runtime: add runtime.debugPinnerV1

Adds runtime.debugPinnerV1 which returns a runtime.Pinner object that
pins itself. This is intended to be used by debuggers in conjunction
with runtime.debugCall to keep heap memory reachable even if it isn't
referenced from anywhere else.

Change-Id: I508ee6a7b103e68df83c96f2e04a0599200300dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/558276
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: aarzilli

src/runtime/asm_amd64.s

@@ -371,8 +371,9 @@ bad_cpu: // show that the program requires a certain microarchitecture level.
 	CALL	runtime·abort(SB)
 	RET
 
-	// Prevent dead-code elimination of debugCallV2, which is
+	// Prevent dead-code elimination of debugCallV2 and debugPinnerV1, which are
 	// intended to be called by debuggers.
+	MOVQ	$runtime·debugPinnerV1<ABIInternal>(SB), AX
 	MOVQ	$runtime·debugCallV2<ABIInternal>(SB), AX
 	RET
 

src/runtime/asm_arm64.s

@@ -96,8 +96,9 @@ nocgo:
 	// start this M
 	BL	runtime·mstart(SB)
 
-	// Prevent dead-code elimination of debugCallV2, which is
+	// Prevent dead-code elimination of debugCallV2 and debugPinnerV1, which are
 	// intended to be called by debuggers.
+	MOVD	$runtime·debugPinnerV1<ABIInternal>(SB), R0
 	MOVD	$runtime·debugCallV2<ABIInternal>(SB), R0
 
 	MOVD	$0, R0

src/runtime/asm_ppc64x.s

@@ -98,9 +98,10 @@ nocgo:
 
 	// start this M
 	BL	runtime·mstart(SB)
-	// Prevent dead-code elimination of debugCallV2, which is
+	// Prevent dead-code elimination of debugCallV2 and debugPinnerV1, which are
 	// intended to be called by debuggers.
 #ifdef GOARCH_ppc64le
+	MOVD	$runtime·debugPinnerV1<ABIInternal>(SB), R31
 	MOVD	$runtime·debugCallV2<ABIInternal>(SB), R31
 #endif
 	MOVD	R0, 0(R0)

src/runtime/debug.go

@@ -124,3 +124,22 @@ func mayMoreStackMove() {
 		gp.stackguard0 = stackForceMove
 	}
 }
+
+// debugPinnerKeepUnpin is used to make runtime.(*Pinner).Unpin reachable.
+var debugPinnerKeepUnpin bool = false
+
+// debugPinnerV1 returns a new Pinner that pins itself. This function can be
+// used by debuggers to easily obtain a Pinner that will not be garbage
+// collected (or moved in memory) even if no references to it exist in the
+// target program. This pinner in turn can be used to extend this property
+// to other objects, which debuggers can use to simplify the evaluation of
+// expressions involving multiple call injections.
+func debugPinnerV1() *Pinner {
+	p := new(Pinner)
+	p.Pin(unsafe.Pointer(p))
+	if debugPinnerKeepUnpin {
+		// Make Unpin reachable.
+		p.Unpin()
+	}
+	return p
+}