runtime: switch debuglog from const-toggled to type-toggled

Currently, the debuglog build tag controls the dlogEnabled const, and
all methods of dlogger first check this const and immediately return
if dlog is not enabled. With constant folding and inlining, this makes
the whole dlog implementation compile away if it's not enabled.

However, we want to be able to test debuglog even when the build tag
isn't set. For that to work, we need a different mechanism.

This CL changes this mechanism so the debuglog build tag instead
controls the type alias for dlogger to be either dloggerImpl or
dloggerFake. These two types have the same method set, but one is just
stubs. This way, the methods of dloggerImpl don't need to be
conditional dlogEnabled, which sets us up to use the now
fully-functional dloggerImpl type in the test.

I confirmed that this change has no effect on the final size of the
cmd/go binary. It does increase the size of the runtime.a file by 0.9%
and make the runtime take ever so slightly longer to compile because
the compiler can no longer simply eliminate the bodies of the all of
dlogger during early deadcode. However, this all gets eliminated by
the linker. I consider this worth it to always get build and test
coverage of debuglog.

Change-Id: I81759e9e1411b7d369a23383a18b022ab7451421
Reviewed-on: https://go-review.googlesource.com/c/go/+/600696
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/runtime/export_debuglog_test.go b/src/runtime/export_debuglog_test.go
index e4b4ab9914..a361c02299 100644
--- a/src/runtime/export_debuglog_test.go
+++ b/src/runtime/export_debuglog_test.go
@@ -12,7 +12,11 @@ const DebugLogBytes = debugLogBytes
 
 const DebugLogStringLimit = debugLogStringLimit
 
-var Dlog = dlog
+type Dlogger = dloggerImpl
+
+func Dlog() *Dlogger {
+	return dlogImpl()
+}
 
 func (l *dloggerImpl) End()                      { l.end() }
 func (l *dloggerImpl) B(x bool) *dloggerImpl     { return l.b(x) }