proxyOnce.Do(func() {
		if cfg.GONOPROXY != "" && cfg.GOPROXY != "direct" {
			proxyOnce.list = append(proxyOnce.list, proxySpec{url: "noproxy"})
		}

		goproxy := cfg.GOPROXY
		for goproxy != "" {
			var url string
			fallBackOnError := false
			if i := strings.IndexAny(goproxy, ",|"); i >= 0 {
				url = goproxy[:i]
				fallBackOnError = goproxy[i] == '|'
				goproxy = goproxy[i+1:]
			} else {
				url = goproxy

env GOPROXY=','
! go get golang.org/x/text
stderr '^go: golang.org/x/text: GOPROXY list is not the empty string, but contains no entries$'

# When GOPROXY=off, fetching modules not matched by GONOPROXY fails.
env GONOPROXY=*/fortune
env GOPROXY=off
! go get golang.org/x/text
stderr '^go: golang.org/x/text: module lookup disabled by GOPROXY=off$'

# GONOPROXY bypasses proxy
[!net:rsc.io] skip
[!git] skip
env GOPRIVATE=none

env sumdb=$GOSUMDB
env proxy=$GOPROXY
env GOPROXY GONOPROXY GOSUMDB GONOSUMDB

# basic fetch (through proxy) works
cp go.mod.orig go.mod
go get rsc.io/fortune@v1.0.0 # note: must use test proxy, does not exist in real world
rm $GOPATH/pkg/mod/cache/download/sumdb # rm sumdb cache but NOT package download cache
rm go.sum

# can fetch by explicit URL
cp go.mod.orig go.mod
env GOSUMDB=$sumdb' '$proxy/sumdb-direct

env GO111MODULE=on
env sumdb=$GOSUMDB
env proxy=$GOPROXY
env GOPROXY GONOPROXY GOSUMDB GONOSUMDB

# rejected proxy fails verification
cp go.mod.orig go.mod
rm go.sum
env GOPROXY=$proxy/sumdb-503
! go get rsc.io/quote
stderr 503

# fetch through working proxy is OK
cp go.mod.orig go.mod
rm go.sum
env GOPROXY=$proxy
go get rsc.io/quote

# repeated fetch works entirely from cache, does not consult sumdb
cp go.mod.orig go.mod

[GOARCH:amd64] stdout '"GOARCH": "arm64"'
[!GOARCH:amd64] stdout '"GOARCH": "amd64"'

env GOPROXY=s
go env -changed GOPROXY
! stdout 'GOPROXY'
env GOPROXY=s2
go env -changed GOPROXY
stdout 'GOPROXY=''?s2''?'

env GOROOT=./b
go env -changed
! stdout 'GOTOOLCHAIN=''?local''?'

--  a/go.env --
GOPROXY=s

--  b/go.env --

# a fallback module mirror.
grep golang.org/x/text go.sum
env GOPATH=$WORK/gopath3
[GOOS:windows] env GOPROXY=file:///$WORK/sumproxy
[!GOOS:windows] env GOPROXY=file://$WORK/sumproxy
! go get golang.org/x/text@v0.3.2
[GOOS:windows] env GOPROXY=file:///$WORK/sumproxy,https://proxy.golang.org
[!GOOS:windows] env GOPROXY=file://$WORK/sumproxy,https://proxy.golang.org
go get golang.org/x/text@v0.3.2

-- supported --

-- go.mod --

env GO111MODULE=on
env proxy=$GOPROXY

# Proxy that can't serve should fail.
env GOPROXY=$proxy/404
! go get rsc.io/quote@v1.0.0
stderr '404 Not Found'

# get should walk down the proxy list past 404 and 410 responses.
env GOPROXY=$proxy/404,$proxy/410,$proxy
go get rsc.io/quote@v1.1.0

# get should not walk past other 4xx errors if proxies are separated with ','.
env GOPROXY=$proxy/403,$proxy
! go get rsc.io/quote@v1.2.0

# Test default GOPROXY and GOSUMDB
[go-builder] env GOPROXY=
[go-builder] env GOSUMDB=
[go-builder] go env GOPROXY
[go-builder] stdout '^https://proxy.golang.org,direct$'
[go-builder] go env GOSUMDB
[go-builder] stdout '^sum.golang.org$'
[go-builder] env GOPROXY=https://proxy.golang.org
[go-builder] go env GOSUMDB
[go-builder] stdout '^sum.golang.org$'

# Download direct from github.

[!net:proxy.golang.org] skip
[!net:sum.golang.org] skip

	# In the Go project's official release GOPROXY defaults to proxy.golang.org,
	# but it may be changed in GOROOT/go.env (such as in third-party
	# distributions).
	#
	# Make sure it is in use here, because the server for releases not served
	# through the proxy (https://golang.org/toolchain?go-get=1) currently only
	# serves the latest patch release for each of the supported stable releases.

[go-builder] env GOPROXY=

# If GOPROXY is 'off', lookups should use whatever pseudo-version is available.
env GOPROXY=off
go mod download -json golang.org/x/text@a1b916ed6726
stdout '"Version": "v0.0.0-20171215141712-a1b916ed6726",'

# If we can re-resolve the commit to a pseudo-version, fetching the commit by
# hash should use the highest such pseudo-version appropriate to the commit.
env GOPROXY=direct
go mod download -json golang.org/x/text@a1b916ed6726