diff: don't use pathname-based diff drivers for symlinks

When we're diffing symlinks, we consider the contents to be
the pathname that the symlink points to. When a user sets up
a userdiff driver like "*.pdf diff=pdf", their "diff.pdf.*"
config generally tells us what to do with the content of
pdf files.

With the current code, we will actually process a symlink
like "link.pdf" using a configured pdf driver, meaning we
are using contents which consist of a pathname with
configuration that is expecting contents that consist of an
actual pdf file.

The most noticeable example of this would have been
textconv; however, it was already protected in its own
textconv-specific code path. We can still see the breakage
with something like "diff.*.binary", though. You could
also see it with diff.*.funcname, though it is a bit harder
to trigger accidentally there.

This patch adds a check for S_ISREG lower in the callstack
than the textconv-specific check, which should block use of
any userdiff config for non-regular files. We can drop the
check in the textconv code, which is now redundant.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1764,8 +1764,14 @@ static void emit_binary_diff(FILE *file, mmfile_t *one, mmfile_t *two, char *pre
 
 static void diff_filespec_load_driver(struct diff_filespec *one)
 {
-	if (!one->driver)
+	/* Use already-loaded driver */
+	if (one->driver)
+		return;
+
+	if (S_ISREG(one->mode))
 		one->driver = userdiff_find_by_path(one->path);
+
+	/* Fallback to default settings */
 	if (!one->driver)
 		one->driver = userdiff_find_by_name("default");
 }
@@ -1813,8 +1819,7 @@ struct userdiff_driver *get_textconv(struct diff_filespec *one)
 {
 	if (!DIFF_FILE_VALID(one))
 		return NULL;
-	if (!S_ISREG(one->mode))
-		return NULL;
+
 	diff_filespec_load_driver(one);
 	if (!one->driver->textconv)
 		return NULL;

t/t4011-diff-symlink.sh

@@ -94,4 +94,30 @@ test_expect_success \
     test_must_fail git diff --no-index pinky brain > output 2> output.err &&
     grep narf output &&
     ! grep error output.err'
+
+test_expect_success SYMLINKS 'setup symlinks with attributes' '
+	echo "*.bin diff=bin" >>.gitattributes &&
+	echo content >file.bin &&
+	ln -s file.bin link.bin &&
+	git add -N file.bin link.bin
+'
+
+cat >expect <<'EOF'
+diff --git a/file.bin b/file.bin
+index e69de29..d95f3ad 100644
+Binary files a/file.bin and b/file.bin differ
+diff --git a/link.bin b/link.bin
+index e69de29..dce41ec 120000
+--- a/link.bin
++++ b/link.bin
+@@ -0,0 +1 @@
++file.bin
+\ No newline at end of file
+EOF
+test_expect_success SYMLINKS 'symlinks do not respect userdiff config by path' '
+	git config diff.bin.binary true &&
+	git diff file.bin link.bin >actual &&
+	test_cmp expect actual
+'
+
 test_done