cmd/go: env -w validates GOTMPDIR value

This change makes go env -w check if GOTMPDIR is an absolute path.

If GOTMPDIR is not an absolute and not existing path there will be an
error at every `work.Builder.Init()`. If `go env` has `-u/-w` as
argument `work.Builder.Init()` is not called.

`go env -w GOTMPDIR=` work in the same way as `go env -u GOTMPDIR`.

Fixes #40932

Change-Id: I6b0662302eeace7f20460b6d26c6e59af1111da2
Reviewed-on: https://go-review.googlesource.com/c/go/+/250198
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Trust: Bryan C. Mills <[email protected]>
Trust: Jay Conrod <[email protected]>

Author: ekalinin

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

@@ -203,10 +203,19 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 	}
 
 	// Do we need to call ExtraEnvVarsCostly, which is a bit expensive?
-	// Only if we're listing all environment variables ("go env")
-	// or the variables being requested are in the extra list.
-	needCostly := true
-	if len(args) > 0 {
+	needCostly := false
+	if *envU || *envW {
+		// We're overwriting or removing default settings,
+		// so it doesn't really matter what the existing settings are.
+		//
+		// Moreover, we haven't validated the new settings yet, so it is
+		// important that we NOT perform any actions based on them,
+		// such as initializing the builder to compute other variables.
+	} else if len(args) == 0 {
+		// We're listing all environment variables ("go env"),
+		// including the expensive ones.
+		needCostly = true
+	} else {
 		needCostly = false
 		for _, arg := range args {
 			switch argKey(arg) {
@@ -269,6 +278,13 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 			}
 		}
 
+		gotmp, okGOTMP := add["GOTMPDIR"]
+		if okGOTMP {
+			if !filepath.IsAbs(gotmp) && gotmp != "" {
+				base.Fatalf("go env -w: GOTMPDIR must be an absolute path")
+			}
+		}
+
 		updateEnvFile(add, nil)
 		return
 	}

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

@@ -24,6 +24,12 @@ stdout GOARCH=
 stdout GOOS=
 stdout GOROOT=
 
+# checking errors
+! go env -w
+stderr 'go env -w: no KEY=VALUE arguments given'
+! go env -u
+stderr 'go env -u: no arguments given'
+
 # go env -w changes default setting
 env root=
 [windows] env root=c:
@@ -97,6 +103,26 @@ stderr 'GOPATH entry cannot start with shell metacharacter'
 ! go env -w GOPATH=./go
 stderr 'GOPATH entry is relative; must be absolute path'
 
+# go env -w rejects invalid GOTMPDIR values
+! go env -w GOTMPDIR=x
+stderr 'go env -w: GOTMPDIR must be an absolute path'
+
+# go env -w should accept absolute GOTMPDIR value
+# and should not create it
+[windows] go env -w GOTMPDIR=$WORK\x\y\z
+[!windows] go env -w GOTMPDIR=$WORK/x/y/z
+! exists $WORK/x/y/z
+# we should be able to clear an env
+go env -u GOTMPDIR
+go env GOTMPDIR
+stdout ^$
+
+[windows] go env -w GOTMPDIR=$WORK\x\y\z
+[!windows] go env -w GOTMPDIR=$WORK/x/y/z
+go env -w GOTMPDIR=
+go env GOTMPDIR
+stdout ^$
+
 # go env -w/-u checks validity of GOOS/ARCH combinations
 env GOOS=
 env GOARCH=