Re: [PATCH 2/9] trace2: convert tr2tls_thread_ctx.thread_name from strbuf to char*

[Jeff Hostetler <git@jeffhostetler.com>]

On 12/21/21 2:22 AM, Junio C Hamano wrote:
> "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Use a 'char *' to hold the thread name rather than a 'struct strbuf'.
>> The thread name is set when the thread is created and should not be
>> be modified afterwards.  Replace the strbuf with an allocated pointer
>> to make that more clear.
> 
> Sounds good.  Use of strbuf is perfectly fine while you compute the
> final value of the string, but as a more permanent location to store
> the result, it often is unsuitable (and strbuf_split_buf() is a prime
> example of how *not* to design your API function around the type).
> 
>> diff --git a/trace2/tr2_tls.c b/trace2/tr2_tls.c
>> index 7da94aba522..cd8b9f2f0a0 100644
>> --- a/trace2/tr2_tls.c
>> +++ b/trace2/tr2_tls.c
>> @@ -35,6 +35,7 @@ struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name,
>>   					     uint64_t us_thread_start)
>>   {
>>   	struct tr2tls_thread_ctx *ctx = xcalloc(1, sizeof(*ctx));
>> +	struct strbuf buf_name = STRBUF_INIT;
>>   
>>   	/*
>>   	 * Implicitly "tr2tls_push_self()" to capture the thread's start
>> @@ -47,12 +48,13 @@ struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name,
>>   
>>   	ctx->thread_id = tr2tls_locked_increment(&tr2_next_thread_id);
>>   
>> -	strbuf_init(&ctx->thread_name, 0);
>>   	if (ctx->thread_id)
>> -		strbuf_addf(&ctx->thread_name, "th%02d:", ctx->thread_id);
>> -	strbuf_addstr(&ctx->thread_name, thread_name);
>> -	if (ctx->thread_name.len > TR2_MAX_THREAD_NAME)
>> -		strbuf_setlen(&ctx->thread_name, TR2_MAX_THREAD_NAME);
>> +		strbuf_addf(&buf_name, "th%02d:", ctx->thread_id);
>> +	strbuf_addstr(&buf_name, thread_name);
>> +	if (buf_name.len > TR2_MAX_THREAD_NAME)
>> +		strbuf_setlen(&buf_name, TR2_MAX_THREAD_NAME);
>> +
>> +	ctx->thread_name = strbuf_detach(&buf_name, NULL);
> 
> This is not exactly a new problem, but if we use a mechanism to
> allow arbitrary long string (like composing with strbuf and
> detaching the resulting string as is), instead of having a fixed
> name[] array embedded in the ctx structure, I wonder if applying the
> maximum length this early makes sense.  Such a truncation would
> allow more than one ctx structures to share the same name, which
> somehow feels error prone, inviting a mistake to use .thread_name
> member as an identifier, when its only intended use is to give a
> human-readable and not necessarily unique label.  Of course, if the
> maximum is reasonably low, like a few dozen bytes, it may even make
> sense to embed an array of the fixed size and not worry about an
> extra pointer.
> 

I'll convert it to a flex-array at the bottom of the CTX structure
and then defer the truncation to the _perf target (which only does
that to keep the columns lined up).

That will simplify things considerably.

Thanks
Jeff