tool/gocross: make gocross behave with pre-release Go toolchains

Also switch the wrapper script to use bash not posix shell. We now
depend on bash elsewhere for saner behavior in esoteric areas, so
might as well use it everywhere for consistency.

Fixes #8425

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

tool/gocross/gocross-wrapper.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env sh
+#!/usr/bin/env bash
 # Copyright (c) Tailscale Inc & AUTHORS
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -6,9 +6,9 @@
 # transparently builds gocross using a "bootstrap" Go toolchain, and
 # then invokes gocross.
 
-set -eu
+set -euo pipefail
 
-if [ "${CI:-}" = "true" ]; then
+if [[ "${CI:-}" == "true" ]]; then
     set -x
 fi
 
@@ -17,7 +17,7 @@ fi
 # accidentally mutate the input environment that will get passed to gocross at
 # the bottom of this script.
 (
-repo_root="$(dirname $0)/../.."
+repo_root="${BASH_SOURCE%/*}/../.."
 
 # Figuring out if gocross needs a rebuild, as well as the rebuild itself, need
 # to happen with CWD inside this repo. Since we're in a subshell entirely
@@ -28,16 +28,21 @@ cd "$repo_root"
 
 toolchain="$HOME/.cache/tailscale-go"
 
-if [ -d "$toolchain" ]; then
+if [[ -d "$toolchain" ]]; then
     # A toolchain exists, but is it recent enough to compile gocross? If not,
     # wipe it out so that the next if block fetches a usable one.
     want_go_minor=$(grep -E '^go ' "go.mod" | cut -f2 -d'.')
-    have_go_minor=$(cut -f2 -d'.' <$toolchain/VERSION)
-    if [ -z "$have_go_minor" -o "$have_go_minor" -lt "$want_go_minor" ]; then
+    have_go_minor=$(head -1 "$toolchain/VERSION" | cut -f2 -d'.')
+    # Shortly before stable releases, we run release candidate
+    # toolchains, which have a non-numeric suffix on the version
+    # number. Remove the rc qualifier, we just care about the minor
+    # version.
+    have_go_minor="${have_go_minor%rc*}"
+    if [[ -z "$have_go_minor" || "$have_go_minor" -lt "$want_go_minor" ]]; then
         rm -rf "$toolchain" "$toolchain.extracted"
     fi
 fi
-if [ ! -d "$toolchain" ]; then
+if [[ ! -d "$toolchain" ]]; then
     mkdir -p "$HOME/.cache"
 
     # We need any Go toolchain to build gocross, but the toolchain also has to
@@ -56,10 +61,10 @@ if [ ! -d "$toolchain" ]; then
         # (we do not build tailscale-go for other targets).
         HOST_OS=$(uname -s | tr A-Z a-z)
         HOST_ARCH="$(uname -m)"
-        if [ "$HOST_ARCH" = "aarch64" ]; then
+        if [[ "$HOST_ARCH" == "aarch64" ]]; then
             # Go uses the name "arm64".
             HOST_ARCH="arm64"
-        elif [ "$HOST_ARCH" = "x86_64" ]; then
+        elif [[ "$HOST_ARCH" == "x86_64" ]]; then
             # Go uses the name "amd64".
             HOST_ARCH="amd64"
         fi
@@ -83,13 +88,13 @@ fi
 gocross_path="gocross"
 gocross_ok=0
 wantver="$(git rev-parse HEAD)"
-if [ -x "$gocross_path" ]; then
+if [[ -x "$gocross_path" ]]; then
 	gotver="$($gocross_path gocross-version 2>/dev/null || echo '')"
-	if [ "$gotver" = "$wantver" ]; then
+	if [[ "$gotver" == "$wantver" ]]; then
 		gocross_ok=1
 	fi
 fi
-if [ "$gocross_ok" = "0" ]; then
+if [[ "$gocross_ok" == "0" ]]; then
     unset GOOS
     unset GOARCH
     unset GO111MODULE
@@ -99,4 +104,4 @@ if [ "$gocross_ok" = "0" ]; then
 fi
 ) # End of the subshell execution.
 
-exec "$(dirname $0)/../../gocross" "$@"
+exec "${BASH_SOURCE%/*}/../../gocross" "$@"