cmd/go/internal/env: add GOCACHEPROG to go env output

For #71059

Change-Id: I4bbdd14d416dc2e6dae3549a84c16dbef9d4e645
Reviewed-on: https://go-review.googlesource.com/c/go/+/640755
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/cache/default.go

@@ -54,8 +54,8 @@ func initDefaultCache() Cache {
 		base.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
 	}
 
-	if v := cfg.Getenv("GOCACHEPROG"); v != "" {
-		return startCacheProg(v, diskCache)
+	if cfg.GOCACHEPROG != "" {
+		return startCacheProg(cfg.GOCACHEPROG, diskCache)
 	}
 
 	return diskCache

src/cmd/go/internal/cfg/cfg.go

@@ -426,6 +426,7 @@ var (
 	GOROOTsrc string
 
 	GOBIN                           = Getenv("GOBIN")
+	GOCACHEPROG, GOCACHEPROGChanged = EnvOrAndChanged("GOCACHEPROG", "")
 	GOMODCACHE, GOMODCACHEChanged   = EnvOrAndChanged("GOMODCACHE", gopathDir("pkg/mod"))
 
 	// Used in envcmd.MkEnv and build ID computations.

src/cmd/go/internal/envcmd/env.go

@@ -85,6 +85,7 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GOAUTH", Value: cfg.GOAUTH, Changed: cfg.GOAUTHChanged},
 		{Name: "GOBIN", Value: cfg.GOBIN},
 		{Name: "GOCACHE"},
+		{Name: "GOCACHEPROG", Value: cfg.GOCACHEPROG, Changed: cfg.GOCACHEPROGChanged},
 		{Name: "GODEBUG", Value: os.Getenv("GODEBUG")},
 		{Name: "GOENV", Value: envFile, Changed: envFileChanged},
 		{Name: "GOEXE", Value: cfg.ExeSuffix},

src/cmd/go/testdata/script/env_gocacheprog.txt

@@ -0,0 +1,42 @@
+# GOCACHEPROG unset
+env GOCACHEPROG=
+
+go env
+stdout 'GOCACHEPROG=''?''?'
+
+go env -changed
+! stdout 'GOCACHEPROG'
+
+go env -changed -json
+! stdout 'GOCACHEPROG'
+
+# GOCACHEPROG set
+[short] skip 'compiles and runs a go program'
+
+go build -o cacheprog$GOEXE cacheprog.go
+
+env GOCACHEPROG=$GOPATH/src/cacheprog$GOEXE
+
+go env
+stdout 'GOCACHEPROG=''?'$GOCACHEPROG'''?'
+
+go env -changed
+stdout 'GOCACHEPROG=''?'$GOCACHEPROG'''?'
+
+go env -changed -json
+stdout '"GOCACHEPROG": "'$GOCACHEPROG'"'
+
+-- cacheprog.go --
+// This is a minimal GOCACHEPROG program that can't actually do anything but exit.
+package main
+
+import (
+    "encoding/json"
+    "os"
+)
+
+func main() {
+    json.NewEncoder(os.Stdout).Encode(map[string][]string{"KnownCommands": {"close"}})
+    var res struct{}
+    json.NewDecoder(os.Stdin).Decode(&res)
+}