending_lf}" -eq 1 ]; then
+		# We are about to print to a dumb terminal or something other
+		# than a terminal. Print a LF character because the last printed
+		# message did not end with one. This technique is not ideal.
+		# For one thing, it can be thwarted by having called a printing
+		# function from a subshell or a shell launched by a subprocess,
+		# because the change to the flag variable would be lost. For
+		# another, it's possible for the user of the library to be
+		# directing STDOUT/STDERR to different places between calls.
+		# Such weaknesses cannot be addressed without some form of IPC.
+		printf '\n'
 	fi
 
-	if _ends_with_newline "${msg}"; then
-		genfun_is_pending_lf=0
+	msg=$*
+	if [ "${genfun_tty}" -lt 2 ]; then
+		if [ "${genfun_tty}" -eq 1 ]; then
+			# Print but do not attempt to save the cursor position.
+			printf ' %s*%s %s%s' "${color}" "${NORMAL}" "${genfun_indent}" "${msg}"
+		else
+			printf ' * %s%s' "${genfun_indent}" "${msg}"
+		fi
+		if _ends_with_newline "${msg}"; then
+			genfun_is_pending_lf=0
+		else
+			# Record the fact that a LF character is pending.
+			genfun_is_pending_lf=1
+		fi
+	elif ! _ends_with_newline "${msg}"; then
+		# Print the message before saving the cursor position with the
+		# DECSC sequence. This is a private mode sequence that is not
+		# defined by ECMA-48. However, it was introduced by DEC for the
+		# VT100 and can be considered as a de-facto standard.
+		printf ' %s*%s %s%s\0337' "${color}" "${NORMAL}" "${genfun_indent}" "${msg}"
 	else
-		genfun_is_pending_lf=1
+		# Print the message without its trailing LF character.
+		msg=${msg%"${genfun_newline}"}
+		printf ' %s*%s %s%s' "${color}" "${NORMAL}" "${genfun_indent}" "${msg}"
+
+		# Determine the current position of the cursor
+		_update_cursor_coords <&1
+
+		if [ "${genfun_y}" -ne "${genfun_rows}" ]; then
+			# Have the terminal save the position of the cursor
+			# with DECSC before printing a LF character to advance
+			# to the next line.
+			printf '\0337\n'
+		else
+			# The cursor is situated on the last row of the
+			# terminal, meaning that vertical scrolling will occur.
+			# Move the cursor up by one row with CUU (ECMA-48 CSI)
+			# before having the terminal save the position of the
+			# cursor with DECSC. Finally, print two LF characters to
+			# advance to the next line.
+			printf '\033[1A\0337\n\n'
+		fi
 	fi
 }
 
@@ -181,7 +241,7 @@ ebegin()
 	if ! yesno "${EINFO_QUIET}"; then
 		msg=$*
 		msg=${msg%"${genfun_newline}"}
-		_eprint "${GOOD}" "${msg} ...${genfun_newline}"
+		_eprint "${GOOD}" "${msg} ..."
 	fi
 }
 
@@ -191,7 +251,7 @@ ebegin()
 #
 _eend()
 {
-	local efunc is_tty msg retval
+	local efunc indent msg offset retval
 
 	efunc=$1
 	shift
@@ -200,28 +260,26 @@ _eend()
 	elif ! is_int "$1" || [ "$1" -lt 0 ]; then
 		ewarn "Invalid argument given to ${GENFUN_CALLER} (the exit status code must be an integer >= 0)"
 		retval=0
-		shift
+		msg=
 	else
 		retval=$1
 		shift
+		msg=$*
 	fi
 
-	if [ -t 1 ]; then
-		is_tty=1
-	else
-		is_tty=0
-	fi
+	# Stash the last known terminal dimensions, if any.
+	set -- "${genfun_cols}" "${genfun_rows}"
+
+	# Check whether STDOUT is a terminal, and how capable it is.
+	_update_tty_level <&1
 
 	if [ "${retval}" -ne 0 ]; then
 		# If a message was given, print it with the specified function.
-		if [ "$#" -gt 0 ]; then
-			msg=$*
-			if _is_visible "${msg}"; then
-				"${efunc}" "${msg}"
-			fi
+		if _is_visible "${msg}"; then
+			"${efunc}" "${msg}"
 		fi
 		# Generate an indicator for ebegin's unsuccessful conclusion.
-		if [ "${is_tty}" -eq 0 ]; then
+		if [ "${genfun_tty}" -eq 0 ]; then
 			msg="[ !! ]"
 		else
 			msg="${BRACKET}[ ${BAD}!!${BRACKET} ]${NORMAL}"
@@ -230,28 +288,65 @@ _eend()
 		return "${retval}"
 	else
 		# Generate an indicator for ebegin's successful conclusion.
-		if [ "${is_tty}" -eq 0 ]; then
+		if [ "${genfun_tty}" -eq 0 ]; then
 			msg="[ ok ]"
 		else
 			msg="${BRACKET}[ ${GOOD}ok${BRACKET} ]${NORMAL}"
 		fi
 	fi
 
