Add MaybeTransaction for better writing speed when supported

CHANGES.md

@@ -23,6 +23,7 @@
   - Add methods `alternative_name`, `is_nullable`, `is_unique`, `default_value` to `Field` ([#561](https://github.com/georust/gdal/pull/561))
   - Add `Defn::geometry_type` ([#562](https://github.com/georust/gdal/pull/562))
   - Add `Defn::field_index` and `Feature::field_index` ([#581](https://github.com/georust/gdal/pull/581))
+  - Add `Dataset::maybe_start_transaction` which always succeeds to use transaction when possible for speed reasons -- no rollback ([#584](https://github.com/georust/gdal/pull/584))
 
 ### Fixed
 

src/test_utils.rs

@@ -136,6 +136,41 @@ pub fn open_gpkg_for_update(path: &Path) -> (TempPath, Dataset) {
     (temp_path, ds)
 }
 
+/// Copies the given file to a temporary file and opens it for writing. When the returned
+/// `TempPath` is dropped, the file is deleted.
+pub fn open_dataset_for_update(path: &Path) -> (TempPath, Dataset) {
+    use std::fs;
+    use std::io::Write;
+
+    let input_data = fs::read(path).unwrap();
+    let (mut file, temp_path) = tempfile::Builder::new()
+        // using the whole filename as suffix should be fine (can't
+        // use .extension() for .shp.zip and such)
+        .suffix(
+            path.file_name()
+                .unwrap_or_default()
+                .to_string_lossy()
+                .as_ref(),
+        )
+        .tempfile()
+        .unwrap()
+        .into_parts();
+    file.write_all(&input_data).unwrap();
+    // Close the temporary file so that Dataset can open it safely even if the filesystem uses
+    // exclusive locking (Windows?).
+    drop(file);
+
+    let ds = Dataset::open_ex(
+        &temp_path,
+        DatasetOptions {
+            open_flags: GDALAccess::GA_Update.into(),
+            ..DatasetOptions::default()
+        },
+    )
+    .unwrap();
+    (temp_path, ds)
+}
+
 /// Assert numerical difference between two expressions is less than
 /// 64-bit machine epsilon or a specified epsilon.
 ///

src/vector/transaction.rs

@@ -179,9 +179,93 @@ impl Dataset {
     }
 }
 
+/// Represents maybe a transaction on a dataset for speed reasons.
+///
+/// It can be committed by calling [`commit`](MaybeTransaction::commit), but unlike [`Transaction`] it can not be rolled back. The transaction part is to make the process speedy when possible. If the [`Dataset`] does not support transaction, it does nothing.
+///
+/// If the transaction is not explicitly committed when it is dropped, it is implicitly committed, but you will not know if it fails.
+///
+/// The transaction holds a mutable borrow on the `Dataset` that it was created from, so during the
+/// lifetime of the transaction you will need to access the dataset by dereferencing the
+/// `MaybeTransaction` through its [`Deref`] or [`DerefMut`] implementations.
+#[derive(Debug)]
+pub struct MaybeTransaction<'a> {
+    dataset: &'a mut Dataset,
+    is_transaction: bool,
+    commit_on_drop: bool,
+}
+
+impl<'a> MaybeTransaction<'a> {
+    fn new(dataset: &'a mut Dataset, is_transaction: bool) -> Self {
+        MaybeTransaction {
+            dataset,
+            is_transaction,
+            commit_on_drop: true,
+        }
+    }
+
+    pub fn is_transaction(&self) -> bool {
+        self.is_transaction
+    }
+
+    /// Commits this transaction.
+    ///
+    /// If the commit fails, will return [`OGRErr::OGRERR_FAILURE`].
+    ///
+    /// Depending on drivers, this may or may not abort layer sequential readings that are active.
+    pub fn commit(mut self) -> Result<()> {
+        if !self.is_transaction {
+            return Ok(());
+        }
+
+        let rv = unsafe { gdal_sys::GDALDatasetCommitTransaction(self.dataset.c_dataset()) };
+        self.commit_on_drop = false;
+        if rv != OGRErr::OGRERR_NONE {
+            return Err(GdalError::OgrError {
+                err: rv,
+                method_name: "GDALDatasetCommitTransaction",
+            });
+        }
+        Ok(())
+    }
+}
+
+impl<'a> Deref for MaybeTransaction<'a> {
+    type Target = Dataset;
+
+    fn deref(&self) -> &Self::Target {
+        self.dataset
+    }
+}
+
+impl<'a> DerefMut for MaybeTransaction<'a> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.dataset
+    }
+}
+
+impl Dataset {
+    pub fn maybe_start_transaction(&mut self) -> MaybeTransaction<'_> {
+        let force = 1;
+        let rv = unsafe { gdal_sys::GDALDatasetStartTransaction(self.c_dataset(), force) };
+
+        MaybeTransaction::new(self, rv == OGRErr::OGRERR_NONE)
+    }
+}
+
+impl<'a> Drop for MaybeTransaction<'a> {
+    fn drop(&mut self) {
+        if self.commit_on_drop {
+            // We silently swallow any errors, because we have no way to report them from a drop
+            // function apart from panicking.
+            unsafe { gdal_sys::GDALDatasetCommitTransaction(self.dataset.c_dataset()) };
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
-    use crate::test_utils::{fixture, open_gpkg_for_update};
+    use crate::test_utils::{fixture, open_dataset_for_update, open_gpkg_for_update};
     use crate::vector::{Geometry, LayerAccess};
     use crate::Dataset;
 
