Re: [PATCH 4/4] commit-graph: close descriptors after mmap

> From: Derrick Stolee <dstolee@microsoft.com>
> Date: Fri, 24 Apr 2020 13:11:13 +0000
> Subject: [PATCH] multi-pack-index: close file descriptor after mmap
>
> We recently discovered that the commit-graph was not closing its
> file descriptor after memory-mapping the file contents. After this
> mmap() succeeds, there is no need to keep the file descriptor open.
> In fact, there is signficant reason to close it so we do not run
> out of descriptors.

> This is entirely my fault for not knowing that we can have an open
> mmap while also closing the descriptor. Some could blame this on
> being a new contributor when the series was first submitted, but
> even years later this is still new information to me.
>
> That made me realize that I used the same pattern when opening a
> multi-pack-index. Since this file is not (yet) incremental, there
> will be at most one descriptor open for this reason. It is still
> worth fixing, especially if we extend the format to be incremental
> like the commit-graph.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  midx.c | 4 +---
>  midx.h | 2 --
>  2 files changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/midx.c b/midx.c
> index 1527e464a7..60d30e873b 100644
> --- a/midx.c
> +++ b/midx.c
> @@ -72,9 +72,9 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
>  	FREE_AND_NULL(midx_name);
>  
>  	midx_map = xmmap(NULL, midx_size, PROT_READ, MAP_PRIVATE, fd, 0);
> +	close(fd);
>  
>  	FLEX_ALLOC_STR(m, object_dir, object_dir);
> -	m->fd = fd;
>  	m->data = midx_map;
>  	m->data_len = midx_size;
>  	m->local = local;
> @@ -190,8 +190,6 @@ void close_midx(struct multi_pack_index *m)
>  		return;
>  
>  	munmap((unsigned char *)m->data, m->data_len);
> -	close(m->fd);
> -	m->fd = -1;
>  
>  	for (i = 0; i < m->num_packs; i++) {
>  		if (m->packs[i])
> diff --git a/midx.h b/midx.h
> index e6fa356b5c..b18cf53bc4 100644
> --- a/midx.h
> +++ b/midx.h
> @@ -12,8 +12,6 @@ struct repository;
>  struct multi_pack_index {
>  	struct multi_pack_index *next;
>  
> -	int fd;
> -
>  	const unsigned char *data;
>  	size_t data_len;