build: add `go env GOROOT` as default GOROOT_BOOTSTRAP value

This change also added the same check in make.bash to make.rc,
which makes sure $GOROOT_BOOTSTRAP != $GOROOT.

Fixes #14339

Change-Id: I2758f4a845bae42ace02492fc6a911f6d6247d26
Reviewed-on: https://go-review.googlesource.com/57753
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/make.bash

@@ -119,7 +119,15 @@ rm -f ./runtime/runtime_defs.go
 echo '##### Building Go bootstrap tool.'
 echo cmd/dist
 export GOROOT="$(cd .. && pwd)"
-GOROOT_BOOTSTRAP=${GOROOT_BOOTSTRAP:-$HOME/go1.4}
+export GOROOT_BOOTSTRAP=${GOROOT_BOOTSTRAP:-$HOME/go1.4}
+for go_exe in $(type -ap go); do
+	if [ ! -x "$GOROOT_BOOTSTRAP/bin/go" ]; then
+		goroot=$(GOROOT='' $go_exe env GOROOT)
+		if [ "$goroot" != "$GOROOT" ]; then
+			GOROOT_BOOTSTRAP=$goroot
+		fi
+	fi
+done
 if [ ! -x "$GOROOT_BOOTSTRAP/bin/go" ]; then
 	echo "ERROR: Cannot find $GOROOT_BOOTSTRAP/bin/go." >&2
 	echo "Set \$GOROOT_BOOTSTRAP to a working Go tree >= Go 1.4." >&2

src/make.rc

@@ -46,11 +46,26 @@ echo cmd/dist
 GOROOT = `{cd .. && pwd}
 if(! ~ $#GOROOT_BOOTSTRAP 1)
 	GOROOT_BOOTSTRAP = $home/go1.4
+for(p in $path){
+	if(! test -x $GOROOT_BOOTSTRAP/bin/go){
+		if(go_exe = `{path=$p whatis go}){
+			goroot = `{GOROOT='' $go_exe env GOROOT}
+			if(! ~ $goroot $GOROOT){
+				GOROOT_BOOTSTRAP = $goroot
+			}
+		}
+	}
+}
 if(! test -x $GOROOT_BOOTSTRAP/bin/go){
 	echo 'ERROR: Cannot find '$GOROOT_BOOTSTRAP'/bin/go.' >[1=2]
 	echo 'Set $GOROOT_BOOTSTRAP to a working Go tree >= Go 1.4.' >[1=2]
 	exit bootstrap
 }
+if(~ $GOROOT_BOOTSTRAP $GOROOT){
+	echo 'ERROR: $GOROOT_BOOTSTRAP must not be set to $GOROOT' >[1=2]
+	echo 'Set $GOROOT_BOOTSTRAP to a working Go tree >= Go 1.4.' >[1=2]
+	exit bootstrap
+}
 rm -f cmd/dist/dist
 GOROOT=$GOROOT_BOOTSTRAP GOOS='' GOARCH='' $GOROOT_BOOTSTRAP/bin/go build -o cmd/dist/dist ./cmd/dist