Re: [PATCH 6/7] reftable/record: use scratch buffer when decoding records

> When decoding log records we need a temporary buffer to decode the
> reflog entry's name, mail address and message. As this buffer is local
> to the function we thus have to reallocate it for every single log
> record which we're about to decode, which is inefficient.
> 
> Refactor the code such that callers need to pass in a scratch buffer,
> which allows us to reuse it for multiple decodes. This reduces the
> number of allocations when iterating through reflogs. Before:
> 
>     HEAP SUMMARY:
>         in use at exit: 13,473 bytes in 122 blocks
>       total heap usage: 2,068,487 allocs, 2,068,365 frees, 305,122,946 bytes allocated
> 
> After:
> 
>     HEAP SUMMARY:
>         in use at exit: 13,473 bytes in 122 blocks
>       total heap usage: 1,068,485 allocs, 1,068,363 frees, 281,122,886 bytes allocated
> 
> Note that this commit also drop some redundant calls to `strbuf_reset()`
> right before calling `decode_string()`. The latter already knows to
> reset the buffer, so there is no need for these.
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  reftable/block.c       |  4 ++-
>  reftable/block.h       |  2 ++
>  reftable/record.c      | 52 ++++++++++++++++++--------------------
>  reftable/record.h      |  5 ++--
>  reftable/record_test.c | 57 ++++++++++++++++++++++++++----------------
>  5 files changed, 68 insertions(+), 52 deletions(-)

> diff --git a/reftable/record.c b/reftable/record.c
> index 7c86877586..060244337f 100644
> --- a/reftable/record.c
> +++ b/reftable/record.c
> @@ -374,7 +374,7 @@ static int reftable_ref_record_encode(const void *rec, struct string_view s,
>  
>  static int reftable_ref_record_decode(void *rec, struct strbuf key,
>  				      uint8_t val_type, struct string_view in,
> -				      int hash_size)
> +				      int hash_size, struct strbuf *scratch)
>  {
>  	struct reftable_ref_record *r = rec;
>  	struct string_view start = in;
> @@ -425,13 +425,12 @@ static int reftable_ref_record_decode(void *rec, struct strbuf key,
>  		break;
>  
>  	case REFTABLE_REF_SYMREF: {
> -		struct strbuf dest = STRBUF_INIT;
> -		int n = decode_string(&dest, in);
> +		int n = decode_string(scratch, in);
>  		if (n < 0) {
>  			return -1;
>  		}
>  		string_view_consume(&in, n);
> -		r->value.symref = dest.buf;
> +		r->value.symref = strbuf_detach(scratch, NULL);
>  	} break;