Sub-transactions API

pgx-tests/src/tests/mod.rs

@@ -42,6 +42,7 @@ mod shmem_tests;
 mod spi_tests;
 mod srf_tests;
 mod struct_type_tests;
+mod subxact_tests;
 mod trigger_tests;
 mod uuid_tests;
 mod variadic_tests;

pgx-tests/src/tests/subxact_tests.rs

@@ -0,0 +1,103 @@
+/*
+Portions Copyright 2019-2021 ZomboDB, LLC.
+Portions Copyright 2021-2022 Technology Concepts & Design, Inc. <[email protected]>
+
+All rights reserved.
+
+Use of this source code is governed by the MIT license that can be found in the LICENSE file.
+*/
+
+#[cfg(any(test, feature = "pg_test"))]
+#[pgx::pg_schema]
+mod tests {
+    #[allow(unused_imports)]
+    use crate as pgx_tests;
+
+    use pgx::prelude::*;
+    use pgx::SpiClient;
+
+    #[pg_test]
+    fn test_subxact_smoketest() {
+        Spi::execute(|c| {
+            c.update("CREATE TABLE a (v INTEGER)", None, None);
+            let c = c.sub_transaction(|xact| {
+                xact.update("INSERT INTO a VALUES (0)", None, None);
+                assert_eq!(
+                    0,
+                    xact.select("SELECT v FROM a", Some(1), None)
+                        .first()
+                        .get_datum::<i32>(1)
+                        .unwrap()
+                );
+                let xact = xact.sub_transaction(|xact| {
+                    xact.update("INSERT INTO a VALUES (1)", None, None);
+                    assert_eq!(
+                        2,
+                        xact.select("SELECT COUNT(*) FROM a", Some(1), None)
+                            .first()
+                            .get_datum::<i32>(1)
+                            .unwrap()
+                    );
+                    xact.rollback()
+                });
+                xact.rollback()
+            });
+            assert_eq!(
+                0,
+                c.select("SELECT COUNT(*) FROM a", Some(1), None)
+                    .first()
+                    .get_datum::<i32>(1)
+                    .unwrap()
+            );
+        })
+    }
+
+    #[pg_test]
+    fn test_commit_on_drop() {
+        Spi::execute(|c| {
+            c.update("CREATE TABLE a (v INTEGER)", None, None);
+            // The type below is explicit to ensure it's commit on drop by default
+            c.sub_transaction(|xact: SubTransaction<SpiClient, CommitOnDrop>| {
+                xact.update("INSERT INTO a VALUES (0)", None, None);
+                // Dropped explicitly for illustration purposes
+                drop(xact);
+            });
+            // Create a new client to check the state
+            Spi::execute(|c| {
+                // The above insert should have been committed
+                assert_eq!(
+                    1,
+                    c.select("SELECT COUNT(*) FROM a", Some(1), None)
+                        .first()
+                        .get_datum::<i32>(1)
+                        .unwrap()
+                );
+            });
+        })
+    }
+
+    #[pg_test]
+    fn test_rollback_on_drop() {
+        Spi::execute(|c| {
+            c.update("CREATE TABLE a (v INTEGER)", None, None);
+            // The type below is explicit to ensure it's commit on drop by default
+            c.sub_transaction(|xact: SubTransaction<SpiClient, CommitOnDrop>| {
+                xact.update("INSERT INTO a VALUES (0)", None, None);
+                let xact = xact.rollback_on_drop();
+                // Dropped explicitly for illustration purposes
+                drop(xact);
+            });
+            // Create a new client to check the state
+            Spi::execute(|c| {
+                // The above insert should NOT have been committed
+                assert_eq!(
+                    0,
+                    c.select("SELECT COUNT(*) FROM a", Some(1), None)
+                        .first()
+                        .get_datum::<i32>(1)
+                        .unwrap()
+                );
+            });
+        })
+    }
+}

pgx/src/lib.rs

@@ -63,6 +63,7 @@ pub mod shmem;
 pub mod spi;
 pub mod spinlock;
 pub mod stringinfo;
+pub mod subxact;
 pub mod trigger_support;
 pub mod tupdesc;
 pub mod varlena;

pgx/src/prelude.rs

@@ -47,3 +47,6 @@ pub use crate::pg_sys::{
     check_for_interrupts, debug1, debug2, debug3, debug4, debug5, ereport, error, function_name,
     info, log, notice, warning, FATAL, PANIC,
 };
+
+// Sub-transactions
+pub use crate::subxact::*;

