Write back preset meta data set by the plugin

The docs don't mention that this is a thing you should support, but
`IStreamAttributes` doesn't make much sense without this.

src/common/serialization/vst3/attribute-list.cpp

@@ -29,6 +29,28 @@ IMPLEMENT_FUNKNOWN_METHODS(YaAttributeList,
                            Steinberg::Vst::IAttributeList::iid)
 #pragma GCC diagnostic pop
 
+tresult YaAttributeList::write_back(
+    Steinberg::Vst::IAttributeList* stream) const {
+    if (!stream) {
+        return Steinberg::kInvalidArgument;
+    }
+
+    for (const auto& [key, value] : attrs_int) {
+        stream->setInt(key.c_str(), value);
+    }
+    for (const auto& [key, value] : attrs_float) {
+        stream->setFloat(key.c_str(), value);
+    }
+    for (const auto& [key, value] : attrs_string) {
+        stream->setString(key.c_str(), u16string_to_tchar_pointer(value));
+    }
+    for (const auto& [key, value] : attrs_binary) {
+        stream->setBinary(key.c_str(), value.data(), value.size());
+    }
+
+    return Steinberg::kResultOk;
+}
+
 tresult PLUGIN_API YaAttributeList::setInt(AttrID id, int64 value) {
     attrs_int[id] = value;
     return Steinberg::kResultOk;

src/common/serialization/vst3/attribute-list.h

@@ -42,6 +42,13 @@ class YaAttributeList : public Steinberg::Vst::IAttributeList {
 
     DECLARE_FUNKNOWN_METHODS
 
+    /**
+     * Write the attribute list a host provided `IAttributeList`. This is used
+     * in `YaBStream::write_back` to write any preset meta data back to the host
+     * for hosts that support it.
+     */
+    tresult write_back(Steinberg::Vst::IAttributeList* stream) const;
+
     virtual tresult PLUGIN_API setInt(AttrID id, int64 value) override;
     virtual tresult PLUGIN_API getInt(AttrID id, int64& value) override;
     virtual tresult PLUGIN_API setFloat(AttrID id, double value) override;

src/common/serialization/vst3/bstream.cpp

@@ -21,6 +21,31 @@
 #include <cassert>
 #include <stdexcept>
 
+/**
+ * These are the meta data keys used for `IStreamAttributes`. We need to keep
+ * track of this because `IAttributeList` has no way to just iterate over the
+ * stored keys. We'll read these from the host if the host supports this
+ * interface, and if the plugin writes an attribute with one of these keys we'll
+ * write the value back to the host.
+ *
+ * TODO: There's also `Steinberg::Vst::PresetAttributes::kFilePathStringType`
+ *       This would require translating between Windows and Unix style paths,
+ *       which we can't easily do outside of Wine. If this ends up being
+ *       important, then we'll have to shell out to `winepath` which is not
+ *       ideal. On the Wine side we can just use the `wine_get_dos_file_name`
+ *       and `wine_get_unix_file_name` functions instead. Requesting this should
+ *       also use a 1024 character buffer.
+ */
+const static char* stream_meta_data_keys[] = {
+    Steinberg::Vst::PresetAttributes::kPlugInName,
+    Steinberg::Vst::PresetAttributes::kPlugInCategory,
+    Steinberg::Vst::PresetAttributes::kInstrument,
+    Steinberg::Vst::PresetAttributes::kStyle,
+    Steinberg::Vst::PresetAttributes::kCharacter,
+    Steinberg::Vst::PresetAttributes::kStateType,
+    Steinberg::Vst::PresetAttributes::kName,
+    Steinberg::Vst::PresetAttributes::kFileName};
+
 YaBStream::YaBStream(){FUNKNOWN_CTOR}
 
 YaBStream::YaBStream(Steinberg::IBStream* stream) {
@@ -61,34 +86,18 @@ YaBStream::YaBStream(Steinberg::IBStream* stream) {
             file_name.emplace(tchar_pointer_to_u16string(vst_string));
         }
 
-        // Copy over all predefined meta data keys. `IAttributeList` does not
-        // offer any interface to enumerate over the stored keys.
-        // TODO: There's also
-        //       `Steinberg::Vst::PresetAttributes::kFilePathStringType`
-        //       This would require translating between Windows and Unix style
-        //       paths, which we can't easily do outside of Wine. If this ends
-        //       up being important, then we'll have to shell out to `winepath`
-        //       which is not ideal. On the Wine side we can just use the
-        //       `wine_get_dos_file_name` and `wine_get_unix_file_name`
-        //       functions instead. Requesting this should also use a 1024
-        //       character buffer.
         attributes.emplace();
-        Steinberg::IPtr<Steinberg::Vst::IAttributeList> stream_attributes_list =
-            stream_attributes->getAttributes();
-        for (const auto& key :
-             {Steinberg::Vst::PresetAttributes::kPlugInName,
-              Steinberg::Vst::PresetAttributes::kPlugInCategory,
-              Steinberg::Vst::PresetAttributes::kInstrument,
-              Steinberg::Vst::PresetAttributes::kStyle,
-              Steinberg::Vst::PresetAttributes::kCharacter,
-              Steinberg::Vst::PresetAttributes::kStateType,
-              Steinberg::Vst::PresetAttributes::kName,
-              Steinberg::Vst::PresetAttributes::kFileName}) {
-            vst_string[0] = 0;
-            if (stream_attributes_list->getString(key, vst_string,
-                                                  sizeof(vst_string)) ==
-                Steinberg::kResultOk) {
-                attributes->setString(key, vst_string);
+        if (Steinberg::IPtr<Steinberg::Vst::IAttributeList>
+                stream_attributes_list = stream_attributes->getAttributes()) {
+            // Copy over all predefined meta data keys. `IAttributeList` does
+            // not offer any interface to enumerate the stored keys.
+            for (const auto& key : stream_meta_data_keys) {
+                vst_string[0] = 0;
+                if (stream_attributes_list->getString(key, vst_string,
+                                                      sizeof(vst_string)) ==
+                    Steinberg::kResultOk) {
+                    attributes->setString(key, vst_string);
+                }
             }
         }
     }
@@ -139,6 +148,18 @@ tresult YaBStream::write_back(Steinberg::IBStream* stream) const {
     //       back after `*::getState()`? I'd assume so, but the docs don't
     //       mention this. If so then we need to always store whether the host
     //       supports `IStreamAttributes`.
+    // Write back any attributes written by the plugin if the host supports
+    // preset meta data
+    if (Steinberg::FUnknownPtr<Steinberg::Vst::IStreamAttributes>
+            stream_attributes = stream;
+        stream_attributes && attributes) {
+        if (Steinberg::IPtr<Steinberg::Vst::IAttributeList>
+                stream_attributes_list = stream_attributes->getAttributes()) {
+            // XXX: If the host somehow preset some attributes, then we're also
+            //      writing those back. This should not cause any issues though.
+            attributes->write_back(stream_attributes_list);
+        }
+    }
 
     return Steinberg::kResultOk;
 }

src/common/serialization/vst3/bstream.h

@@ -58,8 +58,8 @@ class YaBStream : public Steinberg::IBStream,
     DECLARE_FUNKNOWN_METHODS
 
     /**
-     * Write the vector buffer back to an IBStream. After writing the seek
-     * position will be left at the end of the stream.
+     * Write the vector buffer back to a host provided `IBStream`. After writing
+     * the seek position will be left at the end of the stream.
      */
     tresult write_back(Steinberg::IBStream* stream) const;