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diff --git a/widget/cocoa/docs/sdks.md b/widget/cocoa/docs/sdks.md new file mode 100644 index 0000000000..a7fefa7e51 --- /dev/null +++ b/widget/cocoa/docs/sdks.md @@ -0,0 +1,227 @@ +# A primer on macOS SDKs + +## Overview + +A macOS SDK is an on-disk directory that contains header files and meta information for macOS APIs. +Apple distributes SDKs as part of the Xcode app bundle. Each Xcode version comes with one macOS SDK, +the SDK for the most recent released version of macOS at the time of the Xcode release. +The SDK is located at `/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk`. + +Compiling Firefox for macOS requires a macOS SDK. The build system bootstraps an adequate SDK by +default, but you can select a different SDK using the `mozconfig` option `--with-macos-sdk`: + +```text +ac_add_options --with-macos-sdk=/Users/username/SDKs/MacOSX11.3.sdk +``` + +## Supported SDKs + +First off, Firefox runs on 10.12 and above. This is called the "minimum deployment target" and is +independent of the SDK version. + +Our official Firefox builds compiled in CI (continuous integration) currently use the 13.3 SDK (last updated in [bug 1833998](https://bugzilla.mozilla.org/show_bug.cgi?id=1833998)). This is also the minimum supported SDK version for local builds. + +Compiling with different SDKs breaks from time to time. Such breakages should be [reported in Bugzilla](https://bugzilla.mozilla.org/enter_bug.cgi?blocked=mach-busted&bug_type=defect&cc=:spohl,:mstange&component=General&form_name=enter_bug&keywords=regression&op_sys=macOS&product=Firefox%20Build%20System&rep_platform=All) and fixed quickly. + +## Obtaining SDKs + +Sometimes you need an SDK that's different from the one in your Xcode.app, for example +to check whether your code change breaks building with other SDKs, or to verify the +runtime behavior with the SDK used for CI builds. + +The easy but slightly questionable way to obtain an SDK is to download it from a public github repo. + +Here's another option: + + 1. Have your Apple ID login details ready, and bring enough time and patience for a 5GB download. + 2. Check [these tables in the Xcode wikipedia article](https://en.wikipedia.org/wiki/Xcode#Xcode_7.0_-_10.x_(since_Free_On-Device_Development)) + and find an Xcode version that contains the SDK you need. + 3. Look up the Xcode version number on [xcodereleases.com](https://xcodereleases.com/) and click the Download link for it. + 4. Log in with your Apple ID. Then the download should start. + 5. Wait for the 5GB Xcode_*.xip download to finish. + 6. Open the downloaded xip file. This will extract the Xcode.app bundle. + 7. Inside the app bundle, the SDK is at `Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk`. + +## Effects of the SDK version + +An SDK only contains declarations of APIs. It does not contain the implementations for these APIs. + +The implementation of an API is provided by the OS that the app runs on. It is supplied at runtime, +when your app starts up, by the dynamic linker. For example, the AppKit implementation comes +from `/System/Library/Frameworks/AppKit.framework` from the OS that the app is run on, regardless +of what SDK was used when compiling the app. + +In other words, building with a macOS SDK of a higher version doesn't magically make new APIs available +when running on older versions of macOS. And, conversely, building with a lower macOS SDK doesn't limit +which APIs you can use if your app is run on a newer version of macOS, assuming you manage to convince the +compiler to accept your code. + +The SDK used for building an app determines three things: + + 1. Whether your code compiles at all, + 2. which range of macOS versions your app can run on (available deployment targets), and + 3. certain aspects of runtime behavior. + +The first is straightforward: An SDK contains header files. If you call an API that's not declared +anywhere - neither in a header file nor in your own code - then your compiler will emit an error. +(Special case: Calling an unknown Objective-C method usually only emits a warning, not an error.) + +The second aspect, available deployment targets, is usually not worth worrying about: +SDKs have large ranges of supported macOS deployment targets. +For example, the 10.15 SDK supports running your app on macOS versions all the way back to 10.6. +This information is written down in the SDK's `SDKSettings.plist`. + +The third aspect, varying runtime behavior, is perhaps the most insidious and surprising aspect, and is described +in the next