[PATCH 0/2] Fix xdiff's --ignore-space-at-eol handling

[PATCH 0/2] Fix xdiff's --ignore-space-at-eol handling

[Johannes Schindelin]

It turns out that I am not the only Git developer capable of producing
an off-by-one bug ;-)

This patch series fixes a bug where we ignored single-character changes
at the end of the lines when trying to ignore white space at the end of
the lines.

I split the changes into two patches because the fix turned out to have
a much broader scope than the test (which demonstrates just a symptom):
the bug was not in the patience-specific part of the diff code, after all.


Johannes Schindelin (2):
  diff: demonstrate a bug with --patience and --ignore-space-at-eol
  diff: fix a double off-by-one with --ignore-space-at-eol

 t/t4033-diff-patience.sh | 8 ++++++++
 xdiff/xpatience.c        | 2 +-
 xdiff/xutils.c           | 6 ++++--
 3 files changed, 13 insertions(+), 3 deletions(-)

Published-As: https://github.com/dscho/git/releases/tag/patience-v1
-- 
2.9.0.278.g1caae67

base-commit: 5c589a73de4394ad125a4effac227b3aec856fa1

[PATCH 1/2] diff: demonstrate a bug with --patience and --ignore-space-at-eol

[Johannes Schindelin]

When a single character is added to a line, the combination of these
two options results in an empty diff.

This bug was noticed and reported by Naja Melan.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/t4033-diff-patience.sh | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/t/t4033-diff-patience.sh b/t/t4033-diff-patience.sh
index 3c9932e..5f0d0b1 100755
--- a/t/t4033-diff-patience.sh
+++ b/t/t4033-diff-patience.sh
@@ -5,6 +5,14 @@ test_description='patience diff algorithm'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-diff-alternative.sh
 
+test_expect_failure '--ignore-space-at-eol with a single appended character' '
+	printf "a\nb\nc\n" >pre &&
+	printf "a\nbX\nc\n" >post &&
+	test_must_fail git diff --no-index \
+		--patience --ignore-space-at-eol pre post >diff &&
+	grep "^+.*X" diff
+'
+
 test_diff_frobnitz "patience"
 
 test_diff_unique "patience"
-- 
2.9.0.278.g1caae67

[PATCH 2/2] diff: fix a double off-by-one with --ignore-space-at-eol

[Johannes Schindelin]

When comparing two lines, ignoring any whitespace at the end, we first
try to match as many bytes as possible and break out of the loop only
upon mismatch, to let the remainder be handled by the code shared with
the other whitespace-ignoring code paths.

When comparing the bytes, however, we incremented the counters always,
even if the bytes did not match. And because we fall through to  the
space-at-eol handling at that point, it is as if that mismatch never
happened.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/t4033-diff-patience.sh | 2 +-
 xdiff/xpatience.c        | 2 +-
 xdiff/xutils.c           | 6 ++++--
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/t/t4033-diff-patience.sh b/t/t4033-diff-patience.sh
index 5f0d0b1..113304d 100755
--- a/t/t4033-diff-patience.sh
+++ b/t/t4033-diff-patience.sh
@@ -5,7 +5,7 @@ test_description='patience diff algorithm'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-diff-alternative.sh
 
-test_expect_failure '--ignore-space-at-eol with a single appended character' '
+test_expect_success '--ignore-space-at-eol with a single appended character' '
 	printf "a\nb\nc\n" >pre &&
 	printf "a\nbX\nc\n" >post &&
 	test_must_fail git diff --no-index \
diff --git a/xdiff/xpatience.c b/xdiff/xpatience.c
index 04e1a1a..a613efc 100644
--- a/xdiff/xpatience.c
+++ b/xdiff/xpatience.c
@@ -1,6 +1,6 @@
 /*
  *  LibXDiff by Davide Libenzi ( File Differential Library )
- *  Copyright (C) 2003-2009 Davide Libenzi, Johannes E. Schindelin
+ *  Copyright (C) 2003-2016 Davide Libenzi, Johannes E. Schindelin
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
diff --git a/xdiff/xutils.c b/xdiff/xutils.c
index 62cb23d..027192a 100644
--- a/xdiff/xutils.c
+++ b/xdiff/xutils.c
@@ -200,8 +200,10 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
 				return 0;
 		}
 	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
-		while (i1 < s1 && i2 < s2 && l1[i1++] == l2[i2++])
-			; /* keep going */
+		while (i1 < s1 && i2 < s2 && l1[i1] == l2[i2]) {
+			i1++;
+			i2++;
+		}
 	}
 
 	/*
-- 
2.9.0.278.g1caae67

Re: [PATCH 2/2] diff: fix a double off-by-one with --ignore-space-at-eol

[Naja Melan]

Thanks,

you sure are efficient in bug fixing...

good day to you
Naja Melan

Re: [PATCH 2/2] diff: fix a double off-by-one with --ignore-space-at-eol

[Junio C Hamano]

Johannes Schindelin <johannes.schindelin@gmx.de> writes:

> diff --git a/xdiff/xutils.c b/xdiff/xutils.c
> index 62cb23d..027192a 100644
> --- a/xdiff/xutils.c
> +++ b/xdiff/xutils.c
> @@ -200,8 +200,10 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
>  				return 0;
>  		}
>  	} else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL) {
> -		while (i1 < s1 && i2 < s2 && l1[i1++] == l2[i2++])
> -			; /* keep going */
> +		while (i1 < s1 && i2 < s2 && l1[i1] == l2[i2]) {
> +			i1++;
> +			i2++;
> +		}
>  	}

When we notice l1[i1] and l2[i2] does not match, we want i1 and i2
to stay pointing at that unmatch.  The code before this fix however
ends up incrementing them before leaving the loop.

This breakage seems to come from 2344d47f (diff: fix 2 whitespace
issues, 2006-10-12)?  That's quite old and it is somewhat surprising
that nobody complained.

Well spotted.  Will queue.

Thanks.