tool/gocross: make gocross-wrapper.sh keep multiple Go toolchains around

So it doesn't delete and re-pull when switching between branches.

Updates tailscale/corp#17686

Change-Id: Iffb989781db42fcd673c5f03dbd0ce95972ede0f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tool/gocross/gocross-wrapper.sh

@@ -26,40 +26,29 @@ repo_root="${BASH_SOURCE%/*}/../.."
 # being invoked from somewhere else.
 cd "$repo_root"
 
-toolchain="$HOME/.cache/tailscale-go"
+# toolchain, set below, is the root of the Go toolchain we'll use to build
+# gocross.
+#
+# It's set to either an explicit Go toolchain directory (if go.toolchain.rev has
+# a value with a leading slash, for testing new toolchains), or otherwise in the
+# common case it'll be "$HOME/.cache/tsgo/GITHASH" where GITHASH is the contents
+# of the go.toolchain.rev file and the git commit of the
+# https://github.com/tailscale/go release artifact to download.
+toolchain=""
 
-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=""
-    if [[ -f "$toolchain/VERSION" ]]; then
-        have_go_minor=$(head -1 "$toolchain/VERSION" | cut -f2 -d'.')
-    fi
-    # Shortly before stable releases, we run release candidate
-    # toolchains, which have a non-numeric suffix on the version
-    # number. Remove the rc qualifier, we just care about the minor
-    # version.
-    have_go_minor="${have_go_minor%rc*}"
-    if [[ -z "$have_go_minor" || "$have_go_minor" -lt "$want_go_minor" ]]; then
+read -r REV <go.toolchain.rev
+case "$REV" in
+/*)
+    toolchain="$REV"
+    ;;
+*)
+    toolchain="$HOME/.cache/tsgo/$REV"
+    if [[ ! -f "$toolchain.extracted" ]]; then
+        mkdir -p "$HOME/.cache/tsgo"
         rm -rf "$toolchain" "$toolchain.extracted"
-    fi
-fi
-if [[ ! -d "$toolchain" ]]; then
-    mkdir -p "$HOME/.cache"
 
-    # We need any Go toolchain to build gocross, but the toolchain also has to
-    # be reasonably recent because we upgrade eagerly and gocross might not
-    # build with Go N-1. So, if we have no cached tailscale toolchain at all,
-    # fetch the initial one in shell. Once gocross is built, it'll manage
-    # updates.
-    read -r REV <go.toolchain.rev
+        echo "# Downloading Go toolchain $REV" >&2
 
-    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=$(uname -s | tr A-Z a-z)
@@ -71,15 +60,38 @@ if [[ ! -d "$toolchain" ]]; then
             # Go uses the name "amd64".
             HOST_ARCH="amd64"
         fi
-
-        rm -rf "$toolchain" "$toolchain.extracted"
         curl -f -L -o "$toolchain.tar.gz" "https://github.com/tailscale/go/releases/download/build-${REV}/${HOST_OS}-${HOST_ARCH}.tar.gz"
         mkdir -p "$toolchain"
         (cd "$toolchain" && tar --strip-components=1 -xf "$toolchain.tar.gz")
         echo "$REV" >"$toolchain.extracted"
         rm -f "$toolchain.tar.gz"
-        ;;
-    esac
+
+        # Do some cleanup of old toolchains while we're here.
+        for hash in $(find "$HOME/.cache/tsgo" -type f -maxdepth 1 -name '*.extracted' -mtime 90 -exec basename {} \; | sed 's/.extracted$//'); do
+            echo "# Cleaning up old Go toolchain $hash" >&2
+            rm -rf "$HOME/.cache/tsgo/$hash"
+            rm -rf "$HOME/.cache/tsgo/$hash.extracted"
+        done
+    fi
+    ;;
+esac
+
+if [[ -d "$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="$(grep -E '^go ' "go.mod" | cut -f2 -d'.')"
+    have_go_minor=""
+    if [[ -f "$toolchain/VERSION" ]]; then
+        have_go_minor="$(head -1 "$toolchain/VERSION" | cut -f2 -d'.')"
+    fi
+    # 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
 
 # Binaries run with `gocross run` can reinvoke gocross, resulting in a

tool/gocross/toolchain.go

@@ -55,8 +55,13 @@ func readRevFile(path string) (string, error) {
 }
 
 func getToolchain() (toolchainDir, gorootDir string, err error) {
+	rev, err := toolchainRev()
+	if err != nil {
+		return "", "", err
+	}
+
 	cache := filepath.Join(os.Getenv("HOME"), ".cache")
-	toolchainDir = filepath.Join(cache, "tailscale-go")
+	toolchainDir = filepath.Join(cache, "tsgo", rev)
 	gorootDir = filepath.Join(toolchainDir, "gocross-goroot")
 
 	// You might wonder why getting the toolchain also provisions and returns a