fix Issue 16349 - better curl retry for install.sh script

[MartinNowak]

• curl's --retry option isn't really helpful b/c no timeouts are
  activated by default
• use a retry loop with increasing sleep times instead
• enable connection (5s) and download (<1KB/s for 30s) timeouts
  (using --max-time is too tricky b/c of the unknown download sizes)

[dlang-bot]

Fix	Bugzilla	Description
✓	16349	better curl retry for install.sh script

[MartinNowak]

FYI @wilzbach

[wilzbach]

Should we try to cleanup intermediate temp files on failure?
(due to fatal exiting directly the cleanup in fetch won't take place)

[MartinNowak]

There is already a cleanup trap that wipes the whole TMP_ROOT, I'm just deleting intermediate temporaries for cleanliness.

[MartinNowak]

I tested all compilers, and the --activate functionality. Also played a bit w/ tc to simulate a bad network connection, but it's tricky to properly test this. It should be better than before.

[wilzbach]

I tested all compilers

Maybe at some point we should setup a small travis CI for this repo that does this automatically? ;-)

Also played a bit w/ tc to simulate a bad network connection, but it's tricky to properly test this. It should be better than before.

I did the same and also tried it a bit and it looks really good - e.g. I interrupted the network during the download and all the retry and timeouts fire correctly :)

Downloading and unpacking http://downloads.dlang.org/releases/2.x/2.071.1/dmd.2.071.1.linux.tar.xz
##                                                                         3.4%



curl: (28) Operation too slow. Less than 1024 bytes/sec transferred the last 30 seconds

curl: (7) Couldn't connect to server

curl: (7) Couldn't connect to server

curl: (7) Couldn't connect to server

curl: (7) Couldn't connect to server
Failed to download 'http://downloads.dlang.org/releases/2.x/2.071.1/dmd.2.071.1.linux.tar.xz'

So from my side this looks great - thanks for doing this @MartinNowak :)

[MartinNowak]

Let's closely monitor if that fixes the remaining Travis-CI issues.