-	if [ "${is_tty}" -eq 1 ] && [ -n "${genfun_endcol}" ]; then
-		# Should a LF character be pending then print one. The CUU
-		# (ECMA-48 CSI) sequence will move the cursor up by one line
-		# prior to printing the indicator, right-justified.
-		if [ "${genfun_is_pending_lf}" -eq 1 ]; then
-			printf '\n'
-		fi
-		printf '%b %s\n' "${genfun_endcol}" "${msg}"
+	if [ "${genfun_tty}" -lt 2 ]; then
+		printf ' %s\n' "${msg}"
+		genfun_is_pending_lf=0
 	else
+		# Provided that the terminal has not since been resized, it may
+		# be possible to write the indicator on the same row as the
+		# last printed message, even if it were LF-terminated.
+		if [ "${genfun_cols}" -eq "$1" ] && [ "${genfun_rows}" -eq "$2" ]; then
+			# Stash the current position of the cursor.
+			set -- "${genfun_x}" "${genfun_y}"
+
+			# Using the DECRC sequence, restore the cursor position
+			# to wherever it was just after the last message was
+			# printed, but before the trailing LF character, if any.
+			# This is a private mode sequence, and thus not defined
+			# by ECMA-48. However, it was introduced by DEC for the
+			# VT100 and can be considered as a de-facto standard.
+			printf '\0338'
+
+			# Determine the position of the cursor again.
+			_update_cursor_coords <&1
+
+			# Check whether the act of restoring the cursor position
+			# moved it to a different row, excepting the immediately
+			# preceding row. If it did, assume that scrolling has
+			# occurred since printing the last message and move the
+			# cursor back to where it was with CUP (ECMA-48 CSI).
+			offset=$(( $2 - genfun_y ))
+			if [ "${offset}" -lt 0 ] || [ "${offset}" -gt 1 ]; then
+				printf '\033[%d;%dH' "$1" "$2"
+				genfun_x=$1
+			fi
+		fi
+
+		# Calculate the column at which the indicator may be printed.
+		indent=$(( genfun_cols - genfun_x - 6 ))
+
+		# Determine whether the cursor needs to be repositioned.
+		if [ "${indent}" -gt 0 ]; then
+			# Use CHA (ECMA-48 CSI) to move the cursor to the right.
+			printf '\033[%dG' "$(( genfun_x + indent ))"
+		elif [ "${indent}" -lt 0 ]; then
+			# The indent is negative, meaning that there is not
+			# enough room. Arrange for the indicator to be printed
+			# on the next line instead.
+			printf '\n\033[%dG' "$(( genfun_cols - 6 ))"
+		fi
+
+		# Finally, print the indicator.
 		printf ' %s\n' "${msg}"
 	fi
 
-	# Record the fact that a LF character is no longer pending.
-	genfun_is_pending_lf=0
-
 	return "${retval}"
 }
 
@@ -479,6 +574,27 @@ _ends_with_newline() {
 	! case $1 in *"${genfun_newline}") false ;; esac
 }
 
+_update_tty_level() {
+	# Grade the capability of the terminal attached to STDIN (if any) on a
+	# scale of 0 to 2, assigning the resulting value to genfun_tty. If no
+	# terminal is detected, the value shall be 0. If a dumb terminal is
+	# detected, the value shall be 1. If a smart terminal is detected, the
+	# value shall be 2.
+	#
+	# In the case that a smart terminal is detected, its dimensions shall
+	# be assigned to genfun_cols and genfun_rows, and the position of the
+	# cursor shall be assigned to genfun_x and genfun_y. Further, it may
+	# reasonably be assumed that the ECMA-48 CSI and DECSC/DECRC escape
+	# sequences are supported.
+	if [ ! -t 0 ]; then
+		genfun_tty=0
+	elif _has_dumb_terminal || ! _update_winsize || ! _update_cursor_coords; then
+		genfun_tty=1
+	else
+		genfun_tty=2
+	fi
+}
+
 _update_winsize() {
 	# The following use of stty(1) is portable as of POSIX Issue 8. It would
 	# be beneficial to leverage the checkwinsize option in bash but the
@@ -486,7 +602,7 @@ _update_winsize() {
 	# it may eventually become possible to support it.
 	# shellcheck disable=2046
 	set -- $(stty size 2>/dev/null)
-	if is_int "$1" && is_int "$2" && [ "$1" -gt 0 ] && [ "$2" -gt 0 ]; then
+	if [ "$#" -eq 2 ] && is_int "$1" && is_int "$2"; then
 		genfun_rows=$1
 		genfun_cols=$2
 	else
@@ -496,6 +612,19 @@ _update_winsize() {
 	fi
 }
 
+_update_cursor_coords() {
+	# shellcheck disable=2046
+	set -- $(./ecma48-cpr)
+	if [ "$#" -eq 2 ] && is_int "$1" && is_int "$2"; then
+		genfun_y=$1
+		genfun_x=$2
+	else
+		genfun_y=
+		genfun_x=
+		false
+	fi
+}
+
 # This is the main script, please add all functions above this point!
 # shellcheck disable=2034
 RC_GOT_FUNCTIONS="yes"
@@ -530,20 +659,12 @@ else
 	done
 fi
 
-if _has_dumb_terminal || ! _update_winsize; then
-	unset -v genfun_endcol
-else
-	# Set some ECMA-48 CSI sequences (CUU and CUF) for cursor positioning.
-	# These are standard and, conveniently, documented by console_codes(4).
-	genfun_endcol="\\033[A\\033[$(( genfun_cols - 7 ))C"
-fi
-
 if _has_monochrome_terminal || yesno "${RC_NOCOLOR}"; then
 	unset -v BAD BRACKET GOOD HILITE NORMAL WARN
 else
 	# Define some ECMA-48 SGR sequences for color support. These variables
 	# are public, in so far as users of the library may be expanding them.
-	# The sequences are also documented by console_codes(4).
+	# Conveniently, these sequences are documented by console_codes(4).
 	BAD=$(printf '\033[31;01m')
 	BRACKET=$(printf '\033[34;01m')
 	GOOD=$(printf '\033[32;01m')