tool/gocross: adjust Xcode flags to match new Xcode env

Xcode changed how/what data it exports to build steps at some point
recently, so our old way of figuring out the minimum support version
for clang stopped working.

Updates tailscale/corp#4095

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/autoflags.go

@@ -104,10 +104,15 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		if env.IsSet("XCODE_VERSION_ACTUAL") {
 			var xcodeFlags []string
 			// Minimum OS version being targeted, results in
-			// e.g. -mmacosx-version-min=11.3
-			minOSKey := env.Get("DEPLOYMENT_TARGET_CLANG_FLAG_NAME", "")
-			minOSVal := env.Get(env.Get("DEPLOYMENT_TARGET_CLANG_ENV_NAME", ""), "")
-			xcodeFlags = append(xcodeFlags, fmt.Sprintf("-%s=%s", minOSKey, minOSVal))
+			// e.g. -mmacosx-version-min=11.3, -miphoneos-version-min=15.0
+			switch {
+			case env.IsSet("IPHONEOS_DEPLOYMENT_TARGET"):
+				xcodeFlags = append(xcodeFlags, "-miphoneos-version-min="+env.Get("IPHONEOS_DEPLOYMENT_TARGET", ""))
+			case env.IsSet("MACOSX_DEPLOYMENT_TARGET"):
+				xcodeFlags = append(xcodeFlags, "-mmacosx-version-min="+env.Get("MACOSX_DEPLOYMENT_TARGET", ""))
+			default:
+				return nil, nil, fmt.Errorf("invoked by Xcode but couldn't figure out deployment target. Did Xcode change its envvars again?")
+			}
 
 			// Target-specific SDK directory. Must be passed as two
 			// words ("-isysroot PATH", not "-isysroot=PATH").

tool/gocross/autoflags_test.go

@@ -280,9 +280,7 @@ TS_LINK_FAIL_REFLECT=1 (was <nil>)`,
 				"GOOS":                     "darwin",
 				"GOARCH":                   "amd64",
 				"XCODE_VERSION_ACTUAL":     "1300",
-				"DEPLOYMENT_TARGET_CLANG_FLAG_NAME": "mmacosx-version-min",
 				"MACOSX_DEPLOYMENT_TARGET": "11.3",
-				"DEPLOYMENT_TARGET_CLANG_ENV_NAME":  "MACOSX_DEPLOYMENT_TARGET",
 				"SDKROOT":                  "/my/sdk/root",
 			},
 			argv:         []string{"gocross", "build", "./cmd/tailcontrol"},
@@ -308,6 +306,38 @@ TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
 				"./cmd/tailcontrol",
 			},
 		},
+		{
+			name: "darwin_amd64_to_ios_arm64_xcode",
+			env: map[string]string{
+				"GOOS":                       "ios",
+				"GOARCH":                     "arm64",
+				"XCODE_VERSION_ACTUAL":       "1300",
+				"IPHONEOS_DEPLOYMENT_TARGET": "15.0",
+				"SDKROOT":                    "/my/sdk/root",
+			},
+			argv:         []string{"gocross", "build", "./cmd/tailcontrol"},
+			goroot:       "/goroot",
+			nativeGOOS:   "darwin",
+			nativeGOARCH: "amd64",
+
+			envDiff: `CC=cc (was <nil>)
+CGO_CFLAGS=-O3 -std=gnu11 -miphoneos-version-min=15.0 -isysroot /my/sdk/root -arch arm64 (was <nil>)
+CGO_ENABLED=1 (was <nil>)
+CGO_LDFLAGS=-miphoneos-version-min=15.0 -isysroot /my/sdk/root -arch arm64 (was <nil>)
+GOARCH=arm64 (was arm64)
+GOARM=5 (was <nil>)
+GOMIPS=softfloat (was <nil>)
+GOOS=ios (was ios)
+GOROOT=/goroot (was <nil>)
+TS_LINK_FAIL_REFLECT=1 (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 -w",
+				"./cmd/tailcontrol",
+			},
+		},
 		{
 			name:         "linux_amd64_to_linux_amd64_in_goroot",
 			argv:         []string{"go", "build", "./cmd/tailcontrol"},