Fix DNS mirror forward redirects Issue: gmt-admin #297 Incorrect DNS settings for mirrors

[ES-Solar]

Problem

Related to GenericMappingTools/gmt-admin#297

Alias server names such as china and australia expand to Hover DNS
URL-forwards (e.g., http://china.generic-mapping-tools.org) that redirect
all requests to the mirror root — stripping the file path in the process.
This caused every file download to fail: GMT received an HTML directory
listing instead of the requested grid or cache file.

Fix

gmtremote_resolve_redirect() in gmt_remote.c follows the redirect chain
once at session start to discover the real mirror base URL
(e.g., https://mirrors.ustc.edu.cn/gmtdata). gmt_dataserver_url() caches
the result for the process lifetime and uses it for all subsequent file URL
construction. Direct http:// URLs and servers with no redirect are
unaffected.

Test

Added test/gmt_remote/dataserver_alias.sh which forces GMT_DATA_SERVER=oceania,
clears the local cache, downloads @earth_relief_15m_g.grd, and verifies the
grid region with gmt grdinfo -I-.

[joa-quim]

Use the earth_relief_01d_p.grd grid instead. Much smaller.

[joa-quim]

Also, don't you have permission writes to master? Because PRs submitted to master are much easier for us to test.

[ES-Solar]

I updated the test to use the smaller grid.

was used to the updating my fork and thought that was the preferred workflow. I'll change that for the next PR I contribute.

[joa-quim]

Tested it with the Portuguese site and it worked.
Also run the tests locally and all fine.

[ES-Solar]

Should i be concerned about the 4 pull_request failures? Also, there is a note that 'Merging is blocked You're not authorized to push to this branch.' Does my github account need additional privilege to merge the PR?

[joa-quim]

I see only failures in MacOS and are not related to this. Linux is a mess download failures and Windows build is broken. I'll
merge it.

Does my github account need additional privilege to merge the PR?

Yes, and probably means that you must to be added to a group that has that privilege.