From 87f00d76c4815875b1c1100c5cb1bf77b6db54ef Mon Sep 17 00:00:00 2001
From: James Tucker <james@tailscale.com>
Date: Thu, 16 May 2024 10:53:44 -0700
Subject: [PATCH] tool/gocross: treat empty GOOS/GOARCH as native GOOS/GOARCH

Tracking down the side effect can otherwise be a pain, for example on
Darwin an empty GOOS resulted in CGO being implicitly disabled. The user
intended for `export GOOS=` to act like unset, and while this is a
misunderstanding, the main toolchain would treat it this way.

Fixes tailscale/corp#20059

Signed-off-by: James Tucker <james@tailscale.com>
---
 tool/gocross/autoflags.go      |  5 +--
 tool/gocross/autoflags_test.go | 58 ++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/tool/gocross/autoflags.go b/tool/gocross/autoflags.go
index 6bf04d3cc..fd58c1fcb 100644
--- a/tool/gocross/autoflags.go
+++ b/tool/gocross/autoflags.go
@@ -4,6 +4,7 @@
 package main
 
 import (
+	"cmp"
 	"fmt"
 	"runtime"
 	"strings"
@@ -32,8 +33,8 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		subcommand = ""
 
 		cc          = "cc"
-		targetOS    = env.Get("GOOS", nativeGOOS)
-		targetArch  = env.Get("GOARCH", nativeGOARCH)
+		targetOS    = cmp.Or(env.Get("GOOS", ""), nativeGOOS)
+		targetArch  = cmp.Or(env.Get("GOARCH", ""), nativeGOARCH)
 		buildFlags  = []string{"-trimpath"}
 		cgoCflags   = []string{"-O3", "-std=gnu11"}
 		cgoLdflags  []string
diff --git a/tool/gocross/autoflags_test.go b/tool/gocross/autoflags_test.go
index 9f24a131b..438198de7 100644
--- a/tool/gocross/autoflags_test.go
+++ b/tool/gocross/autoflags_test.go
@@ -275,6 +275,64 @@ GOMIPS=softfloat (was <nil>)
 GOOS=darwin (was <nil>)
 GOROOT=/goroot (was <nil>)
 GOTOOLCHAIN=local (was <nil>)
+TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
+			wantArgv: []string{
+				"gocross", "build",
+				"-trimpath",
+				"-tags=tailscale_go,omitidna,omitpemdecrypt",
+				"-ldflags", "-X tailscale.com/version.longStamp=1.2.3-long -X tailscale.com/version.shortStamp=1.2.3 -X tailscale.com/version.gitCommitStamp=abcd -X tailscale.com/version.extraGitCommitStamp=defg",
+				"./cmd/tailcontrol",
+			},
+		},
+		{
+			name: "darwin_arm64_to_darwin_arm64_empty_goos",
+			argv: []string{"gocross", "build", "./cmd/tailcontrol"},
+			env: map[string]string{
+				"GOOS": "",
+			},
+			goroot:       "/goroot",
+			nativeGOOS:   "darwin",
+			nativeGOARCH: "arm64",
+
+			envDiff: `CC=cc (was <nil>)
+CGO_CFLAGS=-O3 -std=gnu11 (was <nil>)
+CGO_ENABLED=1 (was <nil>)
+CGO_LDFLAGS= (was <nil>)
+GOARCH=arm64 (was <nil>)
+GOARM=5 (was <nil>)
+GOMIPS=softfloat (was <nil>)
+GOOS=darwin (was )
+GOROOT=/goroot (was <nil>)
+GOTOOLCHAIN=local (was <nil>)
+TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
+			wantArgv: []string{
+				"gocross", "build",
+				"-trimpath",
+				"-tags=tailscale_go,omitidna,omitpemdecrypt",
+				"-ldflags", "-X tailscale.com/version.longStamp=1.2.3-long -X tailscale.com/version.shortStamp=1.2.3 -X tailscale.com/version.gitCommitStamp=abcd -X tailscale.com/version.extraGitCommitStamp=defg",
+				"./cmd/tailcontrol",
+			},
+		},
+		{
+			name: "darwin_arm64_to_darwin_arm64_empty_goarch",
+			argv: []string{"gocross", "build", "./cmd/tailcontrol"},
+			env: map[string]string{
+				"GOARCH": "",
+			},
+			goroot:       "/goroot",
+			nativeGOOS:   "darwin",
+			nativeGOARCH: "arm64",
+
+			envDiff: `CC=cc (was <nil>)
+CGO_CFLAGS=-O3 -std=gnu11 (was <nil>)
+CGO_ENABLED=1 (was <nil>)
+CGO_LDFLAGS= (was <nil>)
+GOARCH=arm64 (was )
+GOARM=5 (was <nil>)
+GOMIPS=softfloat (was <nil>)
+GOOS=darwin (was <nil>)
+GOROOT=/goroot (was <nil>)
+GOTOOLCHAIN=local (was <nil>)
 TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
 			wantArgv: []string{
 				"gocross", "build",