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 <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>

1 files changed, 20 insertions, 4 deletions

diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index e1f2400f60..59d0ded658 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/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
 	}