tool/gocross: support local toolchain for development

This makes gocross and its bootstrap script understand an absolute
path in go.toolchain.rev to mean "use the given toolchain directly".

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

tool/gocross/gocross-wrapper.sh

@@ -31,6 +31,11 @@ if [ ! -d "$toolchain" ]; then
     # updates.
     read -r REV <$repo_root/go.toolchain.rev
 
+    case "$REV" in
+    /*)
+        toolchain="$REV"
+        ;;
+    *)
         # This works for linux and darwin, which is sufficient
         # (we do not build tailscale-go for other targets).
         HOST_OS=$(uname -s | tr A-Z a-z)
@@ -48,6 +53,8 @@ if [ ! -d "$toolchain" ]; then
         mkdir -p "$toolchain"
         (cd "$toolchain" && tar --strip-components=1 -xf "$toolchain.tar.gz")
         echo "$REV" >"$toolchain.extracted"
+        ;;
+    esac
 fi
 
 # Binaries run with `gocross run` can reinvoke gocross, resulting in a

tool/gocross/toolchain.go

@@ -112,9 +112,18 @@ func ensureToolchain(cacheDir, toolchainDir string) error {
 		return err
 	}
 
+	if filepath.IsAbs(wantRev) {
+		// Local dev toolchain.
+		if err := os.Symlink(wantRev, toolchainDir); err != nil {
+			return err
+		}
+		return nil
+	} else {
 		if err := downloadCachedgo(toolchainDir, wantRev); err != nil {
 			return err
 		}
+	}
+
 	if err := os.WriteFile(stampFile, []byte(wantRev), 0644); err != nil {
 		return err
 	}