sha1_file: pass empty buffer to index empty file

`git add` of an empty file with a filter pops complaints from
`copy_fd` about a bad file descriptor.

This traces back to these lines in sha1_file.c:index_core:

	if (!size) {
		ret = index_mem(sha1, NULL, size, type, path, flags);

The problem here is that content to be added to the index can be
supplied from an fd, or from a memory buffer, or from a pathname. This
call is supplying a NULL buffer pointer and a zero size.

Downstream logic takes the complete absence of a buffer to mean the
data is to be found elsewhere -- for instance, these, from convert.c:

	if (params->src) {
		write_err = (write_in_full(child_process.in, params->src, params->size) < 0);
	} else {
		write_err = copy_fd(params->fd, child_process.in);
	}

~If there's a buffer, write from that, otherwise the data must be coming
from an open fd.~

Perfectly reasonable logic in a routine that's going to write from
either a buffer or an fd.

So change `index_core` to supply an empty buffer when indexing an empty
file.

There's a patch out there that instead changes the logic quoted above to
take a `-1` fd to mean "use the buffer", but it seems to me that the
distinction between a missing buffer and an empty one carries intrinsic
semantics, where the logic change is adapting the code to handle
incorrect arguments.

Signed-off-by: Jim Hill <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jthill

sha1_file.c

@@ -3119,7 +3119,7 @@ static int index_core(unsigned char *sha1, int fd, size_t size,
 	int ret;
 
 	if (!size) {
-		ret = index_mem(sha1, NULL, size, type, path, flags);
+		ret = index_mem(sha1, "", size, type, path, flags);
 	} else if (size <= SMALL_FILE_SIZE) {
 		char *buf = xmalloc(size);
 		if (size == read_in_full(fd, buf, size))

t/t0021-conversion.sh

@@ -204,4 +204,30 @@ test_expect_success EXPENSIVE 'filter large file' '
 	! test -s err
 '
 
+test_expect_success "filter: clean empty file" '
+	git config filter.in-repo-header.clean  "echo cleaned && cat" &&
+	git config filter.in-repo-header.smudge "sed 1d" &&
+
+	echo "empty-in-worktree    filter=in-repo-header" >>.gitattributes &&
+	>empty-in-worktree &&
+
+	echo cleaned >expected &&
+	git add empty-in-worktree &&
+	git show :empty-in-worktree >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success "filter: smudge empty file" '
+	git config filter.empty-in-repo.clean "cat >/dev/null" &&
+	git config filter.empty-in-repo.smudge "echo smudged && cat" &&
+
+	echo "empty-in-repo filter=empty-in-repo" >>.gitattributes &&
+	echo dead data walking >empty-in-repo &&
+	git add empty-in-repo &&
+
+	echo smudged >expected &&
+	git checkout-index --prefix=filtered- empty-in-repo &&
+	test_cmp expected filtered-empty-in-repo
+'
+
 test_done