grep: remove regflags from the public grep_opt API

Refactor calls to the grep machinery to always pass opt.ignore_case &
opt.extended_regexp_option instead of setting the equivalent regflags
bits.

The bug fixed when making -i work with -P in commit 9e3cbc59d5 ("log:
make --regexp-ignore-case work with --perl-regexp", 2017-05-20) was
really just plastering over the code smell which this change fixes.

The reason for adding the extensive commentary here is that I
discovered some subtle complexity in implementing this that really
should be called out explicitly to future readers.

Before this change we'd rely on the difference between
`extended_regexp_option` and `regflags` to serve as a membrane between
our preliminary parsing of grep.extendedRegexp and grep.patternType,
and what we decided to do internally.

Now that those two are the same thing, it's necessary to unset
`extended_regexp_option` just before we commit in cases where both of
those config variables are set. See 84befcd0a4 ("grep: add a
grep.patternType configuration setting", 2012-08-03) for the code and
documentation related to that.

The explanation of why the if/else branches in
grep_commit_pattern_type() are ordered the way they are exists in that
commit message, but I think it's worth calling this subtlety out
explicitly with a comment for future readers.

Even though grep_commit_pattern_type() is the only caller of
grep_set_pattern_type_option() it's simpler to reset the
extended_regexp_option flag in the latter, since 2/3 branches in the
former would otherwise need to reset it, this way we can do it in one
place.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -1169,8 +1169,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 
 	if (!opt.pattern_list)
 		die(_("no pattern given."));
-	if (!opt.fixed && opt.ignore_case)
-		opt.regflags |= REG_ICASE;
 
 	/*
 	 * We have to find "--" in a separate pass, because its presence

grep.c

@@ -35,7 +35,6 @@ void init_grep_defaults(void)
 	memset(opt, 0, sizeof(*opt));
 	opt->relative = 1;
 	opt->pathname = 1;
-	opt->regflags = REG_NEWLINE;
 	opt->max_depth = -1;
 	opt->pattern_type_option = GREP_PATTERN_TYPE_UNSPECIFIED;
 	color_set(opt->color_context, "");
@@ -153,7 +152,6 @@ void grep_init(struct grep_opt *opt, const char *prefix)
 	opt->linenum = def->linenum;
 	opt->max_depth = def->max_depth;
 	opt->pathname = def->pathname;
-	opt->regflags = def->regflags;
 	opt->relative = def->relative;
 	opt->output = def->output;
 
@@ -169,6 +167,24 @@ void grep_init(struct grep_opt *opt, const char *prefix)
 
 static void grep_set_pattern_type_option(enum grep_pattern_type pattern_type, struct grep_opt *opt)
 {
+	/*
+	 * When committing to the pattern type by setting the relevant
+	 * fields in grep_opt it's generally not necessary to zero out
+	 * the fields we're not choosing, since they won't have been
+	 * set by anything. The extended_regexp_option field is the
+	 * only exception to this.
+	 *
+	 * This is because in the process of parsing grep.patternType
+	 * & grep.extendedRegexp we set opt->pattern_type_option and
+	 * opt->extended_regexp_option, respectively. We then
+	 * internally use opt->extended_regexp_option to see if we're
+	 * compiling an ERE. It must be unset if that's not actually
+	 * the case.
+	 */
+	if (pattern_type != GREP_PATTERN_TYPE_ERE &&
+	    opt->extended_regexp_option)
+		opt->extended_regexp_option = 0;
+
 	switch (pattern_type) {
 	case GREP_PATTERN_TYPE_UNSPECIFIED:
 		/* fall through */
@@ -177,7 +193,7 @@ static void grep_set_pattern_type_option(enum grep_pattern_type pattern_type, st
 		break;
 
 	case GREP_PATTERN_TYPE_ERE:
-		opt->regflags |= REG_EXTENDED;
+		opt->extended_regexp_option = 1;
 		break;
 
