runtime: add scan trace for checkfinalizers>1

This change dumps a scan trace (each pointer marked and where it came
from) for the partial GC cycle performed by checkfinalizers mode when
checkfinalizers>1. This is useful for quickly understanding why certain
values are reachable without having to pull out tools like viewcore.

For #72949.

Change-Id: Ic583f80e9558cdfe1c667d27a1d975008dd39a9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/662038
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/mcheckmark.go

@@ -177,6 +177,9 @@ func checkFinalizersAndCleanups() {
 		}
 
 		// Run a checkmark GC using this cleanup and/or finalizer as a root.
+		if debug.checkfinalizers > 1 {
+			print("Scan trace for cleanup/finalizer on ", hex(p), ":\n")
+		}
 		runCheckmark(func(gcw *gcWork) {
 			switch sp.kind {
 			case _KindSpecialFinalizer:
@@ -185,6 +188,9 @@ func checkFinalizersAndCleanups() {
 				gcScanCleanup((*specialCleanup)(unsafe.Pointer(sp)), gcw)
 			}
 		})
+		if debug.checkfinalizers > 1 {
+			println()
+		}
 
 		// Now check to see if the object the special is attached to was marked.
 		// The roots above do not directly mark p, so if it is marked, then p

src/runtime/mgcmark.go

@@ -1665,6 +1665,9 @@ func greyobject(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintp
 			// Already marked.
 			return
 		}
+		if debug.checkfinalizers > 1 {
+			print("  mark ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
+		}
 	} else {
 		if debug.gccheckmark > 0 && span.isFree(objIndex) {
 			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")