Re: [PATCH v2] merge-recursive: check GIT_MERGE_VERBOSITY only once

> On Wed, Oct 25, 2017 at 7:39 AM, Andrey Okoshkin <a.okoshkin@samsung.com> wrote:
>> Check 'GIT_MERGE_VERBOSITY' environment variable only once in
>> init_merge_options().
>> Consequential call of getenv() may return NULL pointer.
> 
> It would be particularly nice to have a more detailed explanation in
> the commit message of the potential problem this patch is trying to
> solve. Given the amount of discussion, thus far, surrounding such a
> simple patch, this cryptic warning about getenv() returning NULL upon
> second invocation is insufficient to explain why this patch is
> desirable; it merely leads to a lot of head-scratching.
> 
>> However the stored pointer to the obtained getenv() result may be invalidated
>> by some other getenv() call as getenv() is not thread-safe.
> 
> This is even more cryptic, as it appears to be arguing for or against
> _something_ (it's not clear what) and it seems to be talking about a
> facet of the existing code, rather than explaining why the updated
> code consumes its 'merge_verbosity' value as early as possible after
> being assigned. Perhaps this part could be reworded something like
> this:
> 
>     Instead, call getenv() only once, storing its value and
>     consulting it as many times as needed. This update takes care
>     to consume the value returned by getenv() without any
>     intervening calls to getenv(), setenv(), unsetenv(), or
>     putenv(), any of which might invalidate the pointer returned
>     by the initial call.
> 
>> Signed-off-by: Andrey Okoshkin <a.okoshkin@samsung.com>
>> Reviewed-by: Stefan Beller <sbeller@google.com>
> 
> As this patch is semantically quite different from the original to
> which Stefan gave his Reviewed-by: (and which other people argued
> against), it might be better omit this footer and let him re-give it
> if he so desires.
> 
>> ---
>> Changes since the previous patch:
>> * no actions are taken between the merge_verbosity assignment and check.
>>  merge-recursive.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/merge-recursive.c b/merge-recursive.c
>> index 1494ffdb8..60084e3a0 100644
>> --- a/merge-recursive.c
>> +++ b/merge-recursive.c
>> @@ -2163,6 +2163,7 @@ static void merge_recursive_config(struct merge_options *o)
>>
>>  void init_merge_options(struct merge_options *o)
>>  {
>> +       const char *merge_verbosity;
>>         memset(o, 0, sizeof(struct merge_options));
>>         o->verbosity = 2;
>>         o->buffer_output = 1;
>> @@ -2171,9 +2172,9 @@ void init_merge_options(struct merge_options *o)
>>         o->renormalize = 0;
>>         o->detect_rename = 1;
>>         merge_recursive_config(o);
>> -       if (getenv("GIT_MERGE_VERBOSITY"))
>> -               o->verbosity =
>> -                       strtol(getenv("GIT_MERGE_VERBOSITY"), NULL, 10);
>> +       merge_verbosity = getenv("GIT_MERGE_VERBOSITY");
>> +       if (merge_verbosity)
>> +               o->verbosity = strtol(merge_verbosity, NULL, 10);
>>         if (o->verbosity >= 5)
>>                 o->buffer_output = 0;
>>         strbuf_init(&o->obuf, 0);
>> --
>> 2.14.3
> 
> 
>