Fixed dropping of GreetingSet

We were incorrectly passing GreetingSet by value, which was causing problems when we tried to free it.

Author: drrb

Diff for: src/main/java/com/github/drrb/javarust/Greeting.java

@@ -49,6 +49,11 @@ protected List<String> getFieldOrder() {
 
     @Override
     public void close() {
+        // Turn off "auto-synch". If it is on, JNA will automatically read all fields
+        // from the struct's memory and update them on the Java object. This synchronization
+        // occurs after every native method call. If it occurs after we drop the struct, JNA
+        // will try to read from the freed memory and cause a segmentation fault.
+        setAutoSynch(false);
         // Send the struct back to rust for the memory to be freed
         Greetings.INSTANCE.dropGreeting(this);
     }

Diff for: src/main/java/com/github/drrb/javarust/GreetingSet.java

@@ -26,18 +26,12 @@
  * A struct that contains an array of structs. This is the Java representation
  * of the GreetingSet struct in Rust (see the Rust code).
  */
-public class GreetingSet extends Structure {
+public class GreetingSet extends Structure implements Closeable {
 
     public static class ByReference extends GreetingSet implements Structure.ByReference {
     }
 
-    public static class ByValue extends GreetingSet implements Structure.ByValue, Closeable {
-
-        @Override
-        public void close() {
-            Greetings.INSTANCE.dropGreetingSet(this);
-        }
-
+    public static class ByValue extends GreetingSet implements Structure.ByValue {
     }
 
     /**
@@ -48,7 +42,7 @@ public void close() {
      * 
      * NB: We need to explicitly specify that the field is a pointer (i.e. we need
      * to use ByReference) because, by default, JNA assumes that struct fields 
-     * are not pointers (i.e. the if you just say "Greeting", JNA assumes 
+     * are not pointers (i.e. if you just say "Greeting", JNA assumes
      * "Greeting.ByValue" here).
      */
     public Greeting.ByReference greetings;
@@ -88,4 +82,20 @@ public List<Greeting> getGreetings() {
     protected List<String> getFieldOrder() {
         return Arrays.asList("greetings", "numberOfGreetings");
     }
+
+    /**
+     * Send the GreetingSet back to Rust to be dropped.
+     *
+     * We do this because JNA doesn't free the memory when the object is garbage collected.
+     */
+    @Override
+    public void close() {
+        // Turn off "auto-synch". If it is on, JNA will automatically read all fields
+        // from the struct's memory and update them on the Java object. This synchronization
+        // occurs after every native method call. If it occurs after we drop the struct, JNA
+        // will try to read from the freed memory and cause a segmentation fault.
+        setAutoSynch(false);
+        // Send the struct back to rust for the memory to be freed
+        Greetings.INSTANCE.dropGreetingSet(this);
+    }
 }

Diff for: src/main/java/com/github/drrb/javarust/Greetings.java

@@ -76,7 +76,7 @@ public interface Greetings extends Library {
      * This is the same as returning a {@link GreetingSet.ByReference}. JNA assumes
      * it's by reference when it's returned from a native function.
      */
-    GreetingSet.ByValue renderGreetings();
+    GreetingSet renderGreetings();
 
     /**
      * Passing a callback that will be called from Rust with individual strings
@@ -116,5 +116,5 @@ interface GreetingSetCallback extends Callback {
     /**
      * Free the memory used by a GreetingSet
      */
-    void dropGreetingSet(GreetingSet.ByValue greetingSet);
+    void dropGreetingSet(GreetingSet greetingSet);
 }

Diff for: src/main/rust/com/github/drrb/javarust/lib/greetings.rs

@@ -158,12 +158,12 @@ pub extern fn sendGreetings(callback: extern "C" fn(&GreetingSet)) {
 /// Example of returning a more complicated struct from Rust
 #[no_mangle]
 #[allow(non_snake_case)]
-pub extern fn renderGreetings() -> GreetingSet {
+pub extern fn renderGreetings() -> Box<GreetingSet> {
     let greetings = vec![ Greeting::new("Hello!"), Greeting::new("Hello again!") ];
 
-    GreetingSet {
+    Box::new(GreetingSet {
         greetings: greetings.into_boxed_slice()
-    }
+    })
 }
 
 #[no_mangle]
@@ -175,7 +175,7 @@ pub extern fn dropGreeting(_: Box<Greeting>) {
 
 #[no_mangle]
 #[allow(non_snake_case)]
-pub extern fn dropGreetingSet(_: GreetingSet) {
+pub extern fn dropGreetingSet(_: Box<GreetingSet>) {
     // Do nothing here. Because we own the GreetingSet here and we're not
     // returning it, Rust will assume we don't want it anymore and clean it up.
 }

Diff for: src/test/java/com/github/drrb/javarust/GreetingsTest.java

@@ -97,7 +97,7 @@ public void apply(GreetingSet.ByReference greetingSet) {
         });
 
         List<String> greetingStrings = new LinkedList<>();
-        for (Greeting greeting: greetings) {
+        for (Greeting greeting : greetings) {
             greetingStrings.add(greeting.getText());
         }
 
@@ -106,7 +106,7 @@ public void apply(GreetingSet.ByReference greetingSet) {
 
     @Test
     public void shouldGetAStructFromRustContainingAnArrayOfStructs() {
-        try (GreetingSet.ByValue result = library.renderGreetings()) {
+        try (GreetingSet result = library.renderGreetings()) {
             List<String> greetings = new LinkedList<>();
             for (Greeting greeting : result.getGreetings()) {
                 greetings.add(greeting.getText());