fix: add calls to extism_function_free to cleanup HostFunctions (#14)

It looks like there isn't a good way to define a destructor in Java, so
this PR adds a `HostFunction.free` function that is automatically called
when a Plugin is freed, if a `HostFunction` is never registered with a
plugin then it should be called manually.

Fixes #13

Author: zshipko

src/main/java/org/extism/sdk/HostFunction.java

@@ -10,6 +10,8 @@ public class HostFunction<T extends HostUserData> {
 
     private final LibExtism.InternalExtismFunction callback;
 
+    private boolean freed;
+
     public final Pointer pointer;
 
     public final String name;
@@ -21,7 +23,7 @@ public class HostFunction<T extends HostUserData> {
     public final Optional<T> userData;
 
     public HostFunction(String name, LibExtism.ExtismValType[] params, LibExtism.ExtismValType[] returns, ExtismFunction f, Optional<T> userData) {
-
+        this.freed = false;
         this.name = name;
         this.params = params;
         this.returns = returns;
@@ -85,8 +87,15 @@ public void setNamespace(String name) {
         }
     }
 
-    HostFunction withNamespace(String name) {
+    public HostFunction withNamespace(String name) {
         this.setNamespace(name);
         return this;
     }
+
+    public void free() {
+        if (!this.freed){
+            LibExtism.INSTANCE.extism_function_free(this.pointer);
+            this.freed = true;
+        }
+    }
 }

src/main/java/org/extism/sdk/LibExtism.java

@@ -62,6 +62,8 @@ Pointer extism_function_new(String name,
                                 Pointer userData,
                                 Pointer freeUserData);
 
+    void extism_function_free(Pointer function);
+
     /**
      * Get the length of an allocated block
      * NOTE: this should only be called from host functions.

src/main/java/org/extism/sdk/Plugin.java

@@ -17,6 +17,8 @@ public class Plugin implements AutoCloseable {
      */
     private final Pointer pluginPointer;
 
+    private final HostFunction[] functions;
+
     /**
      * @param manifestBytes The manifest for the plugin
      * @param functions     The Host functions for th eplugin
@@ -40,13 +42,18 @@ public Plugin(byte[] manifestBytes, boolean withWASI, HostFunction[] functions)
                 withWASI,
                 errormsg);
         if (p == null) {
+            if (functions != null)
+                for (int i = 0; i < functions.length; i++) {
+                   LibExtism.INSTANCE.extism_function_free(functions[i].pointer);
+                }
             int errlen = LibExtism.INSTANCE.strlen(errormsg[0]);
             byte[] msg = new byte[errlen];
             errormsg[0].read(0, msg, 0, errlen);
             LibExtism.INSTANCE.extism_plugin_new_error_free(errormsg[0]);
             throw new ExtismException(new String(msg));
         }
 
+        this.functions = functions;
         this.pluginPointer = p;
     }
 
@@ -114,6 +121,11 @@ protected String error() {
      * Frees a plugin from memory
      */
     public void free() {
+        if (this.functions != null){
+            for (int i = 0; i < this.functions.length; i++) {
+                this.functions[i].free();
+            }
+        }
         LibExtism.INSTANCE.extism_plugin_free(this.pluginPointer);
     }