apply --whitespace=warn/error: diagnose blank at EOF

"git apply" strips new blank lines at EOF under --whitespace=fix option,
but neigher --whitespace=warn nor --whitespace=error paid any attention to
these errors.

Introduce a new whitespace error class, blank-at-eof, to make the
whitespace error handling more consistent.

The patch adds a new "linenr" field to the struct fragment in order to
record which line the hunk started in the input file, but this is needed
solely for reporting purposes.  The detection of this class of whitespace
errors cannot be done while parsing a patch like we do for all the other
classes of whitespace errors.  It instead has to wait until we find where
to apply the hunk, but at that point, we do not have an access to the
original line number in the input file anymore, hence the new field.

Depending on your point of view, this may be a bugfix that makes warn and
error in line with fix.  Or you could call it a new feature.  The line
between them is somewhat fuzzy in this case.

Strictly speaking, triggering more errors than before is a change in
behaviour that is not backward compatible, even though the reason for the
change is because the code was not checking for an error that it should
have.  People who do not want added blank lines at EOF to trigger an error
can disable the new error class.

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

Documentation/config.txt

@@ -389,6 +389,8 @@ core.whitespace::
   error (enabled by default).
 * `indent-with-non-tab` treats a line that is indented with 8 or more
   space characters as an error (not enabled by default).
+* `blank-at-eof` treats blank lines added at the end of file as an error
+  (enabled by default).
 * `cr-at-eol` treats a carriage-return at the end of line as
   part of the line terminator, i.e. with it, `trailing-space`
   does not trigger if the character before such a carriage-return

builtin-apply.c

@@ -126,6 +126,7 @@ struct fragment {
 	const char *patch;
 	int size;
 	int rejected;
+	int linenr;
 	struct fragment *next;
 };
 
@@ -1193,6 +1194,7 @@ static int parse_single_patch(char *line, unsigned long size, struct patch *patc
 		int len;
 
 		fragment = xcalloc(1, sizeof(*fragment));
+		fragment->linenr = linenr;
 		len = parse_fragment(line, size, patch, fragment);
 		if (len <= 0)
 			die("corrupt patch at line %d", linenr);
@@ -2079,17 +2081,24 @@ static int apply_one_fragment(struct image *img, struct fragment *frag,
 	}
 
 	if (applied_pos >= 0) {
-		if (ws_error_action == correct_ws_error &&
-		    new_blank_lines_at_end &&
-		    preimage.nr + applied_pos == img->nr) {
+		if (new_blank_lines_at_end &&
+		    preimage.nr + applied_pos == img->nr &&
+		    (ws_rule & WS_BLANK_AT_EOF) &&
+		    ws_error_action != nowarn_ws_error) {
+			record_ws_error(WS_BLANK_AT_EOF, "+", 1, frag->linenr);
+			if (ws_error_action == correct_ws_error) {
+				while (new_blank_lines_at_end--)
+					remove_last_line(&postimage);
+			}
 			/*
-			 * If the patch application adds blank lines
-			 * at the end, and if the patch applies at the
-			 * end of the image, remove those added blank
-			 * lines.
+			 * We would want to prevent write_out_results()
+			 * from taking place in apply_patch() that follows
+			 * the callchain led us here, which is:
+			 * apply_patch->check_patch_list->check_patch->
+			 * apply_data->apply_fragments->apply_one_fragment
 			 */
-			while (new_blank_lines_at_end--)
-				remove_last_line(&postimage);
+			if (ws_error_action == die_on_ws_error)
+				apply = 0;
 		}
 
 		/*

cache.h

@@ -845,7 +845,8 @@ void shift_tree(const unsigned char *, const unsigned char *, unsigned char *, i
 #define WS_SPACE_BEFORE_TAB	02
 #define WS_INDENT_WITH_NON_TAB	04
 #define WS_CR_AT_EOL           010
-#define WS_DEFAULT_RULE (WS_TRAILING_SPACE|WS_SPACE_BEFORE_TAB)
+#define WS_BLANK_AT_EOF        020
+#define WS_DEFAULT_RULE (WS_TRAILING_SPACE|WS_SPACE_BEFORE_TAB|WS_BLANK_AT_EOF)
 extern unsigned whitespace_rule_cfg;
 extern unsigned whitespace_rule(const char *);
 extern unsigned parse_whitespace_rule(const char *);

t/t4124-apply-ws-rule.sh

@@ -201,4 +201,30 @@ test_expect_success 'blank at end of hunk, not at EOF with --whitespace=fix' '
 	test_cmp expect one
 '
 
+test_expect_success 'blank at EOF with --whitespace=warn' '
+	{ echo a; echo b; echo c; } >one &&
+	git add one &&
+	echo >>one &&
+	cat one >expect &&
+	git diff -- one >patch &&
+
+	git checkout one &&
+	git apply --whitespace=warn patch 2>error &&
+	test_cmp expect one &&
+	grep "new blank line at EOF" error
+'
+
+test_expect_success 'blank at EOF with --whitespace=error' '
+	{ echo a; echo b; echo c; } >one &&
+	git add one &&
+	cat one >expect &&
+	echo >>one &&
+	git diff -- one >patch &&
+
+	git checkout one &&
+	test_must_fail git apply --whitespace=error patch 2>error &&
+	test_cmp expect one &&
+	grep "new blank line at EOF" error
+'
+
 test_done

ws.c

@@ -15,6 +15,7 @@ static struct whitespace_rule {
 	{ "space-before-tab", WS_SPACE_BEFORE_TAB },
 	{ "indent-with-non-tab", WS_INDENT_WITH_NON_TAB },
 	{ "cr-at-eol", WS_CR_AT_EOL },
+	{ "blank-at-eof", WS_BLANK_AT_EOF },
 };
 
 unsigned parse_whitespace_rule(const char *string)
@@ -113,6 +114,11 @@ char *whitespace_error_string(unsigned ws)
 			strbuf_addstr(&err, ", ");
 		strbuf_addstr(&err, "indent with spaces");
 	}
+	if (ws & WS_BLANK_AT_EOF) {
+		if (err.len)
+			strbuf_addstr(&err, ", ");
+		strbuf_addstr(&err, "new blank line at EOF");
+	}
 	return strbuf_detach(&err, NULL);
 }