[swift-build-dev] SwiftPM recompilation - Packages folder

[Daniel Dunbar]

On Mon, Nov 21, 2016 at 2:37 PM, Ricardo N Olivieri <
ricardo.olivieri at us.ibm.com> wrote:

> Thanks Daniel for the response. That's probably the reason (i.e. metadata
> changes in the file inode) for the behavior we are seeing in SwiftPM.
>
> The use case for us is as follows:
>
> We'd like to improve even further the performance of the IBM Bluemix
> buildpack for Swift and one way we could accomplish this is by not
> recompiling the Packages folder as long as the content of that folder has
> not changed. Without getting into too many details about cloud foundry
> buildpacks, for the buildpack to reuse the contents of the Packages folder
> in future re-deployments of the same application to the cloud, the
> buildpack has to copy the Packages folder a different folder outside the
> repo's folder and then copy it back again when the developer redeploys
> his/her application. This is when we noticed that despite the contents of
> the Packages folder had not changed, SwiftPM still recompiled all of the
> dependencies under the Packages folder.


Makes sense, thanks!

>
> Is there an ETA for using content-based signatures instead?


No, unfortunately.

If it is important for your use case, I'd be ok with accepting a PR to
change swift-build-tool to only look at the time/size information to
workaround this in the meantime.

 - Daniel


>
>
>
> Regards,
>   Ricardo Olivieri
>   Software Engineer
>   IBM Swift Engineering at IBM Cloud
>
>
> From:        Daniel Dunbar <daniel at zuster.org>
> To:        Ricardo N Olivieri/Austin/IBM at IBMUS
> Cc:        swift-build-dev at swift.org
> Date:        11/21/2016 03:58 PM
> Subject:        Re: [swift-build-dev] SwiftPM recompilation - Packages
> folder
> Sent by:        daniel.dunbar at gmail.com
> ------------------------------
>
>
>
> swift-build-tool will currently rebuild any file if it detects a change to
> the file inode or device -- my guess is that is what you are seeing here.
>
> My intention is to eventually move to using content-based signatures to
> determine the rebuild conditions (for tools where we know that is safe),
> which would prevent the rebuild in this situation.
>
> Is there a particularly reason it is important to you that this not
> rebuild?
>
>  - Daniel
>
> On Mon, Nov 21, 2016 at 1:54 PM, Ricardo N Olivieri via swift-build-dev <
> *swift-build-dev at swift.org* <swift-build-dev at swift.org>> wrote:
> Hello all,
>
>
> I was wondering if someone could shed some light on how SwiftPM determines
> if it needs to compile again any of the source code under the Packages
> folder. I had thought that the modified timestamp attribute of any source
> code files under the Packages folder was the key for determining if any of
> the Swift package dependencies should be recompiled. However, it seems
> SwiftPM uses additional criteria to make this determination. Here’s the
> test we executed:
>
>
> 1) At T0, we executed swift build. As expected all Swift package
> dependencies were cloned and compiled along with the application’s code.
>
>
>
> 2) At T1, we executed swift build again and nothing was recompiled, as
> expected since no code changes were made.
>
>
> 3) At T2, we made a copy of the Packages folder and place this copy in a
> location outside the repo’s directory. We then deleted the Packages folder
> from the repo’s directory, and, finally, we copied the Packages folder back
> to its original location in the repo’s directory.
>
>
> 4) At T3, we executed again swift build and noticed that SwiftPM
> recompiled each one of the Swift package dependencies in the Packages
> folder, even though no code changes were made. We also double checked that
> the modified and creation timestamps for the Packages folder and folders
> and files in it were not changed. After doing so, we concluded that though
> copying the Packages folder back to the application’s repo did keep the
> original creation and modified timestamp values of all files and folders,
> SwiftPM still recompiled all of the dependencies in the Packages folder.
>
>
> Does anyone know why we are seeing in SwiftPM the behavior described
> above?
>
>
> Regards,
>   Ricardo Olivieri
>
>
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