pgx/src/subxact.rs

@@ -0,0 +1,240 @@
+use crate::{pg_sys, PgMemoryContexts, SpiClient};
+use std::fmt::Debug;
+use std::ops::Deref;
+
+/// Releases a sub-transaction on Drop
+pub trait ReleaseOnDrop {}
+
+/// Sub-transaction's contextual information
+#[derive(Clone, Copy)]
+pub struct Context {
+    memory_context: pg_sys::MemoryContext,
+    // Resource ownership before the transaction
+    //
+    // Based on information from src/backend/utils/resowner/README
+    // as well as practical use of it in src/pl/plpython/plpy_spi.c
+    resource_owner: pg_sys::ResourceOwner,
+}
+
+impl Context {
+    /// Captures the context
+    fn capture() -> Self {
+        // Remember the memory context before starting the sub-transaction
+        let memory_context = PgMemoryContexts::CurrentMemoryContext.value();
+        // Remember resource owner before starting the sub-transaction
+        let resource_owner = unsafe { pg_sys::CurrentResourceOwner };
+        Self { memory_context, resource_owner }
+    }
+}
+
+impl From<Context> for CommitOnDrop {
+    fn from(context: Context) -> Self {
+        CommitOnDrop(context)
+    }
+}
+
+impl From<Context> for RollbackOnDrop {
+    fn from(context: Context) -> Self {
+        RollbackOnDrop(context)
+    }
+}
+
+/// Commits a sub-transaction on Drop
+pub struct CommitOnDrop(Context);
+
+impl Drop for CommitOnDrop {
+    fn drop(&mut self) {
+        unsafe {
+            pg_sys::ReleaseCurrentSubTransaction();
+            pg_sys::CurrentResourceOwner = self.0.resource_owner;
+        }
+        PgMemoryContexts::For(self.0.memory_context).set_as_current();
+    }
+}
+
+impl ReleaseOnDrop for CommitOnDrop {}
+
+/// Rolls back a sub-transaction on Drop
+pub struct RollbackOnDrop(Context);
+
+impl Drop for RollbackOnDrop {
+    fn drop(&mut self) {
+        unsafe {
+            pg_sys::RollbackAndReleaseCurrentSubTransaction();
+            pg_sys::CurrentResourceOwner = self.0.resource_owner;
+        }
+        PgMemoryContexts::For(self.0.memory_context).set_as_current();
+    }
+}
+
+impl ReleaseOnDrop for RollbackOnDrop {}
+
+impl Into<RollbackOnDrop> for CommitOnDrop {
+    fn into(self) -> RollbackOnDrop {
+        let result = RollbackOnDrop(self.0);
+        // IMPORTANT: avoid running Drop (that would commit)
+        std::mem::forget(self);
+        result
+    }
+}
+
+impl Into<CommitOnDrop> for RollbackOnDrop {
+    fn into(self) -> CommitOnDrop {
+        let result = CommitOnDrop(self.0);
+        // IMPORTANT: avoid running Drop (that would roll back)
+        std::mem::forget(self);
+        result
+    }
+}
+
+struct NoOpOnDrop;
+
+impl ReleaseOnDrop for NoOpOnDrop {}
+
+/// Sub-transaction
+///
+/// Can be created by calling `SpiClient::sub_transaction`, `SubTransaction<Parent>::sub_transaction`
+/// or any other implementation of `SubTransactionExt` and obtaining it as an argument to the provided closure.
+///
+/// Unless rolled back or committed explicitly, it'll commit if `Release` generic parameter is `CommitOnDrop`
+/// (default) or roll back if it is `RollbackOnDrop`.
+#[derive(Debug)]
+pub struct SubTransaction<Parent: SubTransactionExt, Release: ReleaseOnDrop = CommitOnDrop> {
+    // Transaction release mechanism (commit, drop)
+    release: Release,
+    // Transaction parent
+    parent: Parent,
+}
+
+impl<Parent: SubTransactionExt, Release: ReleaseOnDrop> SubTransaction<Parent, Release>
+where
+    Release: From<Context>,
+{
+    /// Create a new sub-transaction.
+    fn new(parent: Parent) -> Self {
+        let context = Context::capture();
+        let memory_context = context.memory_context;
+        let release = context.into();
+        unsafe {
+            pg_sys::BeginInternalSubTransaction(std::ptr::null() /* [no] transaction name */);
+        }
+        // Switch to the outer memory context so that all allocations remain
+        // there instead of the sub-transaction's context
+        PgMemoryContexts::For(memory_context).set_as_current();
+        Self { release, parent }
