Consider using GitHub Releases instead of SWI-Prolog Servers

[fnogatz]

Since we constantly get errors in CI environments of third-party projects that fail only because the SWI-Prolog servers are down sometimes, we should consider to use GitHub's releases as the first option to download the source code: https://github.com/SWI-Prolog/swipl-devel/releases

/c @JanWielemaker, as it could be just a temporary server problem.
/c @jonakalkus, who reported this.

[JanWielemaker]

I think is is better to make sure our infrastructure works. Note that the tarballs cannot be done as a github release as it contains both a (subset of) the git submodules and generated files that require extensive infrastructure to create.

Today we had a failure due to one misconfigured backend due to the move to https and the other one failing due to NFS issues. Typically I think problems are rare nowadays. If they are still too often we should get a third backend.

[fnogatz]

Thank you for your answer, Jan. I am using swivm in two projects and all of the last CI failures were due to missing connections to the SWI-Prolog servers. Maybe I will just log the connection errors the next weeks first and then we can decide whether there's a general problem with SWI's servers.

[JanWielemaker]

If you have details I can try to figure out what was wrong.

[fnogatz]

A small list of outage times:

• 2017-06-30T10:26:49Z
• 2017-06-25T07:03:18Z
• 2017-06-25T06:59:51Z
• 2017-06-18T15:18:17Z
• 2017-06-18T15:14:26Z
• 2017-04-09T06:53:23Z

Since I restarted the failing Travis builds later, I have unfortunately no longer a full record of outage times :-/

[JanWielemaker]

Ok. Which requests do you do, how frequently and to which host? If possible, from which ip address?

[fnogatz]

With #16, this issue here is also solved. I still leave it open as a reminder to add a switch for the user to explicitly decide which mirror to use.

[JanWielemaker]

Just a possibly stupid question: why don't you simply use git? That also allows people to install the current HEAD or even a specific hash, as well as any release as these are tagged VX.Y.Z. As there are no more generated files this should work fine now.

[fnogatz]

Obvious question :) I wanted to avoid a mixture of downloading the tarball of swipl-devel, and using git for the submodules. By simply downloading the packages manually, we do not need git for installation.

I will add a way to use only git for both the swipl-devel repository and its submodules in the future (see #12).

[JanWielemaker]

Good 😄 Any plans to make swivm cross-platform? I had some discussion here with Wouter Beek about using conan to deal with the dependencies in a portable way. Should you (or anyone else) be interested to generalise the current build system to become truly OS and toolchain independent, let us join forces.

[fnogatz]

Any plans to make swivm cross-platform?

Not yet. swivm resulted from my needs to test things in different versions of SWI-Prolog. If I only ever need to compile SWI on other OS, I am pretty sure I will add it very soon -- or if someone pays for it 😜