fix(debug): make ptrace sessions reliable on Go tracees

cmd/gasm/debug_linux.go

@@ -223,29 +223,38 @@ REPL commands:
 				return 1
 			}
 		}
-		hits := map[uint64]int{}
-		traps := 0
 		fmt.Printf("gasm debug: coverage run over %d instructions\n", len(instrs))
 		for {
 			for _, bp := range bm.All() {
 				bm.Reinsert(bp.Addr)
 			}
-			// Remove the breakpoint the run is parked on, count the hit,
-			// then continue.
-			regs, rerr := sess.GetRegs()
-			if rerr == nil {
-				if bp := bm.At(regs.GetPC()); bp != nil {
-					bm.Clear(bp.Addr)
-					traps++
-					hits[regs.GetPC()-base]++
-				}
-			}
 			if err := sess.Continue(); err != nil {
 				break // debuggee finished or died
 			}
 			if sess.Exited() {
 				break
 			}
+			regs, rerr := sess.GetRegs()
+			if rerr != nil {
+				break
+			}
+			// HandleTrap restores the original byte, rewinds PC and counts
+			// the hit on the breakpoint itself.  Single-step over the
+			// restored instruction so the reinsertion at the top of the
+			// loop cannot re-trap on the same breakpoint.
+			if bp := bm.HandleTrap(&regs); bp != nil {
+				if err := sess.Step(); err != nil {
+					break
+				}
+			}
+		}
+		hits := map[uint64]int{}
+		traps := 0
+		for _, bp := range bm.All() {
+			if n := bp.Hits(); n > 0 {
+				hits[bp.Addr-base] = n
+				traps += n
+			}
 		}
 		var hit []string
 		var missed []string