build: for default bootstrap, use Go 1.17 if present, falling back to Go 1.4

Preparation for #44505, but safe for Go 1.18.
Also fixes the default build on Macs, at least for
people who have a $HOME/go1.17 or have run

	go install golang.org/dl/go1.17@latest
	go1.17 download

Change-Id: I822f93e75498620fad87db2436376148c42f6bff
Reviewed-on: https://go-review.googlesource.com/c/go/+/369914
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/buildtool.go

@@ -93,10 +93,21 @@ var ignoreSuffixes = []string{
 	"_test.go",
 }
 
+var tryDirs = []string{
+	"sdk/go1.17",
+	"go1.17",
+}
+
 func bootstrapBuildTools() {
 	goroot_bootstrap := os.Getenv("GOROOT_BOOTSTRAP")
 	if goroot_bootstrap == "" {
-		goroot_bootstrap = pathf("%s/go1.4", os.Getenv("HOME"))
+		home := os.Getenv("HOME")
+		goroot_bootstrap = pathf("%s/go1.4", home)
+		for _, d := range tryDirs {
+			if p := pathf("%s/%s", home, d); isdir(p) {
+				goroot_bootstrap = p
+			}
+		}
 	}
 	xprintf("Building Go toolchain1 using %s.\n", goroot_bootstrap)
 

src/make.bash

@@ -153,7 +153,14 @@ if [ "$1" = "-v" ]; then
 fi
 
 goroot_bootstrap_set=${GOROOT_BOOTSTRAP+"true"}
-export GOROOT_BOOTSTRAP=${GOROOT_BOOTSTRAP:-$HOME/go1.4}
+if [ -z "$GOROOT_BOOTSTRAP" ]; then
+	GOROOT_BOOTSTRAP="$HOME/go1.4"
+	for d in sdk/go1.17 go1.17; do
+		if [ -d "$HOME/$d" ]; then
+			GOROOT_BOOTSTRAP="$HOME/$d"
+		fi
+	done
+fi
 export GOROOT="$(cd .. && pwd)"
 IFS=$'\n'; for go_exe in $(type -ap go); do
 	if [ ! -x "$GOROOT_BOOTSTRAP/bin/go" ]; then

src/make.bat

@@ -83,6 +83,8 @@ for /f "tokens=*" %%g in ('where go 2^>nul') do (
 		)
 	)
 )
+if "x%GOROOT_BOOTSTRAP%"=="x" if exist "%HOMEDRIVE%%HOMEPATH%\go1.17" set GOROOT_BOOTSTRAP=%HOMEDRIVE%%HOMEPATH%\go1.17
+if "x%GOROOT_BOOTSTRAP%"=="x" if exist "%HOMEDRIVE%%HOMEPATH%\sdk\go1.17" set GOROOT_BOOTSTRAP=%HOMEDRIVE%%HOMEPATH%\sdk\go1.17
 if "x%GOROOT_BOOTSTRAP%"=="x" set GOROOT_BOOTSTRAP=%HOMEDRIVE%%HOMEPATH%\Go1.4
 
 :bootstrapset

src/make.rc

@@ -55,6 +55,9 @@ goroot_bootstrap_set = 'true'
 if(! ~ $#GOROOT_BOOTSTRAP 1){
 	goroot_bootstrap_set = 'false'
 	GOROOT_BOOTSTRAP = $home/go1.4
+	for(d in sdk/go1.17 go1.17)
+		if(test -d $home/$d)
+			GOROOT_BOOTSTRAP = $home/$d
 }
 for(p in $path){
 	if(! test -x $GOROOT_BOOTSTRAP/bin/go){