Fix issues with zone filling connectivity locking

Two issues found with the locking system used to prevent access to
stale connectivity data during the zone fill process:

1) a std::mutex has undefined behavior if you try to use it to guard
against access from the same thread.  Because of the use of wx event
loops (and coroutines) it is entirely possible, and in some situations
inevitable, that the same thread will try to redraw the ratsnest in the
middle of zone refilling.

2) The mutex was only guarding the ZONE_FILLER::Fill method, but the callers
of that method also do connectivity updates as part of the COMMIT::Push.
Redrawing the ratsnest after the Fill but before the Push will result in
stale connectivity pointers to zone filled areas.

Fixed (1) by switching to a trivial spinlock implementation.  Spinlocks would
generally not be desirable if the contention for the connectivity data crossed
thread boundaries, but at the moment I believe it's guaranteed that the reads
and writes to connectivity that are guarded by this lock happen from the main
UI thread.  The writes are also quite rare compared to reads, and reads are
generally fast, so I'm not really worried about the UI thread spinning for any
real amount of time.

Fixed (2) by moving the locking location up to the call sites of
ZONE_FILLER::Fill.

This issue was quite difficult to reproduce, but I found a fairly reliable way:
It only happens (for me) on Windows, MSYS2 build, with wxWidgets 3.0
It also only happens if I restrict PcbNew to use 2 CPU cores.
With those conditions, I can reproduce the issue described in #6471 by
repeatedly editing a zone properties and changing its net.  The crash is
especially easy to trigger if you press some keys (such as 'e' for edit)
while the progress dialog is displayed.  It's easiest to do this in a debug
build as the slower KiCad is running, the bigger the window is to trigger this
bug.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/6471
Fixes https://gitlab.com/kicad/code/kicad/-/issues/7048

include/core/spinlock.h

@@ -0,0 +1,60 @@
+/*
+* This program source code file is part of KiCad, a free EDA CAD application.
+*
+* Copyright (C) 2021 KiCad Developers, see AUTHORS.txt for contributors.
+*
+* This program is free software: you can redistribute it and/or modify it
+* under the terms of the GNU General Public License as published by the
+* Free Software Foundation, either version 3 of the License, or (at your
+* option) any later version.
+*
+* This program is distributed in the hope that it will be useful, but
+* WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+* General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License along
+* with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef __KICAD_SPINLOCK_H
+#define __KICAD_SPINLOCK_H
+
+#include <atomic>
+
+
+/**
+ * A trivial spinlock implementation with no optimization.  Don't use if congestion is expected!
+ */
+class KISPINLOCK
+{
+public:
+    KISPINLOCK() :
+        m_lock( false )
+    {}
+
+    void lock()
+    {
+        while( m_lock.exchange( true, std::memory_order_acquire ) );
+    }
+
+    bool try_lock()
+    {
+        return !m_lock.exchange( true, std::memory_order_acquire );
+    }
+
+    void unlock()
+    {
+        m_lock.store( false, std::memory_order_release );
+    }
+
+    bool test()
+    {
+        return m_lock.load( std::memory_order_acquire );
+    }
+
+private:
+    std::atomic<bool> m_lock;
+};
+
+#endif

pcbnew/connectivity/connectivity_data.h

@@ -28,6 +28,7 @@
 #define __CONNECTIVITY_DATA_H
 
 #include <core/typeinfo.h>
+#include <core/spinlock.h>
 
 #include <memory>
 #include <mutex>
@@ -261,7 +262,7 @@ public:
         return m_connAlgo;
     }
 
-    std::mutex& GetLock()
+    KISPINLOCK& GetLock()
     {
         return m_lock;
     }
@@ -307,7 +308,7 @@ private:
 
     bool m_skipRatsnest = false;
 
-    std::mutex m_lock;
+    KISPINLOCK m_lock;
 
     /// Map of netcode -> netclass the net is a member of; used for ratsnest painting
     std::map<int, wxString> m_netclassMap;

pcbnew/edit_zone_helpers.cpp

@@ -125,6 +125,8 @@ void PCB_EDIT_FRAME::Edit_Zone_Params( ZONE* aZone )
 
     commit.Stage( pickedList );
 
