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

> Lukas Fleischer <git@cryptocrack.de> 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 <git@cryptocrack.de>
>> ---
>> 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;