Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# Using macOS APIs
+
+With each new macOS release, new APIs are added. Due to the wide range of platforms that Firefox runs on,
+and due to the [wide range of SDKs that we support building with](sdks.md#supported-sdks),
+using macOS APIs in Firefox requires some extra care.
+
+## Availability of APIs, and runtime checks
+
+First of all, if you use an API that is supported by all versions of macOS that Firefox runs on,
+i.e. 10.9 and above, then you don't need to worry about anything:
+The API declaration will be present in any of the supported SDKs, and you don't need any runtime checks.
+
+If you want to use a macOS API that was added after 10.9, then you have to have a runtime check.
+This requirement is completely independent of what SDK is being used for building.
+
+The runtime check [should have the following form](https://developer.apple.com/documentation/macos_release_notes/macos_mojave_10_14_release_notes/appkit_release_notes_for_macos_10_14?language=objc#3014609)
+(replace `10.14` with the appropriate version):
+
+```objc++
+if (@available(macOS 10.14, *)) {
+    // Code for macOS 10.14 or later
+} else {
+    // Code for versions earlier than 10.14.
+}
+```
+
+`@available` guards can be used in Objective-C(++) code.
+(In C++ code, you can use [these `nsCocoaFeatures` methods](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/nsCocoaFeatures.h#21-27) instead.)
+
+For each API, the API declarations in the SDK headers are annotated with `API_AVAILABLE` macros.
+For example, the definition of the `NSVisualEffectMaterial` enum looks like this:
+
+```objc++
+typedef NS_ENUM(NSInteger, NSVisualEffectMaterial) {
+    NSVisualEffectMaterialTitlebar = 3,
+    NSVisualEffectMaterialSelection = 4,
+    NSVisualEffectMaterialMenu API_AVAILABLE(macos(10.11)) = 5,
+   // [...]
+    NSVisualEffectMaterialSheet API_AVAILABLE(macos(10.14)) = 11,
+   // [...]
+} API_AVAILABLE(macos(10.10));
+```
+
+The compiler understands these annotations and makes sure that you wrap all uses of the annotated APIs
+in appropriate `@available` runtime checks.
+
+### Frameworks
+
+In some rare cases, you need functionality from frameworks that are not available on all supported macOS versions.
+Examples of this are `Metal.framework` (added in 10.11) and `MediaPlayer.framework` (added in 10.12.2).
+
+In that case, you can either `dlopen` your framework at runtime ([like we do for MediaPlayer](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/MediaPlayerWrapper.mm#21-27)),
+or you can [use `-weak_framework`](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-107026)
+[like we do for Metal](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/toolkit/library/moz.build#301-304):
+
+```python
+if CONFIG['OS_ARCH'] == 'Darwin':
+    OS_LIBS += [
+        # Link to Metal as required by the Metal gfx-hal backend
+        '-weak_framework Metal',
+    ]
+```
+
+## Using new APIs with old SDKs
+
+If you want to use an API that was introduced after 10.12, you now have one extra thing to worry about.
+In addition to the runtime check [described in the previous section](#using-macos-apis), you also
+have to jump through extra hoops in order to allow the build to succeed, because
+[our build target for Firefox has to remain at 10.12 in order for Firefox to run on macOS versions all the way down to macOS 10.12](sdks.md#supported-sdks).
+
+In order to make the compiler accept your code, you will need to copy some amount of the API declaration
+into your own code. Copy it from the newest recent SDK you can get your hands on.
+The exact procedure varies based on the type of API (enum, objc class, method, etc.),
+but the general approach looks like this:
+
+```objc++
+#if !defined(MAC_OS_VERSION_12_0) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_VERSION_12_0
+@interface NSScreen (NSScreen12_0)
+// https://developer.apple.com/documentation/appkit/nsscreen/3882821-safeareainsets?language=objc&changes=latest_major
+@property(readonly) NSEdgeInsets safeAreaInsets;
+@end
+#endif
+```
+
+See the [Supporting Multiple SDKs](sdks.md#supporting-multiple-sdks) docs for more information on the `MAC_OS_X_VERSION_MAX_ALLOWED` macro.
+
+Keep these three things in mind:
+
+ - Copy only what you need.
+ - Wrap your declaration in `MAC_OS_X_VERSION_MAX_ALLOWED` checks so that, if an SDK is used that
+   already contains these declarations, your declaration does not conflict with the declaration in the SDK.
