Handle integer overflow as a special case in _should_throttle()

At the point that the genfun_time variable overflows, guarantee that the
should_throttle() function behaves as if no throttling should occur
rather than proceed to perform arithmetic based on the result of
deducting genfun_last_time from genfun_time.

Further, guarantee that the should_throttle() function behaves as if no
throttling should occur upon the very first occasion that it is called,
provided that the call to update_time() succeeds.

Finally, add a test case.

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

2 files changed, 62 insertions, 7 deletions

diff --git a/functions.sh b/functions.sh
index 78830c6..e7ef6fe 100644
--- a/functions.sh
+++ b/functions.sh
@@ -756,15 +756,25 @@ _select_by_mtime()
 #
 # Considers the first parameter as a number of centiseconds and determines
 # whether fewer have elapsed since the last occasion on which the function was
-# called.
+# called, or whether the last genfun_time update resulted in integer overflow.
 #
 _should_throttle()
 {
 	_update_time || return
-	if [ "$(( genfun_time - genfun_last_time > $1 ))" -eq 1 ]; then
-		genfun_last_time=${genfun_time}
-		false
-	fi
+
+	_should_throttle()
+	{
+		_update_time || return
+		if [ "$(( (genfun_time < 0 && genfun_last_time >= 0) || genfun_time - genfun_last_time > $1 ))" -eq 1 ]
+		then
+			genfun_last_time=${genfun_time}
+			false
+		fi
+
+	}
+
+	genfun_last_time=${genfun_time}
+	false
 }
 
 #
@@ -817,8 +827,6 @@ _update_columns()
 #
 _update_time()
 {
-	genfun_last_time=0
-
 	# shellcheck disable=3028
 	if [ "${BASH_VERSINFO:-0}" -ge 5 ]; then
 		# shellcheck disable=2034,3045
diff --git a/test-functions b/test-functions
index 1f54208..34ffc41 100755
--- a/test-functions
+++ b/test-functions
@@ -1069,6 +1069,52 @@ test_update_time() {
 	iterate_tests 3 "$@"
 }
 
+test_should_throttle() {
+	local bits max_int
+
+	genfun_time=
+	bits=30
+	while [ "${bits}" -lt 128 ]; do
+		# Dash is buggy and fails to handle $(( 1 << ++bits )).
+		bits=$(( bits + 1 ))
+		case $(( max_int = 1 << bits )) in
+			-*)
+				max_int=$(( max_int - 1 ))
+				genfun_time=$(( max_int - 4 ))
+				break
+		esac
+	done
+
+	if [ ! "${genfun_time}" ]; then
+		bailout "Failed to calculate the maximum possible integer value"
+	fi
+
+	# For the first test, genfun_last_time is not yet known. Therefore, the
+	# return value should always be 1. For the fifth test, integer overflow
+	# is expected to occur. Again, the return value should always be 1.
+	set -- \
+		ge  1  "${max_int}" \
+		eq  0  2            \
+		ge  1  1            \
+		ge  1  0            \
+		ge  1  2            \
+		eq  0  2            \
+		ge  1  1            \
+		ge  1  0
+
+	_update_time() {
+		true
+	}
+
+	callback() {
+		shift
+		test_description="_should_throttle $1 (${genfun_time}, $(( genfun_time += 1 )))"
+		_should_throttle "$1"
+	}
+
+	iterate_tests 3 "$@"
+}
+
 iterate_tests() {
 	local code i j passed slice_width total
 
@@ -1152,6 +1198,7 @@ else
 	test_assign || rc=1
 	test_deref || rc=1
 	test_update_time || rc=1
+	test_should_throttle || rc=1
 fi
 
 cleanup_tmpdir