Re: [PATCH v2] strbuf: add and use strbuf_insertstr()

["René Scharfe" <l.s.r@web.de>]

Am 09.02.20 um 18:36 schrieb Eric Sunshine:
> On Sun, Feb 9, 2020 at 8:45 AM René Scharfe <l.s.r@web.de> wrote:
>> Add a function for inserting a C string into a strbuf.  Use it
>> throughout the source to get rid of magic string length constants and
>> explicit strlen() calls.
>>
>> Like strbuf_addstr(), implement it as an inline function to avoid the
>> implicit strlen() calls to cause runtime overhead.
>>
>> Signed-off-by: René Scharfe <l.s.r@web.de>
>> ---
>> diff --git a/mailinfo.c b/mailinfo.c
>> @@ -570,7 +570,7 @@ static int check_header(struct mailinfo *mi,
>>                 len = strlen("Content-Type: ");
>>                 strbuf_add(&sb, line->buf + len, line->len - len);
>>                 decode_header(mi, &sb);
>> -               strbuf_insert(&sb, 0, "Content-Type: ", len);
>> +               strbuf_insertstr(&sb, 0, "Content-Type: ");
>>                 handle_content_type(mi, &sb);
>
> Meh. We've already computed the length of "Content-Type: " a few lines
> earlier, so taking advantage of that value when inserting the string
> literal is perfectly sensible.

Well, yes, but it would be more sensible if we'd have only a single
string here.  At the source code level we have two string constants that
happen to have the same contents.  Handling them separately is
reasonable, I think.

The compiler is merging those two, and resolves the other strlen() call
at compile time, so the generated code is the same.

> Thus, I'm not convinced that this change is an improvement.

The improvement is to untangle the handling of those two string
constants and to use a C string without having to pass along its
length.  That doesn't make the code clean, yet, admittedly.

> Digging deeper, though, I have to wonder why this bothers inserting
> "Content-Type: " at all. None of the other cases handled by
> check_header() bother re-inserting the header, so why this one? I
> thought it might be because handle_content_type() depends upon the
> header being present, but from my reading, this does not appear to be
> the case. handle_content_type() calls has_attr_value() and
> slurp_attr() to examine the incoming line, but neither of those seem
> to expect any sort of "<Header>: " either. Thus, it appears that the
> insertion of "Content-Type: " is superfluous. If this is indeed the
> case, then rather than converting this to strbuf_insertstr(), I could
> see it being pulled out into a separate patch which merely removes the
> strbuf_insert() call.

Interesting.  It makes sense that handle_content_type() wouldn't need
such a header prefix -- it's only called if its existence in the line
has been confirmed.  And I also don't see a hint in the code that
would justify the insertion.

Do you care to send a follow-up patch (or one against master if you're
not convinced by my reasoning given above)?

Thanks,
René