go: Fix go update

- Change who we get TARGET dir not to be based on GOROOT
- Change display of TARGET using realpath
- Change latest sematnics to use internal /go
- Change latest link location to be inside of repo

README.md

@@ -13,13 +13,13 @@ Scylla Manager consists of tree components:
 
 The desired Go version is specified in `.go-version` file.
 You can install or update Go version by running `make install` in `go` directory.
+It would install the required version alongside your current version assuming that go is extracted from an official tar package.
+If you do not have any Go installed at this point you can pass TARGET variable to specify a directory where to install Go. 
 
 **Procedure**:
 
-1. Define `GOROOT` environment variable as `/PATH/TO/GO/SDKS/latest`
-1. Run `make -C go install`
-
-This would download required go version and install it at `GOROOT`.
+1. Run `make -C go install TARGET=/path/to/go/sdks/goversion` example `make install TARGET=~/tools/go/go1.15.5.linux-amd64`
+1. Define `GOROOT` environment variable as `<git-root>/go/latest`
 
 ## Installing other packages needed for development
 

go/Makefile

@@ -14,15 +14,11 @@ $(GO_BUNDLE):
 
 .PHONY: install
 install: $(GO_BUNDLE)
-install: TARGET=$(shell dirname $(GOROOT))/go$(GO_VERSION).linux-amd64
+install: TARGET=$(shell realpath -m $(shell which go)../../../../../go$(GO_VERSION).linux-amd64)
 install:
-ifndef GOROOT
-	$(error set GOROOT environment variable as '/PATH/TO/GO/SDKS/latest')
-endif
 	@echo "==> Installing Go $(GO_VERSION) to $(TARGET)..."
 	@[ -d $(TARGET) ] || (mkdir -p $(TARGET) && tar -xzf $(GO_BUNDLE) -C $(TARGET))
-	@rm -f `dirname $(TARGET)`/latest
-	@ln -sf $(TARGET)/go `dirname $(TARGET)`/latest
+	@rm -f latest && ln -sf $(TARGET)/go latest
 
 .PHONY: clean
 clean: