Provide programmative transaction for MongoDB reactive with Panache

Fixes #32156

docs/src/main/asciidoc/mongodb-panache.adoc

@@ -771,8 +771,6 @@ To use them with MongoDB with Panache you need to annotate the method that start
 In MongoDB, a transaction is only possible on a replicaset,
 luckily our xref:mongodb.adoc#dev-services[Dev Services for MongoDB] setups a single node replicaset so it is compatible with transactions.
 
-WARNING: Transaction support inside MongoDB with Panache is still experimental.
-
 == Custom IDs
 
 IDs are often a touchy subject. In MongoDB, they are usually auto-generated by the database with an `ObjectId` type.
@@ -1006,7 +1004,28 @@ public Multi<ReactivePerson> streamPersons() {
 
 TIP: `@RestStreamElementType(MediaType.APPLICATION_JSON)` tells RESTEasy Reactive to serialize the object in JSON.
 
-WARNING: Transactions are not supported for Reactive Entities and Repositories.
+=== Reactive transactions
+
+MongoDB offers ACID transactions since version 4.0.
+
+To use them with reactive entities or repositories you need to use `io.quarkus.mongodb.panache.common.reactive.Panache.withTransaction()`.
+
+[source,java]
+----
+@POST
+public Uni<Response> addPerson(ReactiveTransactionPerson person) {
+    return Panache.withTransaction(() -> person.persist().map(v -> {
+        //the ID is populated before sending it to the database
+        String id = person.id.toString();
+        return Response.created(URI.create("/reactive-transaction/" + id)).build();
+    }));
+}
+----
+
+In MongoDB, a transaction is only possible on a replicaset,
+luckily our xref:mongodb.adoc#dev-services[Dev Services for MongoDB] setups a single node replicaset so it is compatible with transactions.
+
+WARNING: Reactive transaction support inside MongoDB with Panache is still experimental.
 
 == Mocking
 

extensions/panache/mongodb-panache-common/runtime/src/main/java/io/quarkus/mongodb/panache/common/reactive/Panache.java

@@ -0,0 +1,117 @@
+package io.quarkus.mongodb.panache.common.reactive;
+
+import java.util.UUID;
+import java.util.function.Supplier;
+
+import org.reactivestreams.Publisher;
+
+import com.mongodb.reactivestreams.client.ClientSession;
+
+import io.quarkus.mongodb.panache.common.runtime.BeanUtils;
+import io.quarkus.mongodb.reactive.ReactiveMongoClient;
+import io.quarkus.vertx.core.runtime.context.VertxContextSafetyToggle;
+import io.smallrye.mutiny.Uni;
+import io.vertx.core.Context;
+import io.vertx.core.Vertx;
+import mutiny.zero.flow.adapters.AdaptersToFlow;
+
+/**
+ * Utility class for reactive MongoDB with Panache.
+ */
+public class Panache {
+    private static final String ERROR_MSG = "MongoDB reactive with Panache requires a safe (isolated) Vert.x sub-context, but the current context hasn't been flagged as such.";
+
+    private static final UUID SESSION_KEY = UUID.randomUUID();
+
+    /**
+     * Performs the given work within the scope of a MongoDB transaction.
+     * The transaction will be rolled back if the work completes with an uncaught exception.
+     *
+     * @param <T> The function's return type
+     * @param work The function to execute in the new transaction
+     * @return the result of executing the function
+     */
+    public static <T> Uni<T> withTransaction(Supplier<Uni<T>> work) {
+        Context context = vertxContext();
+        ClientSession current = context.getLocal(SESSION_KEY);
+        if (current != null && current.hasActiveTransaction()) {
+            // reactive session exists - reuse this session
+            return work.get();
+        } else {
+            // reactive session does not exist - open a new one and close it when the returned Uni completes
+            return Panache.startSession()
+                    .invoke(s -> s.startTransaction())
+                    .invoke(s -> context.putLocal(SESSION_KEY, s))
+                    .chain(s -> work.get())
+                    .call(() -> commitTransaction())
+                    .onFailure().call(() -> abortTransaction())
+                    .eventually(() -> Panache.closeSession());
+        }
+    }
+
+    /**
+     * Allow to access the current MongoDB session.
+     * The session will only exist in the context of a reactive MongoDB with Panache transaction started with
+     * <code>Panache.withTransaction()</code>.
+     *
+     * @see #withTransaction(Supplier)
+     * @return the current ClientSession or null if none.
+     */
+    public static ClientSession getCurrentSession() {
+        Context context = Vertx.currentContext();
+        return context != null ? context.getLocal(SESSION_KEY) : null;
+    }
+
+    private static Uni<?> abortTransaction() {
+        Context context = vertxContext();
+        ClientSession current = context.getLocal(SESSION_KEY);
+        return toUni(current.abortTransaction());
+    }
+
+    private static Uni<?> commitTransaction() {
+        Context context = vertxContext();
+        ClientSession current = context.getLocal(SESSION_KEY);
+        return toUni(current.commitTransaction());
+    }
+
+    private static <T> Uni<T> toUni(Publisher<T> publisher) {
+        Context context = Vertx.currentContext();
+        Uni<T> uni = Uni.createFrom().publisher(AdaptersToFlow.publisher(publisher));
+        if (context != null) {
+            return uni.emitOn(command -> context.runOnContext(x -> command.run()));
+        }
+        return uni;
+    }
+
+    private static Uni<ClientSession> startSession() {
+        ReactiveMongoClient client = BeanUtils.clientFromArc(null, ReactiveMongoClient.class, true);
+        return client.startSession();
+    }
+
+    private static void closeSession() {
+        Context context = vertxContext();
+        ClientSession current = context.getLocal(SESSION_KEY);
+        try {
+            current.close();
+        }
+        finally {
+            context.removeLocal(SESSION_KEY);
+        }
+    }
+
+    /**
+     *
+     * @return the current vertx duplicated context
+     * @throws IllegalStateException If no vertx context is found or is not a safe context as mandated by the
+     *         {@link VertxContextSafetyToggle}
+     */
+    private static Context vertxContext() {
+        Context context = Vertx.currentContext();
+        if (context != null) {
+            VertxContextSafetyToggle.validateContextIfExists(ERROR_MSG, ERROR_MSG);
+            return context;
+        } else {
+            throw new IllegalStateException("No current Vertx context found");
+        }
+    }
+}

