MVT: emit warning when the maximum tile size or feature count is reac…

…hed and the user didn't explicitly set MAX_SIZE or MAX_FEATURES layer creation options

CPL_DEBUG=ON already warns about that, but better be more verbose when
the user didn't explicitly set limitations.

Should help addressing https://mastodon.social/@RegisHaubourg/113567187835337586

autotest/ogr/ogr_mvt.py

@@ -1220,7 +1220,9 @@ def test_ogr_mvt_write_mbtiles():
 
 @pytest.mark.require_driver("SQLite")
 @pytest.mark.require_geos
-def test_ogr_mvt_write_limitations_max_size():
+@pytest.mark.parametrize("implicit_limitation", [True, False])
+@gdaltest.enable_exceptions()
+def test_ogr_mvt_write_limitations_max_size(implicit_limitation, tmp_path):
 
     src_ds = gdal.GetDriverByName("Memory").Create("", 0, 0, 0, gdal.GDT_Unknown)
     lyr = src_ds.CreateLayer("mylayer")
@@ -1244,10 +1246,22 @@ def test_ogr_mvt_write_limitations_max_size():
         out_ds = gdal.VectorTranslate(
             "/vsimem/out.mbtiles",
             src_ds,
-            datasetCreationOptions=["MAX_SIZE=100", "SIMPLIFICATION=1"],
+            datasetCreationOptions=[
+                "@MAX_SIZE_FOR_TEST=101" if implicit_limitation else "MAX_SIZE=101",
+                "SIMPLIFICATION=1",
+            ],
         )
     assert out_ds is not None
-    out_ds = None
+    gdal.ErrorReset()
+    with gdal.quiet_errors():
+        out_ds.Close()
+    if implicit_limitation:
+        assert (
+            gdal.GetLastErrorMsg()
+            == "At least one tile exceeded the default maximum tile size of 101 bytes and was encoded at lower resolution"
+        )
+    else:
+        assert gdal.GetLastErrorMsg() == ""
 
     out_ds = ogr.Open("/vsimem/out.mbtiles")
     assert out_ds is not None
@@ -1441,7 +1455,9 @@ def test_ogr_mvt_write_limitations_max_size_polygon():
 
 @pytest.mark.require_driver("SQLite")
 @pytest.mark.require_geos
-def test_ogr_mvt_write_limitations_max_features():
+@pytest.mark.parametrize("implicit_limitation", [True, False])
+@gdaltest.enable_exceptions()
+def test_ogr_mvt_write_limitations_max_features(implicit_limitation):
 
     src_ds = gdal.GetDriverByName("Memory").Create("", 0, 0, 0, gdal.GDT_Unknown)
     lyr = src_ds.CreateLayer("mylayer")
@@ -1462,10 +1478,21 @@ def test_ogr_mvt_write_limitations_max_features():
         "/vsimem/out.mbtiles",
         src_ds,
         format="MVT",
-        datasetCreationOptions=["MAX_FEATURES=1"],
+        datasetCreationOptions=[
+            "@MAX_FEATURES_FOR_TEST=1" if implicit_limitation else "MAX_FEATURES=1"
+        ],
     )
     assert out_ds is not None
-    out_ds = None
+    gdal.ErrorReset()
+    with gdal.quiet_errors():
+        out_ds.Close()
+    if implicit_limitation:
+        assert (
+            gdal.GetLastErrorMsg()
+            == "At least one tile exceeded the default maximum number of features per tile (1) and was truncated to satisfy it."
+        )
+    else:
+        assert gdal.GetLastErrorMsg() == ""
 
     out_ds = ogr.Open("/vsimem/out.mbtiles")
     assert out_ds is not None

ogr/ogrsf_frmts/mvt/ogrmvtdataset.cpp

@@ -3301,6 +3301,8 @@ class OGRMVTWriterDataset final : public GDALDataset
     CPLString m_osType{"overlay"};
     sqlite3 *m_hDBMBTILES = nullptr;
     OGREnvelope m_oEnvelope;
+    bool m_bMaxTileSizeOptSpecified = false;
+    bool m_bMaxFeaturesOptSpecified = false;
     unsigned m_nMaxTileSize = 500000;
     unsigned m_nMaxFeatures = 200000;
     std::map<std::string, std::string> m_oMapLayerNameToDesc;
@@ -4972,11 +4974,28 @@ std::string OGRMVTWriterDataset::EncodeTile(
     const double dfCompressionRatio =
         static_cast<double>(nSizeAfter) / nSizeBefore;
 
