Fix loss of precision when converting fields to uri in memory provider

Fixes #21316

src/core/providers/memory/qgsmemoryprovider.cpp

@@ -108,7 +108,7 @@ QgsMemoryProvider::QgsMemoryProvider( const QString &uri, const ProviderOptions
     QRegExp reFieldDef( "\\:"
                         "(int|integer|long|int8|real|double|string|date|time|datetime|binary|bool|boolean)" // type
                         "(?:\\((\\-?\\d+)"                // length
-                        "(?:\\,(\\d+))?"                  // precision
+                        "(?:\\,(\\-?\\d+))?"                  // precision
                         "\\))?(\\[\\])?"                  // array
                         "$", Qt::CaseInsensitive );
     QStringList fields = url.allQueryItemValues( QStringLiteral( "field" ) );

src/core/providers/memory/qgsmemoryproviderutils.cpp

@@ -69,15 +69,7 @@ QgsVectorLayer *QgsMemoryProviderUtils::createMemoryLayer( const QString &name,
   }
   for ( const auto &field : fields )
   {
-    QString lengthPrecision;
-    if ( field.length() > 0 && field.precision() > 0 )
-    {
-      lengthPrecision = QStringLiteral( "(%1,%2)" ).arg( field.length() ).arg( field.precision() );
-    }
-    else if ( field.length() > 0 )
-    {
-      lengthPrecision = QStringLiteral( "(%1)" ).arg( field.length() );
-    }
+    const QString lengthPrecision = QStringLiteral( "(%1,%2)" ).arg( field.length() ).arg( field.precision() );
     parts << QStringLiteral( "field=%1:%2%3" ).arg( QString( QUrl::toPercentEncoding( field.name() ) ), memoryLayerFieldType( field.type() ), lengthPrecision );
   }
 

tests/src/python/test_provider_memory.py

@@ -275,6 +275,21 @@ def testLengthPrecisionFromUri(self):
         self.assertEqual(myMemoryLayer.fields().field('size').length(), 12)
         self.assertEqual(myMemoryLayer.fields().field('size').precision(), 9)
 
+        myMemoryLayer = QgsVectorLayer(
+            ('Point?crs=epsg:4326&field=size:double(-1,-1)&index=yes'),
+            'test',
+            'memory')
+
+        self.assertEqual(myMemoryLayer.fields().field('size').length(), -1)
+        self.assertEqual(myMemoryLayer.fields().field('size').precision(), -1)
+
+        myMemoryLayer = QgsVectorLayer(
+            ('Point?crs=epsg:4326&field=size:string(-1)&index=yes'),
+            'test',
+            'memory')
+
+        self.assertEqual(myMemoryLayer.fields().field('size').length(), -1)
+
     def testFromUriWithEncodedField(self):
         """Test we can construct the mem provider from a uri when a field name is encoded"""
         layer = QgsVectorLayer(
@@ -419,6 +434,8 @@ def testCreateMemoryLayer(self):
         for i in range(len(fields)):
             self.assertEqual(layer.fields()[i].name(), fields[i].name())
             self.assertEqual(layer.fields()[i].type(), fields[i].type())
+            self.assertEqual(layer.fields()[i].length(), fields[i].length())
+            self.assertEqual(layer.fields()[i].precision(), fields[i].precision(), fields[i].name())
 
         # unsupported field type
         fields = QgsFields()
@@ -429,6 +446,22 @@ def testCreateMemoryLayer(self):
         self.assertEqual(layer.fields()[0].name(), 'rect')
         self.assertEqual(layer.fields()[0].type(), QVariant.String) # should be mapped to string
 
+        # field precision
+        fields = QgsFields()
+        fields.append(QgsField("string", QVariant.String, len=10))
+        fields.append(QgsField("long", QVariant.LongLong, len=6))
+        fields.append(QgsField("double", QVariant.Double, len=10, prec=7))
+        fields.append(QgsField("double2", QVariant.Double, len=-1, prec=-1))
+        layer = QgsMemoryProviderUtils.createMemoryLayer('my name', fields)
+        self.assertTrue(layer.isValid())
+        self.assertFalse(layer.fields().isEmpty())
+        self.assertEqual(len(layer.fields()), len(fields))
+        for i in range(len(fields)):
+            self.assertEqual(layer.fields()[i].name(), fields[i].name())
+            self.assertEqual(layer.fields()[i].type(), fields[i].type())
+            self.assertEqual(layer.fields()[i].length(), fields[i].length())
+            self.assertEqual(layer.fields()[i].precision(), fields[i].precision())
+
     def testThreadSafetyWithIndex(self):
         layer = QgsVectorLayer('Point?crs=epsg:4326&index=yes&field=pk:integer&field=cnt:int8&field=name:string(0)&field=name2:string(0)&field=num_char:string&key=pk',
                                'test', 'memory')