From 76f00131ff545fa2ab296580a77d2bd0bb40df0c Mon Sep 17 00:00:00 2001
From: MajorCooke <paul.growney22@gmail.com>
Date: Sun, 10 Apr 2016 11:51:40 -0500
Subject: [PATCH] - Fixed: A_CheckBlock's dropoff check ignored the
 specifications of NOACTORS and NOLINES due to P_CheckMove blindly failing if
 P_CheckPosition failed. This lead to false positives such as blocking actors
 being detected when they shouldn't be.

---
 src/p_local.h                     |  9 +++++++--
 src/p_map.cpp                     | 18 +++++++++++++++---
 src/thingdef/thingdef_codeptr.cpp | 12 ++++++++++--
 3 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/src/p_local.h b/src/p_local.h
index 82f4580b9..45b02e8ea 100644
--- a/src/p_local.h
+++ b/src/p_local.h
@@ -186,7 +186,12 @@ enum WARPF
 	WARPF_COPYPITCH			= 0x8000,
 };
 
-
+enum PCM
+{
+	PCM_DROPOFF =		1,
+	PCM_NOACTORS =		1 << 1,
+	PCM_NOLINES =		1 << 2,
+};
 
 
 AActor *P_BlockmapSearch (AActor *mo, int distance, AActor *(*check)(AActor*, int, void *), void *params = NULL);
@@ -221,7 +226,7 @@ void	P_FakeZMovement (AActor *mo);
 bool	P_TryMove(AActor* thing, const DVector2 &pos, int dropoff, const secplane_t * onfloor, FCheckPosition &tm, bool missileCheck = false);
 bool	P_TryMove(AActor* thing, const DVector2 &pos, int dropoff, const secplane_t * onfloor = NULL);
 
-bool	P_CheckMove(AActor *thing, const DVector2 &pos, bool dropoff = false);
+bool	P_CheckMove(AActor *thing, const DVector2 &pos, int flags = 0);
 void	P_ApplyTorque(AActor *mo);
 
 bool	P_TeleportMove(AActor* thing, const DVector3 &pos, bool telefrag, bool modifyactor = true);	// [RH] Added z and telefrag parameters
diff --git a/src/p_map.cpp b/src/p_map.cpp
index 73aa3f73f..a1ef28cc4 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -2410,14 +2410,26 @@ bool P_TryMove(AActor *thing, const DVector2 &pos,
 //
 //==========================================================================
 
-bool P_CheckMove(AActor *thing, const DVector2 &pos, bool dropoff)
+bool P_CheckMove(AActor *thing, const DVector2 &pos, int flags)
 {
 	FCheckPosition tm;
 	double		newz = thing->Z();
 
 	if (!P_CheckPosition(thing, pos, tm))
 	{
-		return false;
+		// Ignore PCM_DROPOFF. Not necessary here: a little later it is.
+		if (!flags || (!(flags & PCM_NOACTORS) && !(flags & PCM_NOLINES)))
+		{
+			return false;
+		}
+		if (!(flags & PCM_NOACTORS) && thing->BlockingMobj)
+		{
+			return false;
+		}
+		if (!(flags & PCM_NOLINES) && thing->BlockingLine)
+		{
+			return false;
+		}
 	}
 
 	if (thing->flags3 & MF3_FLOORHUGGER)
@@ -2469,7 +2481,7 @@ bool P_CheckMove(AActor *thing, const DVector2 &pos, bool dropoff)
 					return false;
 				}
 			}
-			else if (dropoff)
+			else if (flags & PCM_DROPOFF)
 			{
 				const DVector3 oldpos = thing->Pos();
 				thing->SetOrigin(pos.X, pos.Y, newz, true);
diff --git a/src/thingdef/thingdef_codeptr.cpp b/src/thingdef/thingdef_codeptr.cpp
index cc4ffd33b..53685c43b 100644
--- a/src/thingdef/thingdef_codeptr.cpp
+++ b/src/thingdef/thingdef_codeptr.cpp
@@ -6706,8 +6706,16 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckBlock)
 	bool checker = false;
 	if (flags & CBF_DROPOFF)
 	{
+		// Unfortunately, whenever P_CheckMove returned false, that means it could
+		// ignore a variety of flags mainly because of P_CheckPosition. This
+		// results in picking up false positives due to actors or lines being in the way
+		// when they clearly should not be.
+
+		int fpass = PCM_DROPOFF;
+		if (flags & CBF_NOACTORS)	fpass |= PCM_NOACTORS;
+		if (flags & CBF_NOLINES)	fpass |= PCM_NOLINES;
 		mobj->SetZ(pos.Z);
-		checker = P_CheckMove(mobj, pos, true);
+		checker = P_CheckMove(mobj, pos, fpass);
 		mobj->SetZ(oldpos.Z);
 	}
 	else
@@ -6743,7 +6751,7 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckBlock)
 	//[MC] I don't know why I let myself be persuaded not to include a flag.
 	//If an actor is loaded with pointers, they don't really have any options to spare.
 	//Also, fail if a dropoff or a step is too great to pass over when checking for dropoffs.
-
+	
 	if ((!(flags & CBF_NOACTORS) && (mobj->BlockingMobj)) || (!(flags & CBF_NOLINES) && mobj->BlockingLine != NULL) ||
 		((flags & CBF_DROPOFF) && !checker))
 	{