[PATCH v2] diff: do not short-cut CHECK_SIZE_ONLY check in diff_populate_filespec()

[Junio C Hamano <gitster@pobox.com>]

Callers of diff_populate_filespec() can choose to ask only for the
size of the blob without grabbing the blob data, and the function,
after running lstat() when the filespec points at a working tree
file, returns by copying the value in size field of the stat
structure into the size field of the filespec when this is the case.

However, this short-cut cannot be taken if the contents from the
path needs to go through convert_to_git(), whose resulting real blob
data may be different from what is in the working tree file.

As "git diff --quiet" compares the .size fields of filespec
structures to skip content comparison, this bug manifests as a
false "there are differences" for a file that needs eol conversion,
for example.

Reported-by: Mike Crowe <mac@mcrowe.com>
Helped-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * With "test size_only to avoid more expensive would_convert call"
   fix applied.  Also the new test is now in t4xxx that it belongs
   to.

 diff.c                | 19 ++++++++++++++++++-
 t/t4035-diff-quiet.sh |  9 +++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/diff.c b/diff.c
index 059123c5dc..37e60ca601 100644
--- a/diff.c
+++ b/diff.c
@@ -2783,8 +2783,25 @@ int diff_populate_filespec(struct diff_filespec *s, unsigned int flags)
 			s->should_free = 1;
 			return 0;
 		}
-		if (size_only)
+
+		/*
+		 * Even if the caller would be happy with getting
+		 * only the size, we cannot return early at this
+		 * point if the path requires us to run the content
+		 * conversion.
+		 */
+		if (size_only && !would_convert_to_git(s->path))
 			return 0;
+
+		/*
+		 * Note: this check uses xsize_t(st.st_size) that may
+		 * not be the true size of the blob after it goes
+		 * through convert_to_git().  This may not strictly be
+		 * correct, but the whole point of big_file_threshold
+		 * and is_binary check being that we want to avoid
+		 * opening the file and inspecting the contents, this
+		 * is probably fine.
+		 */
 		if ((flags & CHECK_BINARY) &&
 		    s->size > big_file_threshold && s->is_binary == -1) {
 			s->is_binary = 1;
diff --git a/t/t4035-diff-quiet.sh b/t/t4035-diff-quiet.sh
index 461f4bb583..2f1737fcef 100755
--- a/t/t4035-diff-quiet.sh
+++ b/t/t4035-diff-quiet.sh
@@ -152,4 +152,13 @@ test_expect_success 'git diff --quiet ignores stat-change only entries' '
 	test_expect_code 1 git diff --quiet
 '
 
+test_expect_success 'git diff --quiet on a path that need conversion' '
+	echo "crlf.txt text=auto" >.gitattributes &&
+	printf "Hello\r\nWorld\r\n" >crlf.txt &&
+	git add .gitattributes crlf.txt &&
+
+	printf "Hello\r\nWorld\n" >crlf.txt &&
+	git diff --quiet crlf.txt
+'
+
 test_done
-- 
2.12.0-352-gb05ccab5eb