diff: let "git diff -O" read orderfile from any file and fail properly

The -O flag really shouldn't silently fail to do anything when given
a path that it can't read from.

However, it should be able to read from un-mmappable files, such as:

 * pipes/fifos

 * /dev/null:  It's a character device (at least on Linux)

 * ANY empty file:

   Quoting Linux mmap(2), "SUSv3 specifies that mmap() should fail if
   length is 0.  However, in kernels before 2.6.12, mmap() succeeded in
   this case: no mapping was created and the call returned addr.  Since
   kernel 2.6.12, mmap() fails with the error EINVAL for this case."

We especially want "-O/dev/null" to work, since we will be documenting
it as the way to cancel "diff.orderfile" when we add that.

(Note: "-O/dev/null" did have the right effect, since the existing error
handling essentially worked out to "silently ignore the orderfile".  But
this was probably more coincidence than anything else.)

So, lets toss all of that logic to get the file mmapped and just use
strbuf_read_file() instead, which gives us decent error handling
practically for free.

Signed-off-by: Samuel Bronson <naesten@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-order.c

@@ -10,28 +10,21 @@ static int order_cnt;
 
 static void prepare_order(const char *orderfile)
 {
-	int fd, cnt, pass;
+	int cnt, pass;
+	struct strbuf sb = STRBUF_INIT;
 	void *map;
 	char *cp, *endp;
-	struct stat st;
-	size_t sz;
+	ssize_t sz;
 
 	if (order)
 		return;
 
-	fd = open(orderfile, O_RDONLY);
-	if (fd < 0)
-		return;
-	if (fstat(fd, &st)) {
-		close(fd);
-		return;
-	}
-	sz = xsize_t(st.st_size);
-	map = mmap(NULL, sz, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
-	close(fd);
-	if (map == MAP_FAILED)
-		return;
+	sz = strbuf_read_file(&sb, orderfile, 0);
+	if (sz < 0)
+		die_errno(_("failed to read orderfile '%s'"), orderfile);
+	map = strbuf_detach(&sb, NULL);
 	endp = (char *) map + sz;
+
 	for (pass = 0; pass < 2; pass++) {
 		cnt = 0;
 		cp = map;

t/t4056-diff-order.sh

@@ -57,12 +57,38 @@ test_expect_success "no order (=tree object order)" '
 	test_cmp expect_none actual
 '
 
+test_expect_success 'missing orderfile' '
+	rm -f bogus_file &&
+	test_must_fail git diff -Obogus_file --name-only HEAD^..HEAD
+'
+
+test_expect_success POSIXPERM,SANITY 'unreadable orderfile' '
+	>unreadable_file &&
+	chmod -r unreadable_file &&
+	test_must_fail git diff -Ounreadable_file --name-only HEAD^..HEAD
+'
+
+test_expect_success 'orderfile is a directory' '
+	test_must_fail git diff -O/ --name-only HEAD^..HEAD
+'
+
 for i in 1 2
 do
 	test_expect_success "orderfile using option ($i)" '
 		git diff -Oorder_file_$i --name-only HEAD^..HEAD >actual &&
 		test_cmp expect_$i actual
 	'
+
+	test_expect_success PIPE "orderfile is fifo ($i)" '
+		rm -f order_fifo &&
+		mkfifo order_fifo &&
+		{
+			cat order_file_$i >order_fifo &
+		} &&
+		git diff -O order_fifo --name-only HEAD^..HEAD >actual &&
+		wait &&
+		test_cmp expect_$i actual
+	'
 done
 
 test_done