cmd/go/internal/modfetch: always allow Stat for the current toolchain…

… to succeed

This fixes a failure mode observed in TestScript/gotoolchain_version
when building go1.21rc3.

Updates #61259.

Change-Id: Ifb14a5b94b687adea7a26c1155591e3ae75b7a62
Reviewed-on: https://go-review.googlesource.com/c/go/+/509217
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/modfetch/toolchain.go

@@ -98,13 +98,23 @@ func (r *toolchainRepo) Stat(ctx context.Context, rev string) (*RevInfo, error)
 	if !gover.IsValid(v) {
 		return nil, fmt.Errorf("invalid %s version %s", r.path, rev)
 	}
+
 	// If we're asking about "go" (not "toolchain"), pretend to have
 	// all earlier Go versions available without network access:
 	// we will provide those ourselves, at least in GOTOOLCHAIN=auto mode.
 	if r.path == "go" && gover.Compare(v, gover.Local()) <= 0 {
 		return &RevInfo{Version: prefix + v}, nil
 	}
+
+	// Similarly, if we're asking about *exactly* the current toolchain,
+	// we don't need to access the network to know that it exists.
+	if r.path == "toolchain" && v == gover.Local() {
+		return &RevInfo{Version: prefix + v}, nil
+	}
+
 	if gover.IsLang(v) {
+		// We can only use a language (development) version if the current toolchain
+		// implements that version, and the two checks above have ruled that out.
 		return nil, fmt.Errorf("go language version %s is not a toolchain version", rev)
 	}