flipdiff: Add option to fail loudly if patches don't commute

Makefile.am

@@ -394,6 +394,8 @@ TESTS = tests/newline1/run-test \
 	tests/flip17/run-test \
 	tests/flip18/run-test \
 	tests/flip19/run-test \
+	tests/flip-err-no-commute1/run-test \
+	tests/flip-err-no-commute2/run-test \
 	tests/unline1/run-test \
 	tests/nul0/run-test \
 	tests/nul1/run-test \

lib/.gitignore

@@ -68,3 +68,6 @@
 /fseterr.h
 /stdio-consolesafe.c
 /stdio-impl.h
+/memeq.c
+/stdcountof.in.h
+/streq.c

m4/.gitignore

@@ -71,3 +71,6 @@
 /eealloc.m4
 /locale-fr.m4
 /wchar_t.m4
+/memeq.m4
+/stdcountof_h.m4
+/streq.m4

m4/gnulib-cache.m4

@@ -1,4 +1,4 @@
-# Copyright (C) 2002-2025 Free Software Foundation, Inc.
+# Copyright (C) 2002-2026 Free Software Foundation, Inc.
 #
 # This file is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by

src/interdiff.c

@@ -122,6 +122,7 @@ static int no_revert_omitted = 0;
 static int use_colors = 0;
 static int color_option_specified = 0;
 static int debug = 0;
+static int err_no_commute = 0;
 
 static struct patlist *pat_drop_context = NULL;
 
@@ -2009,6 +2010,8 @@ flipdiff (FILE *p1, FILE *p2, FILE *flip1, FILE *flip2)
 				at += this_offset;
 				remove_line (&intermediate, line + 1, at);
 				this_offset--;
+			} else if (err_no_commute) {
+				error (EXIT_FAILURE, 0, "patches don't commute!");
 			}
 		} else if (line[0] == '-') {
 			if (!patch2_removes_line (linenum, offsets,
@@ -2019,6 +2022,8 @@ flipdiff (FILE *p1, FILE *p2, FILE *flip1, FILE *flip2)
 				at += this_offset;
 				insert_line (&intermediate, line + 1,
 					     (size_t) got - 1, at);
+			} else if (err_no_commute) {
+				error (EXIT_FAILURE, 0, "patches don't commute!");
 			}
 			this_offset++;
 		}
@@ -2260,7 +2265,10 @@ syntax (int err)
 "                  (interdiff) When a patch from patch1 is not in patch2,\n"
 "                  don't revert it\n"
 "  --in-place      (flipdiff) Write the output to the original input\n"
-"                  files\n";
+"                  files\n"
+"  --err-no-commute\n"
+"                  (flipdiff) If the patches don't commute, exit with an\n"
+"                  error\n";
 
 	fprintf (err ? stderr : stdout, syntax_str, progname, progname);
 	exit (err);
@@ -2333,6 +2341,7 @@ main (int argc, char *argv[])
 			{"color", 2, 0, 1000 + 'c'},
 			{"decompress", 0, 0, 'z'},
 			{"quiet", 0, 0, 'q'},
+			{"err-no-commute", 0, 0, 1000 + 'E'},
 			{0, 0, 0, 0}
 		};
 		char *end;
@@ -2412,6 +2421,9 @@ main (int argc, char *argv[])
 		case 1000 + 'D':
 			debug = 1;
 			break;
+		case 1000 + 'E':
+			err_no_commute = 1;
+			break;
 		default:
 			syntax(1);
 		}

tests/flip-err-no-commute1/run-test

@@ -0,0 +1,36 @@
+#!/bin/sh
+
+# This is a flipdiff(1) testcase.
+
+
+. ${top_srcdir-.}/tests/common.sh
+
+# Like flip15, but with --err-no-commute.
+# When a line is touched by both patches and --err-no-commute is passed,
+# we must quit with an error, and not make any changes.
+
+cat << EOF > file.orig
+int a;
+EOF
+
+cat << EOF > file
+int a, b;
+EOF
+
+${DIFF} -u file.orig file > patch1
+
+mv -f file file.orig
+cat << EOF > file
+ssize_t a, b;
+EOF
+
+${DIFF} -u file.orig file > patch2
+
+cp patch1 patch1.orig
+cp patch2 patch2.orig
+
+! ${FLIPDIFF} --err-no-commute patch1 patch2 > patch-flipped 2>errors || exit 1
+[ -s errors ] || exit 1
+! [ -s patch-flipped ] || exit 1
+
+exit 0

tests/flip-err-no-commute2/run-test

@@ -0,0 +1,36 @@
+#!/bin/sh
+
+# This is a flipdiff(1) testcase.
+# Like flip1, but with --err-no-commute.
+# Test that we error because the patches don't commute.
+
+
+. ${top_srcdir-.}/tests/common.sh
+
+cat << EOF > file.orig
+a
+b
+c
+EOF
+
+cat << EOF > file
+a
+d
+c
+EOF
+
+${DIFF} -u file.orig file > patch1
+
+mv -f file file.orig
+cat << EOF > file
+a
+c
+EOF
+
+${DIFF} -u file.orig file > patch2
+
+! ${FLIPDIFF} --err-no-commute patch1 patch2 > patch-flipped 2>errors || exit 1
+[ -s errors ] || exit 1
+! [ -s patch-flipped ] || exit 1
+
+exit 0