[Buildroot] [v3 12/13] download: add flock call before dl-wrapper

[Yann E. MORIN <yann.morin.1998@free.fr>]

Maxime, All,

On 2018-03-31 16:24 +0200, Maxime Hadjinlian spake thusly:
> In order to introduce the cache mechanisms, we need to have a lock on
> the $(LIBFOO_DL_DIR), otherwise it would be impossible to do parallel
> download (a shared DL_DIR for two buildroot instances).
> 
> To make sure the directory exists, the mkdir call has been removed from
> the dl-wrapper and put in the infrastructure.
> 
> Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
> ---
>  package/pkg-download.mk     | 4 +++-
>  support/download/dl-wrapper | 1 -
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/package/pkg-download.mk b/package/pkg-download.mk
> index 23f3403098..dff52007e6 100644
> --- a/package/pkg-download.mk
> +++ b/package/pkg-download.mk
> @@ -19,6 +19,7 @@ SSH := $(call qstrip,$(BR2_SSH))
>  export LOCALFILES := $(call qstrip,$(BR2_LOCALFILES))
>  
>  DL_WRAPPER = support/download/dl-wrapper
> +FLOCK = flock $($(PKG)_DL_DIR)/

This means that two downloads running in two concurrent Buildroot
builds, doing a wget from different tarballs from the same package
(eg. foo-1.tar and foo-2.tar) will not be able to complete in parallel,
and will be sequential, while we curently can do that, and it *is* safe
to do so.

We have a few objects we can flock, in  a generic manner:

  - the .stamp_downloaded stanp file: this would allow concurrent
    Buildroot builds of wget downloads, but would not protect against
    concurrent git clones;

  - the $($(PKG)_DL_DIR): would protect against concurrent git clones,
    but forbids concurrent wget of the same package;

And a third and fourth further options:

  - $($(PKG)_DL_DIR)/git: which would be the best of both world, but
    would need to be handled by the dl-wrapper itself when it calls
    the git backend (or hg, svn, cvs... when we later support those).

  - push the flock even further down into each individual backends,
    which would each be responsible to flock only the individual
    commands that require locking.

For the fourth solution, we may be able to shoe-horn the flock call into
the internals _XXX wrappers (e.g. the _git() function of the git
backend).

TBH, I believe that what you propsoe is Good Enough (TM), because the
third, optimised solution, is just getting a bit more complex, as there
is no resource (file or directory) that we can flock, besides the
package's own download dir. The fourth is pusshing into too fine-grained
for being entirely reliable...

So, I'm happy that we go with your proposed patch. But maybe expand the
commit log to explain a bit that restriction.

Regards,
Yann E. MORIN.

>  # DL_DIR may have been set already from the environment
>  ifeq ($(origin DL_DIR),undefined)
> @@ -91,7 +92,8 @@ endif
>  
>  define DOWNLOAD
>  	$(Q)$(if $(filter bzr cvs hg svn,$($(PKG)_SITE_METHOD)),BR_NO_CHECK_HASH_FOR=$(notdir $(1));) \
> -	$(EXTRA_ENV) $(DL_WRAPPER) \
> +	$(Q)mkdir -p $($(PKG)_DL_DIR)/
> +	$(EXTRA_ENV) $(FLOCK) $(DL_WRAPPER) \
>  		-c $($(PKG)_DL_VERSION) \
>  		-f $(notdir $(1)) \
>  		-H $(PKGDIR)/$($(PKG)_RAWNAME).hash \
> diff --git a/support/download/dl-wrapper b/support/download/dl-wrapper
> index 49caf3717b..67e9742767 100755
> --- a/support/download/dl-wrapper
> +++ b/support/download/dl-wrapper
> @@ -50,7 +50,6 @@ main() {
>      if [ -z "${output}" ]; then
>          error "no output specified, use -o\n"
>      fi
> -    mkdir -p "$(dirname "${output}")"
>  
>      # If the output file already exists and:
>      # - there's no .hash file: do not download it again and exit promptly
> -- 
> 2.16.2
> 

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