[MacPorts] #62995: restore_ports fails during migration to macOS 11.[34] (Big Sur) with "requested variants do not match" and also complains "macOS 11 SDK does not appear to be installed"

[MacPorts]

#62995: restore_ports fails during migration to macOS 11.[34] (Big Sur) with
"requested variants do not match" and also complains "macOS 11 SDK does not
appear to be installed"
------------------------+--------------------
 Reporter:  MarkCallow  |      Owner:  (none)
     Type:  defect      |     Status:  new
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:  2.7.1
 Keywords:              |       Port:
------------------------+--------------------
 I am migrating my MacPorts installation from macOS Catalina to Big Sur.
 During `restore_ports.tcl` I've had several errors like the following:

 {{{
 Error: Requested variants "+python39" do not match those the build was
 started with: "+python37".
 Error: Please use the same variants again, or run 'port clean xorg-libxcb'
 first to remove the existing partially completed build.
 Error: See
 /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports
 .org_release_tarballs_ports_x11_xorg-libxcb/xorg-libxcb/main.log for
 details.
 }}}

 I tried just re-running and doing `port clean xorg-libxcb` then re-
 running. There was no change. As only an occasional user of macports, I do
 not understand "variants" nor how to specify them to the port command.


 Also many times during `restore_ports.tcl` the waarning

 {{{
 Warning: The macOS 11 SDK does not appear to be installed. Ports may not
 build correctly.
 Warning: You can install it as part of the Xcode Command Line Tools
 package by running `xcode-select --install'.
 }}}

 is printed. I have Xcode 12.5 installed and I do have the command line
 tools installed. When I run `xcode-select --install` I get the message

 {{{
 xcode-select: error: command line tools are already installed, use
 "Software Update" to install updates
 }}}

-- 
Ticket URL: <https://trac.macports.org/ticket/62995>
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