Make sure you have a current backup before trying the upgrade again.
Something which may be causing the hangup is that if you have a SSD, the installer app is going to try to convert the file system from HFS+ to APFS and if it can't do it, it might give up and stop the upgrade process.
So do you have a SSD? (I would guess yes, if you're running Xcode.) If you do, you should have a file /var/log/install.log On my 2012 Mac Mini (which has a SSD) which I upgraded to High Sierra not that long ago, in this log there is a series of messages that says "APFS Converter: Received Progress: X" where X is some percentage and the last one is 100.000000. There might be a message after these Received Progress messages which indicates that it failed and it's status thereafter. Another place to look in the log is around the message "ROM supports APFS". If you don't have that message, that could be important to. The install.log is likely to be a big file and if you try to scan the file as a whole or interpret every line, your head will likely start to spin (mine does).
So if you have an SSD and it failed because it didn't do the conversion, do you have a preference as to whether or not you want to use APFS or stay in HFS+? If you don't know, your MBP is at the end in terms of what OS it's going to officially be able to run with High Sierra. In which case, maybe just stay with HFS+. If you had a newer computer and were diligent about backups, I'd probably suggest doing APFS. If you want more information, search the web for APFS vs HFS+.
If you have a SSD and want APFS, and the upgrade failed because it couldn't do the conversion to APFS, you're probably going to have to erase the disk, format it in APFS and then install and then migrate. (You will need a backup then!)
If you want to stay on HFS+, there's information on doing that at:
http://osxdaily.com/2017/10/17/how-skip-apfs-macos-high-sierra/
This presumes the installer wasn't deleted, which should happen after a
successful install. If it was deleted, you need to re-download and not automatically start the upgrade, but do it via the commands in the previous link.
If you want to follow the advice in post #2,
https://support.apple.com/en-us/HT201372
If you have a HDD, I had a suggestion in the other thread about using an external HDD or you can try doing the install in safe mode.
You may be wondering why I didn't suggest all of this in the other thread - well - half-way writing this post I started thinking - hmmm - "I wonder if the APFS conversion is playing a role in this". So my original post was going to be quite different.