+    std::lock_guard<KISPINLOCK> lock( GetBoard()->GetConnectivity()->GetLock() );
+
     if( zones_to_refill.size() )
     {
         ZONE_FILLER filler( GetBoard(), &commit );

pcbnew/ratsnest/ratsnest_viewitem.cpp

@@ -63,7 +63,7 @@ const BOX2I RATSNEST_VIEWITEM::ViewBBox() const
 
 void RATSNEST_VIEWITEM::ViewDraw( int aLayer, KIGFX::VIEW* aView ) const
 {
-    std::unique_lock<std::mutex> lock( m_data->GetLock(), std::try_to_lock );
+    std::unique_lock<KISPINLOCK> lock( m_data->GetLock(), std::try_to_lock );
 
     if( !lock )
         return;

pcbnew/tools/zone_create_helper.cpp

@@ -21,6 +21,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <core/spinlock.h>
+#include <connectivity/connectivity_data.h>
 #include <tools/zone_create_helper.h>
 #include <tool/tool_manager.h>
 #include <zone.h>
@@ -168,6 +170,8 @@ void ZONE_CREATE_HELPER::performZoneCutout( ZONE& aZone, ZONE& aCutout )
 
     ZONE_FILLER filler( board, &commit );
 
+    std::lock_guard<KISPINLOCK> lock( board->GetConnectivity()->GetLock() );
+
     if( !filler.Fill( newZones ) )
     {
         commit.Revert();
@@ -207,9 +211,12 @@ void ZONE_CREATE_HELPER::commitZone( std::unique_ptr<ZONE> aZone )
             aZone->HatchBorder();
             bCommit.Add( aZone.get() );
 
+            BOARD* board = m_tool.getModel<BOARD>();
+            std::lock_guard<KISPINLOCK> lock( board->GetConnectivity()->GetLock() );
+
             if( !m_params.m_keepout )
             {
-                ZONE_FILLER filler( m_tool.getModel<BOARD>(), &bCommit );
+                ZONE_FILLER filler( board, &bCommit );
                 std::vector<ZONE*> toFill = { aZone.get() };
 
                 if( !filler.Fill( toFill ) )

pcbnew/tools/zone_filler_tool.cpp

@@ -128,6 +128,8 @@ void ZONE_FILLER_TOOL::FillAllZones( wxWindow* aCaller, PROGRESS_REPORTER* aRepo
     else
         filler.InstallNewProgressReporter( aCaller, _( "Fill All Zones" ), 3 );
 
+    std::lock_guard<KISPINLOCK> lock( board()->GetConnectivity()->GetLock() );
+
     if( filler.Fill( toFill ) )
     {
         commit.Push( _( "Fill Zone(s)" ), false );
@@ -171,6 +173,8 @@ int ZONE_FILLER_TOOL::ZoneFill( const TOOL_EVENT& aEvent )
     ZONE_FILLER filler( board(), &commit );
     filler.InstallNewProgressReporter( frame(), _( "Fill Zone" ), 4 );
 
+    std::lock_guard<KISPINLOCK> lock( board()->GetConnectivity()->GetLock() );
+
     if( filler.Fill( toFill ) )
         commit.Push( _( "Fill Zone(s)" ), false );
     else

pcbnew/zone_filler.cpp

@@ -88,10 +88,6 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
     std::vector<CN_ZONE_ISOLATED_ISLAND_LIST> islandsList;
 
     std::shared_ptr<CONNECTIVITY_DATA> connectivity = m_board->GetConnectivity();
-    std::unique_lock<std::mutex> lock( connectivity->GetLock(), std::try_to_lock );
-
-    if( !lock )
-        return false;
 
     // Rebuild just in case. This really needs to be reliable.
     connectivity->Clear();

pcbnew/zone_filler.h

@@ -44,6 +44,13 @@ public:
 
     void SetProgressReporter( PROGRESS_REPORTER* aReporter );
     void InstallNewProgressReporter( wxWindow* aParent, const wxString& aTitle, int aNumPhases );
+
+    /**
+     * Fills the given list of zones.  Invalidates connectivity - it is up to the caller to obtain
+     * a lock on the connectivity data before calling Fill to prevent access to stale data by other
+     * coroutines (for example, ratsnest redraw).  This will generally be required if a UI-based
+     * progress reporter has been installed.
+     */
     bool Fill( std::vector<ZONE*>& aZones, bool aCheck = false, wxWindow* aParent = nullptr );
 
     bool IsDebug() const { return m_debugZoneFiller; }