Fix issue where string references aren't always completed with a null…

… terminator.

This is particularly easy to reproduce with an empty string - the
existing implementation works most of the time because most characters
don't consume the full four bytes a single UTF-8 character could
consume.

The tests relies on the fact that two consecutive memory allocations
usually end up next to each other. This happens reliably for me on my
machine, but isn't guaranteed. If there's a better way to test this,
I'd love to hear it.

src/main/java/jnr/ffi/Struct.java

@@ -2469,8 +2469,9 @@ public final java.lang.String get() {
 
         public final void set(java.lang.String value) {
             if (value != null) {
-                valueHolder = getRuntime().getMemoryManager().allocateDirect(value.length() * 4);
-                valueHolder.putString(0, value, value.length() * 4, charset);
+                int maxBytes = (value.length() * 4) + 1; // Additional byte required for trailing null termination.
+                valueHolder = getRuntime().getMemoryManager().allocateDirect(maxBytes);
+                valueHolder.putString(0, value, maxBytes, charset);
                 getMemory().putPointer(offset(), valueHolder);
 
             } else {

src/test/java/jnr/ffi/struct/AsciiStringFieldTest.java

@@ -101,4 +101,24 @@ public StructWithStringByRef() {
 
         assertEquals(testValue, struct.stringValue.get());
     }
+
+    public static final class StructWithConsecutiveStringsByRef extends Struct {
+        private final AsciiStringRef firstValue = new AsciiStringRef();
+        private final AsciiStringRef secondValue = new AsciiStringRef();
+
+        public StructWithConsecutiveStringsByRef() {
+            super(runtime);
+        }
+    }
+
+    @Test public void testStructWithConsecutiveStringsByRef() {
+        final String testValue = "some test string";
+        final StructWithConsecutiveStringsByRef struct = new StructWithConsecutiveStringsByRef();
+
+        struct.firstValue.set("");
+        struct.secondValue.set(testValue);
+
+        assertEquals("", struct.firstValue.get());
+        assertEquals(testValue, struct.secondValue.get());
+    }
 }

src/test/java/jnr/ffi/struct/UTF8StringFieldTest.java

@@ -122,4 +122,24 @@ public StructWithStringByRef() {
 
         assertEquals(testValue, struct.stringValue.get());
     }
+
+    public static final class StructWithConsecutiveStringsByRef extends Struct {
+        private final UTF8StringRef firstValue = new UTF8StringRef();
+        private final UTF8StringRef secondValue = new UTF8StringRef();
+
+        public StructWithConsecutiveStringsByRef() {
+            super(runtime);
+        }
+    }
+
+    @Test public void testStructWithConsecutiveStringsByRef() {
+        final String testValue = "some test string";
+        final StructWithConsecutiveStringsByRef struct = new StructWithConsecutiveStringsByRef();
+
+        struct.firstValue.set("");
+        struct.secondValue.set(testValue);
+
+        assertEquals("", struct.firstValue.get());
+        assertEquals(testValue, struct.secondValue.get());
+    }
 }