+    const bool bTooManyFeatures = nFeaturesInTile >= m_nMaxFeatures;
+    if (bTooManyFeatures && !m_bMaxFeaturesOptSpecified)
+    {
+        m_bMaxFeaturesOptSpecified = true;
+        CPLError(CE_Warning, CPLE_AppDefined,
+                 "At least one tile exceeded the default maximum number of "
+                 "features per tile (%u) and was truncated to satisfy it.",
+                 m_nMaxFeatures);
+    }
+
     // If the tile size is above the allowed values or there are too many
     // features, then sort by descending area / length until we get to the
     // limit.
     bool bTooBigTile = oTileBuffer.size() > m_nMaxTileSize;
-    const bool bTooManyFeatures = nFeaturesInTile >= m_nMaxFeatures;
+    if (bTooBigTile && !m_bMaxTileSizeOptSpecified)
+    {
+        m_bMaxTileSizeOptSpecified = true;
+        CPLError(CE_Warning, CPLE_AppDefined,
+                 "At least one tile exceeded the default maximum tile size of "
+                 "%u bytes and was encoded at lower resolution",
+                 m_nMaxTileSize);
+    }
 
     GUInt32 nExtent = m_nExtent;
     while (bTooBigTile && !bTooManyFeatures && nExtent >= 256)
@@ -6087,14 +6106,30 @@ GDALDataset *OGRMVTWriterDataset::Create(const char *pszFilename, int nXSize,
     poDS->m_nBuffer = static_cast<unsigned>(atoi(CSLFetchNameValueDef(
         papszOptions, "BUFFER", CPLSPrintf("%u", 5 * poDS->m_nExtent / 256))));
 
-    poDS->m_nMaxTileSize =
-        std::max(100U, static_cast<unsigned>(atoi(CSLFetchNameValueDef(
-                           papszOptions, "MAX_SIZE",
-                           CPLSPrintf("%u", poDS->m_nMaxTileSize)))));
-    poDS->m_nMaxFeatures =
-        std::max(1U, static_cast<unsigned>(atoi(CSLFetchNameValueDef(
-                         papszOptions, "MAX_FEATURES",
-                         CPLSPrintf("%u", poDS->m_nMaxFeatures)))));
+    {
+        const char *pszMaxSize = CSLFetchNameValue(papszOptions, "MAX_SIZE");
+        poDS->m_bMaxTileSizeOptSpecified = pszMaxSize != nullptr;
+        pszMaxSize = CSLFetchNameValueDef(papszOptions, "@MAX_SIZE_FOR_TEST",
+                                          pszMaxSize);
+        if (pszMaxSize)
+        {
+            poDS->m_nMaxTileSize =
+                std::max(100U, static_cast<unsigned>(atoi(pszMaxSize)));
+        }
+    }
+
+    {
+        const char *pszMaxFeatures =
+            CSLFetchNameValue(papszOptions, "MAX_FEATURES");
+        poDS->m_bMaxFeaturesOptSpecified = pszMaxFeatures != nullptr;
+        pszMaxFeatures = CSLFetchNameValueDef(
+            papszOptions, "@MAX_FEATURES_FOR_TEST", pszMaxFeatures);
+        if (pszMaxFeatures)
+        {
+            poDS->m_nMaxFeatures =
+                std::max(1U, static_cast<unsigned>(atoi(pszMaxFeatures)));
+        }
+    }
 
     poDS->m_osName =
         CSLFetchNameValueDef(papszOptions, "NAME", CPLGetBasename(pszFilename));