section. + +## Runtime differences based on macOS SDK version + +When a new version of macOS is released, existing APIs can change their behavior. +These changes are usually described in the AppKit release notes: + + - [macOS 10.15 release notes](https://developer.apple.com/documentation/macos_release_notes/macos_catalina_10_15_release_notes?language=objc) + - [macOS 10.14 AppKit release notes](https://developer.apple.com/documentation/macos_release_notes/macos_mojave_10_14_release_notes/appkit_release_notes_for_macos_10_14?language=objc) + - [macOS 10.13 AppKit release notes](https://developer.apple.com/library/archive/releasenotes/AppKit/RN-AppKit/) + - [macOS 10.12 and older AppKit release notes](https://developer.apple.com/library/archive/releasenotes/AppKit/RN-AppKitOlderNotes/) + +Sometimes, these differences in behavior have the potential to break existing apps. In those instances, +Apple often provides the old (compatible) behavior until the app is re-built with the new SDK, expecting +developers to update their apps so that they work with the new behavior, at the same time as +they update to the new SDK. + +Here's an [example from the 10.13 release notes](https://developer.apple.com/library/archive/releasenotes/AppKit/RN-AppKit/#10_13NSCollectionView%20Responsive%20Scrolling): + +> Responsive Scrolling in NSCollectionViews is enabled only for apps linked on or after macOS 10.13. + +Here, "linked on or after macOS 10.13" means "linked against the macOS 10.13 SDK or newer". + +Apple's expectation is that you upgrade to the new macOS version when it is released, download a new +Xcode version when it is released, synchronize these updates across the machines of all developers +that work on your app, use the SDK in the newest Xcode to compile your app, and make changes to your +app to be compatible with any behavior changes whenever you update Xcode. +This expectation does not always match reality. It definitely doesn't match what we're doing with Firefox. + +For Firefox, SDK-dependent compatibility behaviors mean that developers who build Firefox locally +can see different runtime behavior than the users of our CI builds, if they use a different SDK than +the SDK used in CI. +That is, unless we change the Firefox code so that it has the same behavior regardless of SDK version. +Often this can be achieved by using APIs in a way that's more in line with the API's recommended use. + +For example, we've had cases of +[broken placeholder text in search fields](https://bugzilla.mozilla.org/show_bug.cgi?id=1273106), +[missing](https://bugzilla.mozilla.org/show_bug.cgi?id=941325) or [double-drawn focus rings](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/widget/cocoa/nsNativeThemeCocoa.mm#149-169), +[a startup crash](https://bugzilla.mozilla.org/show_bug.cgi?id=1516437), +[fully black windows](https://bugzilla.mozilla.org/show_bug.cgi?id=1494022), +[fully gray windows](https://bugzilla.mozilla.org/show_bug.cgi?id=1576113#c4), +[broken vibrancy](https://bugzilla.mozilla.org/show_bug.cgi?id=1475694), and +[broken colors in dark mode](https://bugzilla.mozilla.org/show_bug.cgi?id=1578917). + +In most of these cases, the breakage was either very minor, or it was caused by Firefox doing things +that were explicitly discouraged, like creating unexpected NSView hierarchies, or relying on unspecified +implementation details. (With one exception: In 10.14, HiDPI-aware `NSOpenGLContext` rendering in +layer-backed windows simply broke.) + +And in all of these cases, it was the SDK-dependent compatibility behavior that protected our users from being +exposed to the breakage. Our CI builds continued to work because they were built with an older SDK. + +We have addressed all known cases of breakage when building Firefox with newer SDKs. +I am not aware of any current instances of this problem as of this writing (June 2020). + +For more information about how these compatibility tricks work, +read the [Overriding SDK-dependent runtime behavior](#overriding-sdk-dependent-runtime-behavior) section. + +## Supporting multiple SDKs + +As described under [Supported SDKs](#supported-sdks), Firefox can be built with a wide variety of SDK versions. + +This ability comes at the cost of some manual labor; it requires some well-placed `#ifdefs` and +copying of header definitions. + +Every SDK defines the macro `MAC_OS_X_VERSION_MAX_ALLOWED` with a value that matches the SDK version, +in the SDK's `AvailabilityMacros.h` header. This header also defines version constants like `MAC_OS_X_VERSION_10_12`. +For example, I have a version of