Re: [PATCH v2 06/11] commit-graph: simplify chunk writes into loop

From: Derrick Stolee <stolee@gmail.com>
To: "René Scharfe" <l.s.r@web.de>,
	"SZEDER Gábor via GitGitGadget" <gitgitgadget@gmail.com>,
	git@vger.kernel.org
Cc: me@ttaylorr.com, szeder.dev@gmail.com,
	Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH v2 06/11] commit-graph: simplify chunk writes into loop
Date: Thu, 25 Jun 2020 10:59:22 -0400	[thread overview]
Message-ID: <467ecf3b-2635-5d92-c8f9-9e89dd6580ee@gmail.com> (raw)
In-Reply-To: <675f46df-c57c-bcd6-3f6a-fdd40b2f3066@web.de>

On 6/25/2020 3:25 AM, René Scharfe wrote:
> Am 23.06.20 um 19:47 schrieb SZEDER Gábor via GitGitGadget:
>> From: =?UTF-8?q?SZEDER=20G=C3=A1bor?= <szeder.dev@gmail.com>
>>
>> In write_commit_graph_file() we now have one block of code filling the
>> array of 'struct chunk_info' with the IDs and sizes of chunks to be
>> written, and an other block of code calling the functions responsible
>> for writing individual chunks.  In case of optional chunks like Extra
>> Edge List an Base Graphs List there is also a condition checking
>> whether that chunk is necessary/desired, and that same condition is
>> repeated in both blocks of code. Other, newer chunks have similar
>> optional conditions.
>>
>> Eliminate these repeated conditions by storing the function pointers
>> responsible for writing individual chunks in the 'struct chunk_info'
>> array as well, and calling them in a loop to write the commit-graph
>> file.  This will open up the possibility for a bit of foolproofing in
>> the following patch.
> 
> You can do that without storing function pointers by selecting the
> function to use based on the chunk ID -- like parse_commit_graph() does
> on the read side.  Advantage: You don't need to press all write
> functions into the same mold and can keep their individual signatures.

I do think that the loop without a switch statement is valuable.
It focuses the updates for new chunks to be localized to the
section that calculates the offset values.

>>
>> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
>> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
>> ---
>>  commit-graph.c | 31 +++++++++++++++++++------------
>>  1 file changed, 19 insertions(+), 12 deletions(-)
>>
>> diff --git a/commit-graph.c b/commit-graph.c
>> index f33bfe49b3..086fc2d070 100644
>> --- a/commit-graph.c
>> +++ b/commit-graph.c
>> @@ -1555,9 +1555,13 @@ static int write_graph_chunk_base(struct hashfile *f,
>>  	return 0;
>>  }
>>
>> +typedef int (*chunk_write_fn)(struct hashfile *f,
>> +			      struct write_commit_graph_context *ctx);
>> +
>>  struct chunk_info {
>>  	uint32_t id;
>>  	uint64_t size;
>> +	chunk_write_fn write_fn;
>>  };
>>
>>  static int write_commit_graph_file(struct write_commit_graph_context *ctx)
>> @@ -1615,27 +1619,34 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
>>
>>  	chunks[0].id = GRAPH_CHUNKID_OIDFANOUT;
>>  	chunks[0].size = GRAPH_FANOUT_SIZE;
>> +	chunks[0].write_fn = write_graph_chunk_fanout;
>>  	chunks[1].id = GRAPH_CHUNKID_OIDLOOKUP;
>>  	chunks[1].size = hashsz * ctx->commits.nr;
>> +	chunks[1].write_fn = write_graph_chunk_oids;
>>  	chunks[2].id = GRAPH_CHUNKID_DATA;
>>  	chunks[2].size = (hashsz + 16) * ctx->commits.nr;
>> +	chunks[2].write_fn = write_graph_chunk_data;
>>  	if (ctx->num_extra_edges) {
>>  		chunks[num_chunks].id = GRAPH_CHUNKID_EXTRAEDGES;
>>  		chunks[num_chunks].size = 4 * ctx->num_extra_edges;
>> +		chunks[num_chunks].write_fn = write_graph_chunk_extra_edges;
>>  		num_chunks++;
>>  	}
>>  	if (ctx->changed_paths) {
>>  		chunks[num_chunks].id = GRAPH_CHUNKID_BLOOMINDEXES;
>>  		chunks[num_chunks].size = sizeof(uint32_t) * ctx->commits.nr;
>> +		chunks[num_chunks].write_fn = write_graph_chunk_bloom_indexes;
>>  		num_chunks++;
>>  		chunks[num_chunks].id = GRAPH_CHUNKID_BLOOMDATA;
>>  		chunks[num_chunks].size = sizeof(uint32_t) * 3
>>  					  + ctx->total_bloom_filter_data_size;
>> +		chunks[num_chunks].write_fn = write_graph_chunk_bloom_data;
>>  		num_chunks++;
>>  	}
>>  	if (ctx->num_commit_graphs_after > 1) {
>>  		chunks[num_chunks].id = GRAPH_CHUNKID_BASE;
>>  		chunks[num_chunks].size = hashsz * (ctx->num_commit_graphs_after - 1);
>> +		chunks[num_chunks].write_fn = write_graph_chunk_base;
>>  		num_chunks++;
>>  	}
>>
>> @@ -1671,19 +1682,15 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
>>  			progress_title.buf,
>>  			num_chunks * ctx->commits.nr);
>>  	}
>> -	write_graph_chunk_fanout(f, ctx);
>> -	write_graph_chunk_oids(f, ctx);
>> -	write_graph_chunk_data(f, ctx);
>> -	if (ctx->num_extra_edges)
>> -		write_graph_chunk_extra_edges(f, ctx);
>> -	if (ctx->changed_paths) {
>> -		write_graph_chunk_bloom_indexes(f, ctx);
>> -		write_graph_chunk_bloom_data(f, ctx);
>> -	}
>> -	if (ctx->num_commit_graphs_after > 1 &&
>> -	    write_graph_chunk_base(f, ctx)) {
>> -		return -1;
>> +
>> +	for (i = 0; i < num_chunks; i++) {
>> +		if (chunks[i].write_fn(f, ctx)) {
>> +			error(_("failed writing chunk with id %"PRIx32""),
>> +			      chunks[i].id);
> 
> Of all the write functions only write_graph_chunk_base() can return
> non-zero and it already prints an error message in that case ("failed to
> write correct number of base graph ids").  Why add this one?

Ok, we can require the chunk methods to write an error() message with
appropriate context and simply return -1 here.

Thanks,
-Stolee