apply: fix new-style empty context line triggering incomplete-line check

A new-style unified context diff represents an empty context line
with an empty line (instead of a line with a single SP on it).  The
code to check whitespace errors in an incoming patch is designed to
omit the first byte of a line (typically SP, "-", or "+") and pass the
remainder of the line to the whitespace checker.

Usually we do not pass a context line to the whitespace error checker,
but when we are correcting errors, we do.  This "remove the first
byte and send the remainder" strategy of checking a line ended up
sending a zero-length string to the whitespace checker when seeing a
new-style empty context line, which caused the whitespace checker to
say "ah, you do not even have a newline at the end!", leading to an
"incomplete line" in the middle of the patch!

Fix this by pretending that we got a traditional empty context line
when we drive the whitespace checker.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

apply.c

@@ -1796,8 +1796,16 @@ static int parse_fragment(struct apply_state *state,
 			trailing++;
 			check_old_for_crlf(patch, line, len);
 			if (!state->apply_in_reverse &&
-			    state->ws_error_action == correct_ws_error)
-				check_whitespace(state, line, len, patch->ws_rule);
+			    state->ws_error_action == correct_ws_error) {
+				const char *test_line = line;
+				int test_len = len;
+				if (*line == '\n') {
+					test_line = " \n";
+					test_len = 2;
+				}
+				check_whitespace(state, test_line, test_len,
+						 patch->ws_rule);
+			}
 			break;
 		case '-':
 			if (!state->apply_in_reverse)

t/t4124-apply-ws-rule.sh

@@ -561,6 +561,22 @@ test_expect_success 'check incomplete lines (setup)' '
 	git config core.whitespace incomplete-line
 '
 
+test_expect_success 'no incomplete context line (not an error)' '
+	test_when_finished "rm -f sample*-i patch patch-new target" &&
+	test_write_lines 1 2 3 "" 4 5 >sample-i &&
+	test_write_lines 1 2 3 "" 0 5 >sample2-i &&
+	cat sample-i >target &&
+	git add target &&
+	cat sample2-i >target &&
+	git diff-files -p target >patch &&
+	sed -e "s/^ $//" <patch >patch-new &&
+
+	cat sample-i >target &&
+	git apply --whitespace=fix <patch-new 2>error &&
+	test_cmp sample2-i target &&
+	test_must_be_empty error
+'
+
 test_expect_success 'incomplete context line (not an error)' '
 	(test_write_lines 1 2 3 4 5 && printf 6) >sample-i &&
 	(test_write_lines 1 2 3 0 5 && printf 6) >sample2-i &&