Re: [RFC PATCH v3 2/3] grep: make PCRE2 aware of custom allocator

["René Scharfe" <l.s.r@web.de>]

Am 06.08.19 um 18:36 schrieb Carlo Marcelo Arenas Belón:
> 94da9193a6 (grep: add support for PCRE v2, 2017-06-01) didn't include
> a way to override the system allocator, and so it is incompatible with
> USE_NED_ALLOCATOR.  The problem was made visible when an attempt to
> avoid a leak in a data structure that is created by the library was
> passed to NED's free for disposal triggering a segfault in Windows.
>
> PCRE2 requires the use of a general context to override the allocator
> and therefore, there is a lot more code needed than in PCRE1, including
> a couple of wrapper functions.
>
> Extend the grep API with a "destructor" that could be called to cleanup
> any objects that were created and used globally.
>
> Update builtin/grep to use that new API, but any other future
> users should make sure to have matching grep_init/grep_destroy calls
> if they are using the pattern matching functionality (currently only
> relevant when using both NED and PCRE2)
>
> Move some of the logic that was before done per thread (in the workers)
> into an earlier phase to avoid degrading performance

Which logic is moved?  In the patch I basically only see additions.

>, but as the use
> of PCRE2 with NED is better understood it is expected more of its
> functions will be instructed to use the custom allocator as well as
> was done in the original code[1] this work was based on.
>
> [1] https://public-inbox.org/git/3397e6797f872aedd18c6d795f4976e1c579514b.1565005867.git.gitgitgadget@gmail.com/

I'm not sure I understand that part.  Do you mean there are gaps of
knowledge about nedmalloc and/or PCRE2 and/or their interaction?  And
someone is working on closing those gaps, and is going to submit more
patches in the process?

Your patch uses a custom global context only if USE_NED_ALLOCATOR is
defined, while [1] does it unconditionally.  The latter is easier to
debug and requires less preprocessor directives.  What's the upside
of your approach?

>
> Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
> ---
>  builtin/grep.c |  1 +
>  grep.c         | 36 +++++++++++++++++++++++++++++++++++-
>  grep.h         |  1 +
>  3 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/builtin/grep.c b/builtin/grep.c
> index 560051784e..e49c20df60 100644
> --- a/builtin/grep.c
> +++ b/builtin/grep.c
> @@ -1145,5 +1145,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
>  		run_pager(&opt, prefix);
>  	clear_pathspec(&pathspec);
>  	free_grep_patterns(&opt);
> +	grep_destroy();
>  	return !hit;
>  }
> diff --git a/grep.c b/grep.c
> index 0154998695..3e5bdf94a6 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -16,6 +16,22 @@ static int grep_source_is_binary(struct grep_source *gs,
>
>  static struct grep_opt grep_defaults;
>
> +#ifdef USE_LIBPCRE2
> +static pcre2_general_context *pcre2_global_context;
> +
> +#ifdef USE_NED_ALLOCATOR
> +static void *pcre2_malloc(PCRE2_SIZE size, MAYBE_UNUSED void *memory_data)
> +{
> +	return malloc(size);

Should this be xmalloc() to get consistent out-of-memory handling?

> +}
> +
> +static void pcre2_free(void *pointer, MAYBE_UNUSED void *memory_data)
> +{
> +	return free(pointer);
> +}
> +#endif
> +#endif
> +
>  static const char *color_grep_slots[] = {
>  	[GREP_COLOR_CONTEXT]	    = "context",
>  	[GREP_COLOR_FILENAME]	    = "filename",
> @@ -153,6 +169,7 @@ int grep_config(const char *var, const char *value, void *cb)
>   *
>   * If using PCRE make sure that the library is configured
>   * to use the right allocator (ex: NED)
> + * if any object is created it should be cleaned up in grep_destroy()
>   */
>  void grep_init(struct grep_opt *opt, struct repository *repo, const char *prefix)
>  {
> @@ -188,6 +205,13 @@ void grep_init(struct grep_opt *opt, struct repository *repo, const char *prefix
>  		color_set(opt->colors[i], def->colors[i]);
>  }
>
> +void grep_destroy(void)
> +{
> +#ifdef USE_LIBPCRE2
> +	pcre2_general_context_free(pcre2_global_context);
> +#endif
> +}
> +
>  static void grep_set_pattern_type_option(enum grep_pattern_type pattern_type, struct grep_opt *opt)
>  {
>  	/*
> @@ -319,6 +343,11 @@ void append_header_grep_pattern(struct grep_opt *opt,
>  void append_grep_pattern(struct grep_opt *opt, const char *pat,
>  			 const char *origin, int no, enum grep_pat_token t)
>  {
> +#if defined(USE_LIBPCRE2) && defined(USE_NED_ALLOCATOR)
> +	if (!pcre2_global_context && opt->ignore_case && has_non_ascii(pat))
> +		pcre2_global_context = pcre2_general_context_create(
> +					pcre2_malloc, pcre2_free, NULL);
> +#endif
>  	append_grep_pat(opt, pat, strlen(pat), origin, no, t);
>  }
>
> @@ -507,9 +536,14 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
>
>  	p->pcre2_compile_context = NULL;
>
> +	/* pcre2_global_context is initialized in append_grep_pattern */
>  	if (opt->ignore_case) {
>  		if (has_non_ascii(p->pattern)) {
> -			character_tables = pcre2_maketables(NULL);
> +#ifdef USE_NED_ALLOCATOR
> +			if (!pcre2_global_context)
> +				BUG("pcre2_global_context uninitialized");

[1] initializes on demand.  Why not do that?  To avoid race conditions
that would lead to occasional double-allocation of the global context?

> +#endif
> +			character_tables = pcre2_maketables(pcre2_global_context);
>  			p->pcre2_compile_context = pcre2_compile_context_create(NULL);

Don't you want to pass pcre2_global_context here as well?  And [1] even
uses it in some more places.

Oh, that's the "expected more" when "better understood" part from the
commit message, right?

Basically I'd expect the custom global context to be used for all PCRE2
allocations; I can't think of a reason for mixing allocators (e.g.
system malloc for PCRE2 regexes and nedmalloc for everything else).

>  			pcre2_set_character_tables(p->pcre2_compile_context, character_tables);
>  		}
> diff --git a/grep.h b/grep.h
> index 1875880f37..526c2db9ef 100644
> --- a/grep.h
> +++ b/grep.h
> @@ -189,6 +189,7 @@ struct grep_opt {
>  void init_grep_defaults(struct repository *);
>  int grep_config(const char *var, const char *value, void *);
>  void grep_init(struct grep_opt *, struct repository *repo, const char *prefix);
> +void grep_destroy(void);
>  void grep_commit_pattern_type(enum grep_pattern_type, struct grep_opt *opt);
>
>  void append_grep_pat(struct grep_opt *opt, const char *pat, size_t patlen, const char *origin, int no, enum grep_pat_token t);
>