fix: ST_AsMVTGeom corrupts Protobuf for Linestrings

src/spatial/modules/mvt/mvt_module.cpp

@@ -609,15 +609,15 @@ class MVTFeatureBuilder {
 					const auto y = CastDouble(cursor.Read<double>());
 					cursor.Skip(vertex_space); // Skip z and m if present
 
-					if (vertex_idx == 0) {
-						geometry.push_back((1 & 0x7) | (1 << 3)); // MoveTo, 1 part
-						geometry.push_back(protozero::encode_zigzag32(x - cursor_x));
-						geometry.push_back(protozero::encode_zigzag32(y - cursor_y));
-						geometry.push_back((2 & 0x7) | ((vertex_count - 2) << 3)); // LineTo, part count
-					} else {
-						geometry.push_back(protozero::encode_zigzag32(x - cursor_x));
-						geometry.push_back(protozero::encode_zigzag32(y - cursor_y));
-					}
+				if (vertex_idx == 0) {
+					geometry.push_back((1 & 0x7) | (1 << 3)); // MoveTo, 1 part
+					geometry.push_back(protozero::encode_zigzag32(x - cursor_x));
+					geometry.push_back(protozero::encode_zigzag32(y - cursor_y));
+					geometry.push_back((2 & 0x7) | ((vertex_count - 1) << 3)); // LineTo, part count
+				} else {
+					geometry.push_back(protozero::encode_zigzag32(x - cursor_x));
+					geometry.push_back(protozero::encode_zigzag32(y - cursor_y));
+				}
 
 					cursor_x = x;
 					cursor_y = y;

test/sql/mvt/st_asmvt_linestring.test

@@ -0,0 +1,157 @@
+# name: test/sql/mvt/st_asmvt_linestring.test
+# group: [mvt]
+
+require spatial
+
+# Test LINESTRING encoding
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": geom},
+		'lines'
+	) as mvt
+	FROM (
+		SELECT
+			st_geomfromtext('LINESTRING(0 0, 100 100, 200 0)') as geom
+	)
+) TO '__TEST_DIR__/test_linestring.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_linestring.mvt');
+----
+1
+
+# Test MULTI_LINESTRING encoding
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": geom},
+		'multilines'
+	) as mvt
+	FROM (
+		SELECT
+			st_geomfromtext('MULTILINESTRING((0 0, 100 100, 200 0), (300 0, 400 100, 500 0))') as geom
+	)
+) TO '__TEST_DIR__/test_multilinestring.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_multilinestring.mvt');
+----
+1
+
+# Test LINESTRING with ST_AsMVTGeom (clipping can produce MULTI_LINESTRING)
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": ST_AsMVTGeom(
+			geom,
+			ST_Extent(ST_MakeEnvelope(0, 0, 1000, 1000)),
+			4096,
+			256,
+			true
+		)},
+		'clipped_lines'
+	) as mvt
+	FROM (
+		SELECT
+			st_geomfromtext('LINESTRING(100 100, 500 500, 900 100)') as geom
+	)
+) TO '__TEST_DIR__/test_clipped_linestring.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_clipped_linestring.mvt');
+----
+1
+
+# Test LINESTRING crossing tile boundary (produces MULTI_LINESTRING after clipping)
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": ST_AsMVTGeom(
+			geom,
+			ST_Extent(ST_MakeEnvelope(0, 0, 1000, 1000)),
+			4096,
+			256,
+			true
+		)},
+		'crossing_lines'
+	) as mvt
+	FROM (
+		SELECT
+			st_geomfromtext('LINESTRING(-500 500, 500 500, 1500 500)') as geom
+	)
+) TO '__TEST_DIR__/test_crossing_linestring.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_crossing_linestring.mvt');
+----
+1
+
+# Test multiple LINESTRINGs with various lengths
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": geom, "id": id},
+		'various_lines',
+		4096,
+		'geom',
+		'id'
+	) as mvt
+	FROM (
+		SELECT
+			row_number() over () as id,
+			st_geomfromtext('LINESTRING(' || (x*100) || ' ' || (y*100) || ', ' || (x*100+50) || ' ' || (y*100+50) || ', ' || (x*100+100) || ' ' || (y*100) || ')') as geom
+		FROM range(0, 10) as r(x),
+			 range(0, 10) as rr(y)
+	)
+) TO '__TEST_DIR__/test_various_linestrings.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_various_linestrings.mvt');
+----
+100
+
+# Test global scale dataset scenario (like Natural Earth roads)
+# This simulates the case where geometries at low zoom levels span large areas
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": ST_AsMVTGeom(
+			geom,
+			ST_Extent(ST_TileEnvelope(2, 1, 1)),
+			4096,
+			256,
+			false
+		)},
+		'global_lines'
+	) as mvt
+	FROM (
+		SELECT
+			st_geomfromtext('LINESTRING(-10000000 5000000, 0 0, 10000000 -5000000)') as geom
+	)
+) TO '__TEST_DIR__/test_global_linestring.mvt' (FORMAT BLOB);
+
+query I
+select count(*) from st_read('__TEST_DIR__/test_global_linestring.mvt');
+----
+1
+
+# Test that LINESTRING with attributes can be read back
+statement ok
+COPY (
+	SELECT st_asmvt(
+		{"geom": geom, "name": name},
+		'roads'
+	) as mvt
+	FROM (
+		VALUES
+			(st_geomfromtext('MULTILINESTRING((100 100, 500 500), (600 600, 900 900))'), 'road1'),
+			(st_geomfromtext('LINESTRING(200 200, 800 800)'), 'road2')
+	) t(geom, name)
+) TO '__TEST_DIR__/test_roads.mvt' (FORMAT BLOB);
+
+query II
+select count(*), count(name) from st_read('__TEST_DIR__/test_roads.mvt');
+----
+2	2
+