Proposal for fix.

src/db/db/dbLayoutQuery.cc

@@ -663,6 +663,26 @@ public:
     }
   }
 
+  db::Instance instance () const
+  {
+    return mp_parent->sorted_inst_ptr (std::distance (mp_parent->begin_sorted_insts (), m_inst));
+  }
+
+  void validate_instance ()
+  {
+    //  during modification or delete, instances may vanish while iterating,
+    //  hence we need to check during iteration
+    while (m_inst != m_inst_end && !mp_parent->is_valid (instance ())) {
+      ++m_inst;
+    }
+  }
+
+  void next_valid_instance ()
+  {
+    ++m_inst;
+    validate_instance ();
+  }
+
   virtual void reset (FilterStateBase *previous)
   {
     FilterStateBase::reset (previous);
@@ -707,6 +727,8 @@ public:
         m_inst = mp_parent->begin_sorted_insts ();
         m_inst_end = mp_parent->end_sorted_insts ();
 
+        validate_instance ();
+
         while (m_inst != m_inst_end) {
 
           db::cell_index_type cid = (*m_inst)->object ().cell_index ();
@@ -714,9 +736,9 @@ public:
             break;
           }
 
-          ++m_inst;
+          next_valid_instance ();
           while (m_inst != m_inst_end && (*m_inst)->object ().cell_index () == cid) {
-            ++m_inst;
+            next_valid_instance ();
           }
 
         }
@@ -760,14 +782,14 @@ public:
 
         if (m_instance_mode != ExplodedInstances || m_array_iter.at_end ()) {
 
-          if (! m_reading && m_i != mp_parent->sorted_inst_ptr (std::distance (mp_parent->begin_sorted_insts (), m_inst))) {
+          if (! m_reading && m_i != instance ()) {
             m_ignored.insert (m_i);
           }
 
           do {
 
             db::cell_index_type cid = (*m_inst)->object ().cell_index ();
-            ++m_inst;
+            next_valid_instance ();
 
             if (m_inst != m_inst_end && (*m_inst)->object ().cell_index () != cid) {
 
@@ -778,9 +800,9 @@ public:
                   break;
                 }
 
-                ++m_inst;
+                next_valid_instance ();
                 while (m_inst != m_inst_end && (*m_inst)->object ().cell_index () == cid) {
-                  ++m_inst;
+                  next_valid_instance ();
                 }
 
               }
@@ -788,7 +810,7 @@ public:
             }
 
             if (! m_reading && m_inst != m_inst_end) {
-              m_i = mp_parent->sorted_inst_ptr (std::distance (mp_parent->begin_sorted_insts (), m_inst));
+              m_i = instance ();
             } else {
               break;
             }

src/db/unit_tests/dbLayoutQueryTests.cc

@@ -30,6 +30,8 @@
 #include "dbPCellDeclaration.h"
 #include "dbLibrary.h"
 #include "dbLibraryManager.h"
+#include "dbReader.h"
+#include "tlStream.h"
 
 
 static std::string q2s_var (db::Layout &g, const std::string &query, const std::string &pname, const char *sep = ",")
@@ -1011,6 +1013,39 @@ void init_layout (db::Layout &g)
   c2.insert (db::array <db::CellInst, db::Trans> (db::CellInst (c5.cell_index ()), tt));
 }
 
+void init_layout2 (db::Layout &g)
+{
+  g = db::Layout ();
+
+  tl::InputStream stream (tl::testdata () + "/gds/issue-1671.gds");
+  db::Reader reader (stream);
+  reader.read (g, db::LoadLayoutOptions ());
+  return; // @@@
+
+  g.insert_layer (0, db::LayerProperties ("l0"));
+  g.insert_layer (1, db::LayerProperties ("l1"));
+  g.insert_layer (2, db::LayerProperties ("l2"));
+  db::Cell &c1 (g.cell (g.add_cell ("c1")));
+  db::Cell &c2 (g.cell (g.add_cell ("c2")));
+  db::Cell &c3 (g.cell (g.add_cell ("c3")));
+  c2.shapes (0).insert (db::Box (0, 1, 2, 3));
+  c2.shapes (1).insert (db::Polygon (db::Box (0, 1, 2, 3)));
+  c2.shapes (1).insert (db::Edge (db::Point (0, 1), db::Point (2, 3)));
+  c2.shapes (2).insert (db::Text ("hallo", db::Trans (db::Vector (10, 11))));
+  c1.shapes (1).insert (db::Box (0, 10, 10, 30));
+
+  db::FTrans f (1, true);
+  db::Vector p (-10, 20);
+  db::Trans t (f.rot (), p);
+  db::Vector pp (10, -20);
+  db::Trans tt (0, pp);
+
+  c3.insert (db::array <db::CellInst, db::Trans> (db::CellInst (c2.cell_index ()), t));
+  c3.insert (db::array <db::CellInst, db::Trans> (db::CellInst (c2.cell_index ()), tt));
+
+  c2.insert (db::array <db::CellInst, db::Trans> (db::CellInst (c1.cell_index ()), t));
+}
+
 TEST(4)
 {
   db::Layout g;
@@ -1203,7 +1238,8 @@ TEST(53)
 {
   if (! db::default_editable_mode ()) { return; }
 
-  db::Layout g;
+  db::Manager m;
+  db::Layout g (&m);
   init_layout (g);
 
   {
@@ -1252,6 +1288,18 @@ TEST(53)
     s = q2s_var (iq, "path_names", ";");
     EXPECT_EQ (s, "c1;c2x;c3;c4;c5x");
   }
+
+  init_layout (g);
+
+  {
+    //  triggers issue-1671 (with transaction)
+    db::Transaction trans (&m, "test 53");
+    std::string s;
+    db::LayoutQuery q ("delete instances of ...c1 pass");
+    db::LayoutQueryIterator iq (q, &g);
+    s = q2s_var (iq, "path_names", ";");
+    EXPECT_EQ (s, "c2x,c1;c2x,c1;c2x,c4,c1;c2x,c4,c1;c2x,c4,c3,c5x,c1");
+  }
 }
 
 TEST(61)