test: Test less detection when given a file path

less called as `less /path/file` shows up in `ps` with the path
appended; basename-ing the whole command line returns the file name
instead of `less`, so batpipe fails to detect less and disables color.
The `less` shim is extended to honor a piped LESSOPEN and report itself
as `less` (like the fish/nu shims) so the test can exercise this.

test/shimexec/less

@@ -12,10 +12,27 @@ EOF
 exit 0
 fi
 
-FILES=()
-while [[ $# -gt 0 ]]; do
-    -*) : ;;
-    *)  FILES+=("$1")
+# Re-exec with argv[0] set to `less` (like the fish/nu shims) so this process
+# is reported as `less` by `ps`. batpipe identifies its parent process by name;
+# a plain script would show up as `bash .../less` and hide us.
+if [[ -z "${_LESS_SHIM_REEXEC:-}" ]]; then
+	export _LESS_SHIM_REEXEC=1
+	exec -a "${SHIM_ARGV0:-less}" bash "$0" "$@"
+fi
+
+# Collect file operands, ignoring options -- like the real less CLI.
+files=()
+for arg; do
+	[[ "$arg" == -* ]] || files+=("$arg")
 done
 
-cat "${FILES[@]}"
+# Honor an input-pipe LESSOPEN preprocessor (`|command %s`) the way real less
+# does: feed each file through it. Otherwise just print the file.
+for file in "${files[@]}"; do
+	if [[ "${LESSOPEN-}" == "|"* ]]; then
+		command="${LESSOPEN#|}"
+		eval "${command//%s/$file}"
+	else
+		cat "$file"
+	fi
+done

test/suite/batpipe.sh

@@ -42,6 +42,23 @@ test:detected_nu_shell() {
 	grep '^\$env' <<< "$output" >/dev/null || fail 'Detected wrong shell when checking parent process.'
 }
 
+test:detected_less_with_path_argument() {
+	description "Test it detects less when less opens a file by an absolute path."
+
+	# Drive less, letting it invoke batpipe as its LESSOPEN preprocessor, with a
+	# file opened by an absolute path -- exactly how files are normally opened
+	# (`less /home/user/file.txt`). less's command line then ends in that path;
+	# batpipe must detect "less" from the executable name only. Taking the
+	# basename of the *whole* command line yields the file's basename instead,
+	# so batpipe wrongly concludes it is not inside less and disables color.
+	export BATPIPE_DEBUG=1
+	export LESSOPEN="|$(batpipe_path) %s"
+
+	output="$(less "${PWD}/file.txt" 2>&1)"
+	grep 'BATPIPE_INSIDE_LESS: true' <<< "$output" >/dev/null \
+		|| fail "Did not detect less as the parent when opening a file by an absolute path."
+}
+
 test:viewer_gzip() {
 	description "Test it can view .gz files."
 	command -v "gunzip" &>/dev/null || skip "Test requires gunzip."