Subject: Re: [PATCH] blame.c: fix garbled error message

From: Junio C Hamano <[email-masked]>
Date: Mon, 12 Jan 2015 11:08:26 -0800

Lukas Fleischer <[email-masked]> writes:

> The helper functions prepare_final() and prepare_initial() return a
> pointer to a string that is a member of an object in the revs->pending
> array. This array is later rebuilt when running prepare_revision_walk()
> which potentially transforms the pointer target into a bogus string. Fix
> this by maintaining a copy of the original string.
>
> Signed-off-by: Lukas Fleischer <[email-masked]>
> ---
> The bug manifests when running `git blame HEAD^ -- nonexistent.file`.

Before 1da1e07c (clean up name allocation in prepare_revision_walk,
2014-10-15), these strings used to be non-volatile; they were instead
leaked more or less deliberately.  But these days, these strings are
cleared, so your patch is absolutely the right thing to do.

Thanks for catching and fixing.  This fix needs to go to the 2.2.x
maintenance track.

> Note that I could have reduced code churn a little by moving the
> xstrdup() invocations to the call sites. However, I think that the
> return value of these functions should not depend on the consistency of
> a volatile data structure. On the other hand, you might also argue that
> there currently are only two call sites and that the functions are
> marked static, so if you prefer the less intrusive version, please let
> me know.

FWIW, I agree that this can be argued either way.

If we had a common low-level API that is used by short-term users to
grab these names, it would be reasonable to make it the callers'
responsibility to strdup() the return values for safekeeping if they
want to keep using them long after the function returns.  But I
agree that prepare_final/prepare_initial are not such a low-level
common API functions.  I do not care too much about who does the
strdup(), either the callers of prepare_* or the callee.

>
>  builtin/blame.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/builtin/blame.c b/builtin/blame.c
> index 303e217..34d6f4f 100644
> --- a/builtin/blame.c
> +++ b/builtin/blame.c
> @@ -2390,7 +2390,7 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
>  	return commit;
>  }
>  
> -static const char *prepare_final(struct scoreboard *sb)
> +static char *prepare_final(struct scoreboard *sb)
>  {
>  	int i;
>  	const char *final_commit_name = NULL;
> @@ -2415,10 +2415,10 @@ static const char *prepare_final(struct scoreboard *sb)
>  		sb->final = (struct commit *) obj;
>  		final_commit_name = revs->pending.objects[i].name;
>  	}
> -	return final_commit_name;
> +	return xstrdup(final_commit_name);
>  }
>  
> -static const char *prepare_initial(struct scoreboard *sb)
> +static char *prepare_initial(struct scoreboard *sb)
>  {
>  	int i;
>  	const char *final_commit_name = NULL;
> @@ -2445,7 +2445,7 @@ static const char *prepare_initial(struct scoreboard *sb)
>  	}
>  	if (!final_commit_name)
>  		die("No commit to dig down to?");
> -	return final_commit_name;
> +	return xstrdup(final_commit_name);
>  }
>  
>  static int blame_copy_callback(const struct option *option, const char *arg, int unset)
> @@ -2489,7 +2489,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
>  	struct origin *o;
>  	struct blame_entry *ent = NULL;
>  	long dashdash_pos, lno;
> -	const char *final_commit_name = NULL;
> +	char *final_commit_name = NULL;
>  	enum object_type type;
>  
>  	static struct string_list range_list;
> @@ -2786,6 +2786,8 @@ parse_done:
>  
>  	assign_blame(&sb, opt);
>  
> +	free(final_commit_name);
> +
>  	if (incremental)
>  		return 0;