currently I'm trying to debug golang implementation of ethereum(link), because my core interest is in developing new consensus algorithm (i.e. modify the open source Golang code from github).
However, I'm having problem with the location/path of the source code. When I put the folder(i.e. go-ethereum) outside of the $GOPATH and then try to compile&debug geth(go-ethereum/cmd/geth/main.go) it shows the following error: Use of internal package is not allowed.
From that error message, I figured out that the import github.com/ethereum/go-ethereum was not importing my source, and instead it was getting code from internet(like other libraries). Which of course is definitely what I shouldn't do when I'm trying to modify the github.com/ethereum/go-ethereum package code.
So, my workaround was to clone the source code into $GOPATH/src/github.com/ethereum/go-ethereum and followed this answer, and Goland IDE started compiling&debugging without error (wasn't able to go build ./cmd/geth/main.go though due to error undefined: configFileFlag...)
Thereby now I've got a working debugger that can debug with my source code modification, but this doesn't look like ideal source code structure.
The question is:
Is putting source code inside $GOPATH(because of internals) a proper approach? If so, what if I was using go-ethereum package from another project?(Fortunately I'm not, but I'm curious) Do I have to stash&revert changes I made to the code?
Yes, the folder structure you ended up with is the right one.
Code should be under $GOPATH/src as you describe.
But note that $GOPATH is not a fixed folder in your system, you can have multiple projects under different folders and change the value of $GOPATH accordingly depending what you are working on (or have multiple console terminals open, each with its own $GOPATH value).