grep: fix match highlighting for combined patterns with context lines

When git grep is run with combined patterns such as '-e p1 --and -e
p2' and surrounding context lines are requested, the output contains
incorrectly highlighted matches.

Consider the following output (highlighted matches are surrounded by '*'
characters):
    $ cat testfile
    foo a
    foo b
    foo bar
    baz bar foo
    bar x
    bar y
    $ git grep -n -C2 -e foo --and -e bar testfile
    testfile-1-*foo* a
    testfile-2-*foo* b
    testfile:3:*foo* *bar*
    testfile:4:baz *bar* *foo*
    testfile-5-*bar* x
    testfile-6-*bar* y

Lines 1, 2, 5 and 6 do not match the combined patterns, they only
contain incorrectly highlighted 'false positives'.

Modify the show_line() function in grep.c to highlight matches only on
lines that match the combined pattern. Do not highlight matches on lines
that provide only context or contain only the function name of the match.

The output of the same command after the change:
    $ git grep -n -C2 -e foo --and -e bar testfile
    testfile-1-foo a
    testfile-2-foo b
    testfile:3:*foo* *bar*
    testfile:4:baz *bar* *foo*
    testfile-5-bar x
    testfile-6-bar y

Also avoid unnecessary regmatch on lines we do not split by
moving the code to paint a single line piece-by-piece only
to apply to the matched line.

Signed-off-by: Zoltan Klinger <zoltan.klinger@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -1112,31 +1112,33 @@ static void show_line(struct grep_opt *opt, char *bol, char *eol,
 		output_sep(opt, sign);
 	}
 	if (opt->color) {
-		regmatch_t match;
-		enum grep_context ctx = GREP_CONTEXT_BODY;
-		int ch = *eol;
-		int eflags = 0;
+		if (sign == ':') {
+			/* paint the hits on matched lines */
+			regmatch_t match;
+			enum grep_context ctx = GREP_CONTEXT_BODY;
+			int ch = *eol;
+			int eflags = 0;
 
-		if (sign == ':')
 			line_color = opt->color_selected;
-		else if (sign == '-')
+			*eol = '\0';
+			while (next_match(opt, bol, eol, ctx, &match, eflags)) {
+				if (match.rm_so == match.rm_eo)
+					break;
+
+				output_color(opt, bol, match.rm_so, line_color);
+				output_color(opt, bol + match.rm_so,
+					     match.rm_eo - match.rm_so,
+					     opt->color_match);
+				bol += match.rm_eo;
+				rest -= match.rm_eo;
+				eflags = REG_NOTBOL;
+			}
+			*eol = ch;
+		} else if (sign == '-') {
 			line_color = opt->color_context;
-		else if (sign == '=')
+		} else if (sign == '=') {
 			line_color = opt->color_function;
-		*eol = '\0';
-		while (next_match(opt, bol, eol, ctx, &match, eflags)) {
-			if (match.rm_so == match.rm_eo)
-				break;
-
-			output_color(opt, bol, match.rm_so, line_color);
-			output_color(opt, bol + match.rm_so,
-				     match.rm_eo - match.rm_so,
-				     opt->color_match);
-			bol += match.rm_eo;
-			rest -= match.rm_eo;
-			eflags = REG_NOTBOL;
 		}
-		*eol = ch;
 	}
 	output_color(opt, bol, rest, line_color);
 	opt->output(opt, "\n", 1);

t/t7810-grep.sh

@@ -1195,4 +1195,94 @@ test_expect_success LIBPCRE 'grep -P "^ "' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+space-line without leading space1
+space: line <RED>with <RESET>leading space1
+space: line <RED>with <RESET>leading <RED>space2<RESET>
+space: line <RED>with <RESET>leading space3
+space:line without leading <RED>space2<RESET>
+EOF
+
+test_expect_success 'grep --color -e A -e B with context' '
+	test_config color.grep.context		normal &&
+	test_config color.grep.filename		normal &&
+	test_config color.grep.function		normal &&
+	test_config color.grep.linenumber	normal &&
+	test_config color.grep.match		red &&
+	test_config color.grep.selected		normal &&
+	test_config color.grep.separator	normal &&
+
+	git grep --color=always -C2 -e "with " -e space2  space |
+	test_decode_color >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+space-line without leading space1
+space- line with leading space1
+space: line <RED>with <RESET>leading <RED>space2<RESET>
+space- line with leading space3
+space-line without leading space2
+EOF
+
+test_expect_success 'grep --color -e A --and -e B with context' '
+	test_config color.grep.context		normal &&
+	test_config color.grep.filename		normal &&
+	test_config color.grep.function		normal &&
+	test_config color.grep.linenumber	normal &&
+	test_config color.grep.match		red &&
+	test_config color.grep.selected		normal &&
+	test_config color.grep.separator	normal &&
+
+	git grep --color=always -C2 -e "with " --and -e space2  space |
+	test_decode_color >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+space-line without leading space1
+space: line <RED>with <RESET>leading space1
+space- line with leading space2
+space: line <RED>with <RESET>leading space3
+space-line without leading space2
+EOF
+
+test_expect_success 'grep --color -e A --and --not -e B with context' '
+	test_config color.grep.context		normal &&
+	test_config color.grep.filename		normal &&
+	test_config color.grep.function		normal &&
+	test_config color.grep.linenumber	normal &&
+	test_config color.grep.match		red &&
+	test_config color.grep.selected		normal &&
+	test_config color.grep.separator	normal &&
+
+	git grep --color=always -C2 -e "with " --and --not -e space2  space |
+	test_decode_color >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
+hello.c-#include <stdio.h>
+hello.c=int main(int argc, const char **argv)
+hello.c-{
+hello.c:	pr<RED>int<RESET>f("<RED>Hello<RESET> world.\n");
+hello.c-	return 0;
+hello.c-	/* char ?? */
+hello.c-}
+EOF
+
+test_expect_success 'grep --color -e A --and -e B -p with context' '
+	test_config color.grep.context		normal &&
+	test_config color.grep.filename		normal &&
+	test_config color.grep.function		normal &&
+	test_config color.grep.linenumber	normal &&
+	test_config color.grep.match		red &&
+	test_config color.grep.selected		normal &&
+	test_config color.grep.separator	normal &&
+
+	git grep --color=always -p -C3 -e int --and -e Hello --no-index hello.c |
+	test_decode_color >actual &&
+	test_cmp expected actual
+'
+
 test_done