[Buildroot] [PATCH 2/5] support/download: properly use temp files

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Yann, Arnout,

On Mon, 7 Jul 2014 23:38:02 +0200, Yann E. MORIN wrote:

> >  Not really related to this patch, but why do we need this ${tmp_dl} to begin
> > with? Especially since we're already "occupying" a tempfile in DL_DIR anyway.
> 
> The idea was not to pollute BR2_DL_DIR, in case the download fails.
> Hence this dance:
>   - download to a disposable area (BUILD_DIR);
>   - move to a temp file in BR2_DL_DIR;
>   - atomically rename to the final file.

And also because $(DL_DIR) and $(BUILD_DIR) might be on different
filesystems, so the rename/move of the file from $(BUILD_DIR) (where it
was downloaded) to $(DL_DIR) may not be atomic. Hence the idea is to:

 - Download in $(BUILD_DIR)

 - Move to a temporary file in $(DL_DIR). This operation may not be
   atomic if $(DL_DIR) is not on the same filesystem as $(BUILD_DIR)

 - Finally rename the temporary file to the expected file name in
   $(DL_DIR). Since we're in the same directory, it's guaranteed to be
   atomic.

This allows to ensure that when the final file appears in $(DL_DIR),
we're sure the download is finished, and that therefore concurrently
executing Buildroot instances will either not see the downloaded file,
or see a fully completed downloaded file.

Or am I missing the point of the discussion here?

Thanks,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com