+ - Include the `API_AVAILABLE` annotations so that the compiler can protect you from accidentally
+   calling the API on unsupported macOS versions.
+
+Our current code does not always follow the `API_AVAILABLE` advice, but it should.
+
+### Enum types and C structs
+
+If you need a new enum type or C struct, copy the entire type declaration and wrap it in the appropriate ifdefs. Example:
+
+```objc++
+#if !defined(MAC_OS_X_VERSION_10_12_2) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_12_2
+typedef NS_ENUM(NSUInteger, MPNowPlayingPlaybackState) {
+    MPNowPlayingPlaybackStateUnknown = 0,
+    MPNowPlayingPlaybackStatePlaying,
+    MPNowPlayingPlaybackStatePaused,
+    MPNowPlayingPlaybackStateStopped,
+    MPNowPlayingPlaybackStateInterrupted
+} MP_API(ios(11.0), tvos(11.0), macos(10.12.2), watchos(5.0));
+#endif
+```
+### New enum values for existing enum type
+
+If the enum type itself already exists, but gained a new value, define the value in an unnamed enum:
+
+```objc++
+#if !defined(MAC_OS_X_VERSION_10_12) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_12
+enum { NSVisualEffectMaterialSelection = 4 };
+#endif
+```
+
+(This is an example of an interesting case: `NSVisualEffectMaterialSelection` is available starting with
+macOS 10.10, but it's only defined in SDKs starting with the 10.12 SDK.)
+
+### Objective-C classes
+
+For a new Objective-C class, copy the entire `@interface` declaration and wrap it in the appropriate ifdefs.
+
+I haven't personally tested this. If this does not compile (or maybe link?), you can use the following workaround:
+
+ - Define your methods and properties as a category on `NSObject`.
+ - Look up the class at runtime using `NSClassFromString()`.
+ - If you need to create a subclass, do it at runtime using `objc_allocateClassPair` and `class_addMethod`.
+   [Here's an example of that.](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/VibrancyManager.mm#44-60)
+
+### Objective-C properties and methods on an existing class
+
+If an Objective-C class that already exists gains a new method or property, you can "add" it to the
+existing class declaration with the help of a [category](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html):
+
+```objc++
+@interface ExistingClass (YourMadeUpCategoryName)
+// methods and properties here
+@end
+```
+
+### Functions
+
+With free-standing functions I'm not entirely sure what to do.
+In theory, copying the declarations from the new SDK headers should work. Example:
+
+```objc++
+extern "C" {
+  __attribute__((warn_unused_result)) bool
+SecTrustEvaluateWithError(SecTrustRef trust, CFErrorRef _Nullable * _Nullable CF_RETURNS_RETAINED error)
+    API_AVAILABLE(macos(10.14), ios(12.0), tvos(12.0), watchos(5.0));
+
+  __nullable
+CFDataRef SecCertificateCopyNormalizedSubjectSequence(SecCertificateRef certificate)
+    __OSX_AVAILABLE_STARTING(__MAC_10_12_4, __IPHONE_10_3);
+}
+```
+
+I'm not sure what the linker or the dynamic linker do when the symbol is not available.
+Does this require [`__attribute__((weak_import))` annotations](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html#//apple_ref/doc/uid/20002378-107262-CJBJAEID)?
+
+And maybe this is where .tbd files in the SDK come in? So that the linker knows which symbols to allow?
+So then that part cannot be worked around by copying code from headers.
+
+Anyway, what always works is the pure runtime approach:
+
+ 1. Define types for the functions you need, but not the functions themselves.
+ 2. At runtime, look up the functions using `dlsym`.
+
+## Notes on Rust
+
+If you call macOS APIs from Rust code, you're kind of on your own. Apple does not provide any Rust
+"headers", so there isn't really an SDK to speak of. So you have to supply your own API declarations
+anyway, regardless of what SDK is being used for building.
+
+In a way, you're side-stepping some of the build time trouble. You don't need to worry about any
+`#ifdefs` because there are no system headers you could conflict with.
+
+On the other hand, you still need to worry about API availability at runtime.
+And in Rust, there are no [availability attributes](https://clang.llvm.org/docs/AttributeReference.html#availability)
+on your API declarations, and there are no
+[`@available` runtime check helpers](https://clang.llvm.org/docs/LanguageExtensions.html#objective-c-available),
+and the compiler cannot warn you if you call APIs outside of availability checks.