Bug 1530506 - Add a generic writable property bag wrapper for Rust code. r=nika

This commit adds a `storage_variant::HashPropertyBag` type that
exposes an idiomatic Rust interface for `nsIWritablePropertyBag`.

Differential Revision: https://phabricator.services.mozilla.com/D21062

--HG--
extra : moz-landing-system : lando

storage/variant/src/bag.rs

@@ -0,0 +1,94 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use nserror::{nsresult, NS_OK};
+use nsstring::nsString;
+use xpcom::{getter_addrefs, interfaces::nsIWritablePropertyBag, RefPtr};
+
+use crate::VariantType;
+
+extern "C" {
+    fn NS_NewHashPropertyBag(bag: *mut *const nsIWritablePropertyBag) -> libc::c_void;
+}
+
+/// A hash property bag backed by storage variant values.
+pub struct HashPropertyBag(RefPtr<nsIWritablePropertyBag>);
+
+impl Default for HashPropertyBag {
+    fn default() -> HashPropertyBag {
+        // This is safe to unwrap because `NS_NewHashPropertyBag` is infallible.
+        let bag = getter_addrefs(|p| {
+            unsafe { NS_NewHashPropertyBag(p) };
+            NS_OK
+        }).unwrap();
+        HashPropertyBag(bag)
+    }
+}
+
+impl HashPropertyBag {
+    /// Creates an empty property bag.
+    #[inline]
+    pub fn new() -> HashPropertyBag {
+        HashPropertyBag::default()
+    }
+
+    /// Returns the value for a property name. Fails with `NS_ERROR_FAILURE`
+    /// if the property doesn't exist, or `NS_ERROR_CANNOT_CONVERT_DATA` if the
+    /// property exists, but is not of the value type `V`.
+    pub fn get<K, V>(&self, name: K) -> Result<V, nsresult>
+    where
+        K: AsRef<str>,
+        V: VariantType,
+    {
+        getter_addrefs(|p| unsafe { self.0.GetProperty(&*nsString::from(name.as_ref()), p) })
+            .and_then(|v| V::from_variant(v.coerce()))
+    }
+
+    /// Returns the value for a property name, or the default if not set or
+    /// not of the value type `V`.
+    #[inline]
+    pub fn get_or_default<K, V>(&self, name: K) -> V
+    where
+        K: AsRef<str>,
+        V: VariantType + Default,
+    {
+        self.get(name).unwrap_or_default()
+    }
+
+    /// Sets a property with the name to the value, overwriting any previous
+    /// value.
+    pub fn set<K, V>(&mut self, name: K, value: V)
+    where
+        K: AsRef<str>,
+        V: VariantType,
+    {
+        let v = value.into_variant();
+        unsafe {
+            // This is safe to unwrap because
+            // `nsHashPropertyBagBase::SetProperty` only returns an error if `v`
+            // is a null pointer.
+            self.0
+                .SetProperty(&*nsString::from(name.as_ref()), v.coerce())
+                .to_result()
+                .unwrap()
+        }
+    }
+
+    /// Deletes a property with the name. Returns `true` if the property
+    /// was previously in the bag, `false` if not.
+    pub fn delete(&mut self, name: impl AsRef<str>) -> bool {
+        unsafe {
+            self.0
+                .DeleteProperty(&*nsString::from(name.as_ref()))
+                .to_result()
+                .is_ok()
+        }
+    }
+
+    /// Returns a reference to the backing `nsIWritablePropertyBag`.
+    #[inline]
+    pub fn bag(&self) -> &nsIWritablePropertyBag {
+        &self.0
+    }
+}

storage/variant/src/lib.rs

@@ -7,11 +7,15 @@ extern crate nserror;
 extern crate nsstring;
 extern crate xpcom;
 
+mod bag;
+
 use libc::{c_double, int64_t, uint16_t};
 use nserror::{nsresult, NS_OK};
 use nsstring::{nsACString, nsAString, nsCString, nsString};
 use xpcom::{getter_addrefs, interfaces::nsIVariant, RefPtr};
 
+pub use crate::bag::HashPropertyBag;
+
 extern "C" {
     fn NS_GetDataType(variant: *const nsIVariant) -> uint16_t;
     fn NS_NewStorageNullVariant(result: *mut *const nsIVariant);

xpcom/ds/nsHashPropertyBag.cpp

@@ -14,6 +14,16 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/Move.h"
 
+extern "C" {
+
+// This function uses C linkage because it's exposed to Rust to support the
+// `HashPropertyBag` wrapper in the `storage_variant` crate.
+void NS_NewHashPropertyBag(nsIWritablePropertyBag** aBag) {
+  MakeRefPtr<nsHashPropertyBag>().forget(aBag);
+}
+
+}  // extern "C"
+
 /*
  * nsHashPropertyBagBase implementation.
  */