 	case GREP_PATTERN_TYPE_FIXED:
@@ -207,6 +223,11 @@ void grep_commit_pattern_type(enum grep_pattern_type pattern_type, struct grep_o
 	else if (opt->pattern_type_option != GREP_PATTERN_TYPE_UNSPECIFIED)
 		grep_set_pattern_type_option(opt->pattern_type_option, opt);
 	else if (opt->extended_regexp_option)
+		/*
+		 * This branch *must* happen after setting from the
+		 * opt->pattern_type_option above, we don't want
+		 * grep.extendedRegexp to override grep.patternType!
+		 */
 		grep_set_pattern_type_option(GREP_PATTERN_TYPE_ERE, opt);
 }
 
@@ -572,7 +593,7 @@ static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
 	struct strbuf sb = STRBUF_INIT;
 	int err;
-	int regflags = opt->regflags;
+	int regflags = REG_NEWLINE;
 
 	basic_regex_quote_buf(&sb, p->pattern);
 	if (opt->ignore_case)
@@ -591,12 +612,12 @@ static void compile_fixed_regexp(struct grep_pat *p, struct grep_opt *opt)
 
 static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 {
-	int icase, ascii_only;
+	int ascii_only;
 	int err;
+	int regflags = REG_NEWLINE;
 
 	p->word_regexp = opt->word_regexp;
 	p->ignore_case = opt->ignore_case;
-	icase	       = opt->regflags & REG_ICASE || p->ignore_case;
 	ascii_only     = !has_non_ascii(p->pattern);
 
 	/*
@@ -614,10 +635,10 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 	if (opt->fixed ||
 	    has_null(p->pattern, p->patternlen) ||
 	    is_fixed(p->pattern, p->patternlen))
-		p->fixed = !icase || ascii_only;
+		p->fixed = !p->ignore_case || ascii_only;
 
 	if (p->fixed) {
-		p->kws = kwsalloc(icase ? tolower_trans_tbl : NULL);
+		p->kws = kwsalloc(p->ignore_case ? tolower_trans_tbl : NULL);
 		kwsincr(p->kws, p->pattern, p->patternlen);
 		kwsprep(p->kws);
 		return;
@@ -641,7 +662,11 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 		return;
 	}
 
-	err = regcomp(&p->regexp, p->pattern, opt->regflags);
+	if (p->ignore_case)
+		regflags |= REG_ICASE;
+	if (opt->extended_regexp_option)
+		regflags |= REG_EXTENDED;
+	err = regcomp(&p->regexp, p->pattern, regflags);
 	if (err) {
 		char errbuf[1024];
 		regerror(err, &p->regexp, errbuf, 1024);

grep.h

@@ -162,7 +162,6 @@ struct grep_opt {
 	char color_match_selected[COLOR_MAXLEN];
 	char color_selected[COLOR_MAXLEN];
 	char color_sep[COLOR_MAXLEN];
-	int regflags;
 	unsigned pre_context;
 	unsigned post_context;
 	unsigned last_shown;

revision.c

@@ -1362,7 +1362,6 @@ void init_revisions(struct rev_info *revs, const char *prefix)
 	init_grep_defaults();
 	grep_init(&revs->grep_filter, prefix);
 	revs->grep_filter.status_only = 1;
-	revs->grep_filter.regflags = REG_NEWLINE;
 
 	diff_setup(&revs->diffopt);
 	if (prefix && !revs->diffopt.prefix) {
@@ -2022,7 +2021,6 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->grep_filter.pattern_type_option = GREP_PATTERN_TYPE_ERE;
 	} else if (!strcmp(arg, "--regexp-ignore-case") || !strcmp(arg, "-i")) {
 		revs->grep_filter.ignore_case = 1;
-		revs->grep_filter.regflags |= REG_ICASE;
 		DIFF_OPT_SET(&revs->diffopt, PICKAXE_IGNORE_CASE);
 	} else if (!strcmp(arg, "--fixed-strings") || !strcmp(arg, "-F")) {
 		revs->grep_filter.pattern_type_option = GREP_PATTERN_TYPE_FIXED;