From 592dbcf6579f6903d8ff4f306bb699b3148e2a49 Mon Sep 17 00:00:00 2001
From: Sai Asish Y <say.apm35@gmail.com>
Date: Thu, 18 Jun 2026 11:38:42 -0700
Subject: [PATCH] flipdiff: avoid crash on non-commuting patches

Signed-off-by: Sai Asish Y <say.apm35@gmail.com>
---
 Makefile.am           |  1 +
 src/interdiff.c       |  9 +++++++++
 tests/flip20/run-test | 30 ++++++++++++++++++++++++++++++
 3 files changed, 40 insertions(+)
 create mode 100755 tests/flip20/run-test

diff --git a/Makefile.am b/Makefile.am
index 55e0717..edc8c5b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -286,6 +286,7 @@ TESTS = tests/newline1/run-test \
 	tests/flip17/run-test \
 	tests/flip18/run-test \
 	tests/flip19/run-test \
+	tests/flip20/run-test \
 	tests/unline1/run-test \
 	tests/nul0/run-test \
 	tests/nul1/run-test \
diff --git a/src/interdiff.c b/src/interdiff.c
index 96f5a92..a3c1ecb 100644
--- a/src/interdiff.c
+++ b/src/interdiff.c
@@ -1661,6 +1661,15 @@ remove_line (struct lines_info *lines, const char *line,
 		}
 	}
 
+	if (!kill) {
+		/* The line we were asked to remove is not present (for
+		 * example when the two patches do not commute).  There is
+		 * nothing to do. */
+		if (debug)
+			printf ("Nothing to remove at %lu: %s", n, line);
+		return;
+	}
+
 	at = kill;
 	while (at) {
 		at->n--;
diff --git a/tests/flip20/run-test b/tests/flip20/run-test
new file mode 100755
index 0000000..85c5e08
--- /dev/null
+++ b/tests/flip20/run-test
@@ -0,0 +1,30 @@
+#!/bin/sh
+
+# This is a flipdiff(1) testcase.
+# Test: non-commuting patches where the second patch removes a line that
+# the first patch had added.  flipdiff used to crash on this input.
+
+
+. ${top_srcdir-.}/tests/common.sh
+
+cat << EOF > file.orig
+a
+b
+EOF
+
+cat << EOF > file
+a-m
+b-y
+EOF
+
+${DIFF} -u file.orig file > patch1
+
+mv -f file file.orig
+cat << EOF > file
+a
+b-y
+EOF
+
+${DIFF} -u file.orig file > patch2
+
+${FLIPDIFF} patch1 patch2 > patch-flipped || exit 1