add detailed jts debugging info

planetiler-core/src/main/java/com/onthegomap/planetiler/FeatureMerge.java

@@ -19,6 +19,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.function.Function;
+import org.geotools.geometry.jts.WKTWriter2;
 import org.locationtech.jts.algorithm.Area;
 import org.locationtech.jts.geom.CoordinateSequence;
 import org.locationtech.jts.geom.Envelope;
@@ -28,6 +29,7 @@
 import org.locationtech.jts.geom.LinearRing;
 import org.locationtech.jts.geom.Polygon;
 import org.locationtech.jts.geom.Polygonal;
+import org.locationtech.jts.geom.TopologyException;
 import org.locationtech.jts.index.strtree.STRtree;
 import org.locationtech.jts.operation.buffer.BufferOp;
 import org.locationtech.jts.operation.buffer.BufferParameters;
@@ -410,7 +412,7 @@ public static Collection<List<VectorTile.Feature>> groupByAttrs(
    * Merges nearby polygons by expanding each individual polygon by {@code buffer}, unioning them, and contracting the
    * result.
    */
-  private static Geometry bufferUnionUnbuffer(double buffer, List<Geometry> polygonGroup) {
+  private static Geometry bufferUnionUnbuffer(double buffer, List<Geometry> polygonGroup) throws GeometryException {
     /*
      * A simpler alternative that might initially appear faster would be:
      *
@@ -424,11 +426,27 @@ private static Geometry bufferUnionUnbuffer(double buffer, List<Geometry> polygo
      * The following approach is slower most of the time, but faster on average because it does
      * not choke on dense nearby polygons:
      */
-    for (int i = 0; i < polygonGroup.size(); i++) {
-      polygonGroup.set(i, buffer(buffer, polygonGroup.get(i)));
+    List<Geometry> buffered = new ArrayList<>(polygonGroup.size());
+    for (Geometry geometry : polygonGroup) {
+      buffered.add(buffer(buffer, geometry));
+    }
+    Geometry merged = GeoUtils.createGeometryCollection(buffered);
+    try {
+      merged = union(merged);
+    } catch (TopologyException e) {
+      throw new GeometryException("buffer_union_failure", "Error unioning buffered polygons", e).addDetails(() -> {
+        var wktWriter = new WKTWriter2();
+        return """
+          Original polygons: %s
+          Buffer: %f
+          Buffered: %s
+          """.formatted(
+          wktWriter.write(GeoUtils.createGeometryCollection(polygonGroup)),
+          buffer,
+          wktWriter.write(GeoUtils.createGeometryCollection(buffered))
+        );
+      });
     }
-    Geometry merged = GeoUtils.createGeometryCollection(polygonGroup);
-    merged = union(merged);
     merged = unbuffer(buffer, merged);
     return merged;
   }

planetiler-core/src/main/java/com/onthegomap/planetiler/collection/FeatureGroup.java

@@ -488,7 +488,7 @@ private void postProcessAndAddLayerFeatures(VectorTile encoder, String layer,
         // log failures, only throwing when it's a fatal error
         if (e instanceof GeometryException geoe) {
           geoe.log(stats, "postprocess_layer",
-            "Caught error postprocessing features for " + layer + " layer on " + tileCoord);
+            "Caught error postprocessing features for " + layer + " layer on " + tileCoord, config.logJtsExceptions());
         } else if (e instanceof Error err) {
           LOGGER.error("Caught fatal error postprocessing features {} {}", layer, tileCoord, e);
           throw err;

planetiler-core/src/main/java/com/onthegomap/planetiler/config/PlanetilerConfig.java

@@ -58,7 +58,8 @@ public record PlanetilerConfig(
   String debugUrlPattern,
   Path tmpDir,
   Path tileWeights,
-  double maxPointBuffer
+  double maxPointBuffer,
+  boolean logJtsExceptions
 ) {
 
   public static final int MIN_MINZOOM = 0;
@@ -208,7 +209,8 @@ public static PlanetilerConfig from(Arguments arguments) {
         "Max tile pixels to include points outside tile bounds. Set to a lower value to reduce tile size for " +
           "clients that handle label collisions across tiles (most web and native clients). NOTE: Do not reduce if you need to support " +
           "raster tile rendering",
-        Double.POSITIVE_INFINITY)
+        Double.POSITIVE_INFINITY),
+      arguments.getBoolean("log_jts_exceptions", "Emit verbose details to debug JTS geometry errors", false)
     );
   }
 

planetiler-core/src/main/java/com/onthegomap/planetiler/geo/GeometryException.java

@@ -1,6 +1,7 @@
 package com.onthegomap.planetiler.geo;
 
 import com.onthegomap.planetiler.stats.Stats;
+import java.util.function.Supplier;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -14,6 +15,7 @@ public class GeometryException extends Exception {
 
   private final String stat;
   private final boolean nonFatal;
+  private Supplier<String> detailsSupplier;
 
   /**
    * Constructs a new exception with a detailed error message caused by {@code cause}.
@@ -51,6 +53,11 @@ public GeometryException(String stat, String message, boolean nonFatal) {
     this.nonFatal = nonFatal;
   }
 
+  public GeometryException addDetails(Supplier<String> detailsSupplier) {
+    this.detailsSupplier = detailsSupplier;
+    return this;
+  }
+
   /** Returns the unique code for this error condition to use for counting the number of occurrences in stats. */
   public String stat() {
     return stat;
@@ -72,6 +79,17 @@ void logMessage(String log) {
     assert nonFatal : log; // make unit tests fail if fatal
   }
 
+
+  /** Logs the error but if {@code logDetails} is true, then also prints detailed debugging info. */
+  public void log(Stats stats, String statPrefix, String logPrefix, boolean logDetails) {
+    if (logDetails && detailsSupplier != null) {
+      stats.dataError(statPrefix + "_" + stat());
+      logMessage(logPrefix + ": " + getMessage() + "\n" + detailsSupplier.get());
+    } else {
+      log(stats, statPrefix, logPrefix);
+    }
+  }
+
   /**
    * An error that we expect to encounter often so should only be logged at {@code TRACE} level.
    */