[PATCH] sha1_file: pass empty buffer to index empty file

`git add` of an empty file with a filter currently 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 <redacted>
---
 sha1_file.c                        |  2 +-
 t/t2205-add-empty-filtered-file.sh | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100755 t/t2205-add-empty-filtered-file.sh

diff --git a/sha1_file.c b/sha1_file.c
index f860d67..61e2735 100644
--- a/sha1_file.c
+++ b/sha1_file.c

@@ -3186,7 +3186,7 @@ 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))

diff --git a/t/t2205-add-empty-filtered-file.sh b/t/t2205-add-empty-filtered-file.sh
new file mode 100755
index 0000000..28903c4
--- /dev/null
+++ b/t/t2205-add-empty-filtered-file.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+test_description='adding empty filtered file'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	echo "* filter=test" >>.gitattributes &&
+	git config filter.test.clean cat &&
+	git config filter.test.smudge cat &&
+	git add . &&
+	git commit -m-
+
+'
+
+test_expect_success "add of empty filtered file produces no complaints" '
+	touch emptyfile &&
+	git add emptyfile 2>out &&
+	test -e out -a ! -s out
+'
+test_done

-- 
2.4.1.4.gd9c648d