Re: BUG: git grep behave oddly with alternatives

["René Scharfe" <l.s.r@web.de>]

Am 06.01.23 um 10:09 schrieb Jeff King:
> On Wed, Jan 04, 2023 at 05:36:21PM +0100, René Scharfe wrote:
>
>>> I didn't test, but just from looking at the patch I'd expect this to
>>> affect other parts of Git besides git-grep. E.g., "git log --grep".
>>> Which raises two questions:
>>>
>>>  - would a more generalized name be better? USE_REG_ENHANCED or
>>>    something? That might be _too_ general, but see below.
>>>
>>>  - should this cover other cases? Grepping for "regcomp", would people
>>>    want this to behave consistently for "git config --get-regexp", or
>>>    diff funcnames, and so on?
>>>
>>> If so, then I could envision a USE_REG_ENHANCED which just wraps the
>>> system regcomp and adds the REG_ENHANCED flag when REG_EXTENDED is not
>>> set?
>>
>> Good point.  I don't know what people want, though.  re_format(7) on
>> macOS/BSD and regex(7) on Linux call basic REs "obsolete" and extended
>> REs "modern", so they seem to push people away from the old kind,
>> enhanced or not.
>
> Oh, good point. I was just grepping for regcomp(), but of course any
> case which is already passing REG_EXTENDED would not be affected anyway.
> And most places are already using that. E.g., the config code always
> does so, and it looks like pickaxe "-G" does so.
>
> For diffs, we have diff.*.xfuncname, which uses EREs. We do still
> support regular "funcname" for backwards compatibility, but we only
> document the extended version. Ironically, that option was introduced
> because BREs did not portably support things like alternation, even with
> the "enhanced" syntax. ;) See 45d9414fa5 (diff.*.xfuncname which uses
> "extended" regex's for hunk header selection, 2008-09-18).
>
> So I think we are embracing the "everyone should use EREs" mentality
> already. The only spots I see that use BREs are:
>
>   - grep.c, which handles "git grep" and "git log --grep"
>
>   - line-range.c, presumably for "-L" function matching
>
>   - deprecated non-ERE funcname patterns
>
> Your patch is handling the first, which is by the far most important. I
> would be OK leaving the others as-is, but I also wouldn't mind a patch
> that works at the regcomp() level to make things automatically
> consistent.

There's also the code handling "git log -i -S nonäscii" in
diffcore_pickaxe(), but it quotes special characters and thus
effectively does a fixed-string search, so you're right in omitting it
above.

REG_ENHANCED on macOS affects REG_EXTENDED as well.  It unlocks e.g.
non-greedy repetitions and inline comments.  Sounds nice, but also
potentially surprising.  I was unable to find a current version of
the re_format(7) manpage online, unfortunately.

Apple's latest version of Git sets NO_REGEX and thus uses
compat/regex, if I read their source correctly:

https://github.com/apple-oss-distributions/Git/blob/Git-128/src/git/Makefile#L1302

The easiest and most consistent option would be to do the same.  But
we can't do that, because it would break multibyte support, which was
fixed by 1819ad327b (grep: fix multibyte regex handling under macOS,
2022-08-26), which started to use the system regex functions again.

Which begs the question: Isn't that a problem for the platforms that
still have to use NO_REGEX?  Shouldn't we fix compat/regex?

Anyway, here's an attempt at a more general, but still targeted fix
for macOS:

--- >8 ---
Subject: [PATCH] use enhanced basic regular expressions on macOS

When 1819ad327b (grep: fix multibyte regex handling under macOS,
2022-08-26) started to use the native regex library instead of Git's
own (compat/regex/), it lost support for alternation in basic
regular expressions.

Bring it back by enabling the flag REG_ENHANCED on macOS when
compiling basic regular expressions.

Reported-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
---
 Makefile                  | 9 +++++++++
 compat/regcomp_enhanced.c | 9 +++++++++
 config.mak.uname          | 1 +
 git-compat-util.h         | 5 +++++
 4 files changed, 24 insertions(+)
 create mode 100644 compat/regcomp_enhanced.c

diff --git a/Makefile b/Makefile
index db447d0738..46e30be673 100644
--- a/Makefile
+++ b/Makefile
@@ -289,6 +289,10 @@ include shared.mak
 # Define NO_REGEX if your C library lacks regex support with REG_STARTEND
 # feature.
 #
+# Define USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS if your C library provides
+# the flag REG_ENHANCED and you'd like to use it to enable enhanced basic
+# regular expressions.
+#
 # Define HAVE_DEV_TTY if your system can open /dev/tty to interact with the
 # user.
 #
@@ -2037,6 +2041,11 @@ endif
 ifdef NO_REGEX
 	COMPAT_CFLAGS += -Icompat/regex
 	COMPAT_OBJS += compat/regex/regex.o
+else
+ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+	COMPAT_CFLAGS += -DUSE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+	COMPAT_OBJS += compat/regcomp_enhanced.o
+endif
 endif
 ifdef NATIVE_CRLF
 	BASIC_CFLAGS += -DNATIVE_CRLF
diff --git a/compat/regcomp_enhanced.c b/compat/regcomp_enhanced.c
new file mode 100644
index 0000000000..84193ce53b
--- /dev/null
+++ b/compat/regcomp_enhanced.c
@@ -0,0 +1,9 @@
+#include "../git-compat-util.h"
+#undef regcomp
+
+int git_regcomp(regex_t *preg, const char *pattern, int cflags)
+{
+	if (!(cflags & REG_EXTENDED))
+		cflags |= REG_ENHANCED;
+	return regcomp(preg, pattern, cflags);
+}
diff --git a/config.mak.uname b/config.mak.uname
index d63629fe80..7d25995265 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -147,6 +147,7 @@ ifeq ($(uname_S),Darwin)
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	HAVE_NS_GET_EXECUTABLE_PATH = YesPlease
 	CSPRNG_METHOD = arc4random
+	USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS = YesPlease

 	# Workaround for `gettext` being keg-only and not even being linked via
 	# `brew link --force gettext`, should be obsolete as of
diff --git a/git-compat-util.h b/git-compat-util.h
index 76e4b11131..1efa834089 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1338,6 +1338,11 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 	return regexec(preg, buf, nmatch, pmatch, eflags | REG_STARTEND);
 }

+#ifdef USE_ENHANCED_BASIC_REGULAR_EXPRESSIONS
+int git_regcomp(regex_t *preg, const char *pattern, int cflags);
+#define regcomp git_regcomp
+#endif
+
 #ifndef DIR_HAS_BSD_GROUP_SEMANTICS
 # define FORCE_DIR_SET_GID S_ISGID
 #else
--
2.39.0