extensions/panache/mongodb-panache-common/runtime/src/main/java/io/quarkus/mongodb/panache/common/reactive/runtime/CommonReactivePanacheQueryImpl.java

@@ -5,6 +5,7 @@
 import java.util.OptionalInt;
 import java.util.Set;
 
+import org.bson.BsonDocument;
 import org.bson.Document;
 import org.bson.conversions.Bson;
 
@@ -13,6 +14,7 @@
 import com.mongodb.client.model.CountOptions;
 
 import io.quarkus.mongodb.FindOptions;
+import io.quarkus.mongodb.panache.common.reactive.Panache;
 import io.quarkus.mongodb.panache.common.runtime.MongoPropertyUtil;
 import io.quarkus.mongodb.reactive.ReactiveMongoCollection;
 import io.quarkus.panache.common.Page;
@@ -168,9 +170,11 @@ public Uni<Long> count() {
                 countOptions.collation(collation);
             }
 
-            count = mongoQuery == null
-                    ? collection.countDocuments()
-                    : collection.countDocuments(mongoQuery, countOptions);
+            if (Panache.getCurrentSession() != null) {
+                count = collection.countDocuments(Panache.getCurrentSession(), getQuery(), countOptions);
+            } else {
+                count = collection.countDocuments(getQuery(), countOptions);
+            }
         }
         return count;
     }
@@ -184,7 +188,8 @@ public <T extends Entity> Uni<List<T>> list() {
     @SuppressWarnings("unchecked")
     public <T extends Entity> Multi<T> stream() {
         FindOptions options = buildOptions();
-        return mongoQuery == null ? collection.find(options) : collection.find(mongoQuery, options);
+        return Panache.getCurrentSession() != null ? collection.find(Panache.getCurrentSession(), getQuery(), options)
+                : collection.find(getQuery(), options);
     }
 
     public <T extends Entity> Uni<T> firstResult() {
@@ -194,14 +199,18 @@ public <T extends Entity> Uni<T> firstResult() {
 
     public <T extends Entity> Uni<Optional<T>> firstResultOptional() {
         FindOptions options = buildOptions(1);
-        Multi<T> results = mongoQuery == null ? collection.find(options) : collection.find(mongoQuery, options);
+        Multi<T> results = Panache.getCurrentSession() != null
+                ? collection.find(Panache.getCurrentSession(), getQuery(), options)
+                : collection.find(getQuery(), options);
         return results.collect().first().map(o -> Optional.ofNullable(o));
     }
 
     @SuppressWarnings("unchecked")
     public <T extends Entity> Uni<T> singleResult() {
         FindOptions options = buildOptions(2);
-        Multi<T> results = mongoQuery == null ? collection.find(options) : collection.find(mongoQuery, options);
+        Multi<T> results = Panache.getCurrentSession() != null
+                ? collection.find(Panache.getCurrentSession(), getQuery(), options)
+                : collection.find(getQuery(), options);
         return results.collect().asList().map(list -> {
             if (list.size() != 1) {
                 throw new PanacheQueryException("There should be only one result");
@@ -213,7 +222,9 @@ public <T extends Entity> Uni<T> singleResult() {
 
     public <T extends Entity> Uni<Optional<T>> singleResultOptional() {
         FindOptions options = buildOptions(2);
-        Multi<T> results = mongoQuery == null ? collection.find(options) : collection.find(mongoQuery, options);
+        Multi<T> results = Panache.getCurrentSession() != null
+                ? collection.find(Panache.getCurrentSession(), getQuery(), options)
+                : collection.find(getQuery(), options);
         return results.collect().asList().map(list -> {
             if (list.size() == 2) {
                 throw new PanacheQueryException("There should be no more than one result");
@@ -258,4 +269,8 @@ private FindOptions buildOptions(int maxResults) {
         }
         return options.limit(maxResults);
     }
+
+    private Bson getQuery() {
+        return mongoQuery == null ? new BsonDocument() : mongoQuery;
+    }
 }