@@ -241,4 +325,54 @@ mod tests {
         let mut ds = Dataset::open(fixture("roads.geojson")).unwrap();
         assert!(ds.start_transaction().is_err());
     }
+
+    #[test]
+    fn test_maybe_start_transaction() {
+        let (_temp_path, mut ds) = open_gpkg_for_update(&fixture("poly.gpkg"));
+        let txn = ds.maybe_start_transaction();
+        assert!(txn.is_transaction());
+        let mut ds = Dataset::open(fixture("roads.geojson")).unwrap();
+        let txn = ds.maybe_start_transaction();
+        assert!(!txn.is_transaction());
+    }
+
+    #[test]
+    fn test_maybe_transaction_commit() {
+        let (_temp_path, mut ds) = open_gpkg_for_update(&fixture("poly.gpkg"));
+        let orig_feature_count = ds.layer(0).unwrap().feature_count();
+
+        let txn = ds.maybe_start_transaction();
+        let mut layer = txn.layer(0).unwrap();
+        layer.create_feature(polygon()).unwrap();
+        assert!(txn.commit().is_ok());
+
+        assert_eq!(ds.layer(0).unwrap().feature_count(), orig_feature_count + 1);
+    }
+
+    #[test]
+    fn test_maybe_transaction_commit_unsupported() {
+        let (_temp_path, mut ds) = open_dataset_for_update(&fixture("roads.geojson"));
+        let orig_feature_count = ds.layer(0).unwrap().feature_count();
+
+        let txn = ds.maybe_start_transaction();
+        let mut layer = txn.layer(0).unwrap();
+        layer.create_feature(polygon()).unwrap();
+        assert!(txn.commit().is_ok());
+
+        assert_eq!(ds.layer(0).unwrap().feature_count(), orig_feature_count + 1);
+    }
+
+    #[test]
+    fn test_maybe_transaction_implicit_commit() {
+        let (_temp_path, mut ds) = open_gpkg_for_update(&fixture("poly.gpkg"));
+        let orig_feature_count = ds.layer(0).unwrap().feature_count();
+
+        {
+            let txn = ds.maybe_start_transaction();
+            let mut layer = txn.layer(0).unwrap();
+            layer.create_feature(polygon()).unwrap();
+        } // txn is dropped here.
+
+        assert_eq!(ds.layer(0).unwrap().feature_count(), orig_feature_count + 1);
+    }
 }