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, CommitOnDrop> {
+    /// Commit the transaction, returning its parent
+    pub fn commit(self) -> Parent {
+        // `Self::do_nothing_on_drop()` will commit as `Release` is `CommitOnDrop`
+        self.do_nothing_on_drop().parent
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, RollbackOnDrop> {
+    /// Commit the transaction, returning its parent
+    pub fn commit(self) -> Parent {
+        // Make sub-transaction commit on drop and then use `commit`
+        self.commit_on_drop().commit()
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, RollbackOnDrop> {
+    /// Rollback the transaction, returning its parent
+    pub fn rollback(self) -> Parent {
+        // `Self::do_nothing_on_drop()` will roll back as `Release` is `RollbackOnDrop`
+        self.do_nothing_on_drop().parent
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, CommitOnDrop> {
+    /// Rollback the transaction, returning its parent
+    pub fn rollback(self) -> Parent {
+        // Make sub-transaction roll back on drop and then use `rollback`
+        self.rollback_on_drop().rollback()
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, CommitOnDrop> {
+    /// Make this sub-transaction roll back on drop
+    pub fn rollback_on_drop(self) -> SubTransaction<Parent, RollbackOnDrop> {
+        SubTransaction { parent: self.parent, release: self.release.into() }
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransaction<Parent, RollbackOnDrop> {
+    /// Make this sub-transaction commit on drop
+    pub fn commit_on_drop(self) -> SubTransaction<Parent, CommitOnDrop> {
+        SubTransaction { parent: self.parent, release: self.release.into() }
+    }
+}
+
+impl<Parent: SubTransactionExt, Release: ReleaseOnDrop> SubTransaction<Parent, Release> {
+    /// Make this sub-transaction do nothing on drop
+    ///
+    /// Releases the sub-transaction based on `Release` generic parameter. Further
+    /// dropping of the sub-transaction will not do anything.
+    fn do_nothing_on_drop(self) -> SubTransaction<Parent, NoOpOnDrop> {
+        SubTransaction { parent: self.parent, release: NoOpOnDrop }
+    }
+}
+
+// This allows SubTransaction to be de-referenced to SpiClient
+impl<'conn, Release: ReleaseOnDrop> Deref for SubTransaction<SpiClient<'conn>, Release> {
+    type Target = SpiClient<'conn>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.parent
+    }
+}
+
+// This allows a SubTransaction of a SubTransaction to be de-referenced to SpiClient
+impl<Parent: SubTransactionExt, Release: ReleaseOnDrop> Deref
+    for SubTransaction<SubTransaction<Parent>, Release>
+{
+    type Target = Parent;
+
+    fn deref(&self) -> &Self::Target {
+        &self.parent.parent
+    }
+}
+
+/// Trait that allows creating a sub-transaction off any type
+pub trait SubTransactionExt {
+    /// Parent's type
+    ///
+    /// In most common cases, it'll be equal to `Self`. However, in some cases
+    /// it may be desirable to use a different type to achieve certain goals.
+    type Parent: SubTransactionExt;
+
+    /// Consume `self` and execute a closure with a sub-transaction
+    ///
+    /// If further use of the given sub-transaction is necessary, it must
+    /// be returned by the closure alongside with its intended result. Otherwise,
+    /// the sub-transaction be released when dropped.
+    fn sub_transaction<F: FnOnce(SubTransaction<Self::Parent>) -> R, R>(self, f: F) -> R
+    where
+        Self: Sized;
+}
+
+impl<'a> SubTransactionExt for SpiClient<'a> {
+    type Parent = Self;
+    fn sub_transaction<F: FnOnce(SubTransaction<Self::Parent>) -> R, R>(self, f: F) -> R
+    where
+        Self: Sized,
+    {
+        f(SubTransaction::new(self))
+    }
+}
+
+impl<Parent: SubTransactionExt> SubTransactionExt for SubTransaction<Parent> {
+    type Parent = Self;
+    fn sub_transaction<F: FnOnce(SubTransaction<Self::Parent>) -> R, R>(self, f: F) -> R
+    where
+        Self: Sized,
+    {
+        f(SubTransaction::new(self))
+    }
+}