the 10.12 SDK which contains the line + +```cpp +#define MAC_OS_X_VERSION_MAX_ALLOWED MAC_OS_X_VERSION_10_12_4 +``` + +The name `MAC_OS_X_VERSION_MAX_ALLOWED` is rather misleading; a better name would be +`MAC_OS_X_VERSION_MAX_KNOWN_BY_SDK`. Compiling with an old SDK *does not* prevent apps from running +on newer versions of macOS. + +With the help of the `MAC_OS_X_VERSION_MAX_ALLOWED` macro, we can make our code adapt to the SDK that's +being used. Here's [an example](https://searchfox.org/mozilla-central/rev/9ad88f80aeedcd3cd7d7f63be07f577861727054/toolkit/xre/MacApplicationDelegate.mm#345-351) where the 10.14 SDK changed the signature of +[an `NSApplicationDelegate` method](https://developer.apple.com/documentation/appkit/nsapplicationdelegate/1428471-application?language=objc): + +```objc++ +- (BOOL)application:(NSApplication*)application + continueUserActivity:(NSUserActivity*)userActivity +#if defined(MAC_OS_X_VERSION_10_14) && MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_14 + restorationHandler:(void (^)(NSArray<id<NSUserActivityRestoring>>*))restorationHandler { +#else + restorationHandler:(void (^)(NSArray*))restorationHandler { +#endif + ... +} +``` + +We can also use this macro to supply missing API definitions in such a way that +they don't conflict with the definitions from the SDK. +This is described in the "Using macOS APIs" document, under [Using new APIs with old SDKs](./macos-apis.md#using-new-apis-with-old-sdks). + +## Overriding SDK-dependent runtime behavior + +This section contains some more details on the compatibility tricks that cause different runtime +behavior dependent on the SDK, as described in +[Runtime differences based on macOS SDK version](#runtime-differences-based-on-macos-sdk-version). + +### How it works + +AppKit is the one system framework I know of that employs these tricks. Let's explore how AppKit makes this work, +by going back to the [NSCollectionView example](https://developer.apple.com/library/archive/releasenotes/AppKit/RN-AppKit/#10_13NSCollectionView%20Responsive%20Scrolling) from above: + +> Responsive Scrolling in NSCollectionViews is enabled only for apps linked on or after macOS 10.13. + +For each of these SDK-dependent behavior differences, both the old and the new behavior are implemented +in the version of AppKit that ships with the new macOS version. +At runtime, AppKit selects one of the behaviors based on the SDK version, with a call to +`_CFExecutableLinkedOnOrAfter()`. This call checks the SDK version of the main executable of the +process that's running AppKit code; in our case that's the `firefox` or `plugin-container` executable. +The SDK version is stored in the mach-o headers of the executable by the linker. + +One interesting design aspect of AppKit's compatibility tricks is the fact that most of these behavior differences +can be toggled with a "user default" preference. +For example, the "responsive scrolling in NSCollectionViews" behavior change can be controlled with +a user default with the name "NSCollectionViewPrefetchingEnabled". +The SDK check only happens if "NSCollectionViewPrefetchingEnabled" is not set to either YES or NO. + +More precisely, this example works as follows: + + - `-[NSCollectionView prepareContentInRect:]` is the function that supports both the old and the new behavior. + - It calls `_NSGetBoolAppConfig` for the value "NSCollectionViewPrefetchingEnabled", and also supplies a "default + value function". + - If the user default is not set, the default value function is called. This function has the name + `NSCollectionViewPrefetchingEnabledDefaultValueFunction`. + - `NSCollectionViewPrefetchingEnabledDefaultValueFunction` calls `_CFExecutableLinkedOnOrAfter(13)`. + +You can find many similar toggles if you list the AppKit symbols that end in `DefaultValueFunction`, +for example by executing `nm /System/Library/Frameworks/AppKit.framework/AppKit | grep DefaultValueFunction`. + +### Overriding SDK-dependent runtime behavior + +You can set these preferences programmatically, in a way that `_NSGetBoolAppConfig()` can pick them up, +for example with [`registerDefaults`](https://developer.apple.com/documentation/foundation/nsuserdefaults/1417065-registerdefaults?language=objc) +or like this: + +```objc++ +[[NSUserDefaults standardUserDefaults] setBool:YES forKey:@"NSViewAllowsRootLayerBacking"]; +``` + +The AppKit release notes mention this ability but ask for it to only be used for debugging purposes: + +> In some cases, we provide defaults (preferences) settings which can be used to get the old or new behavior, +> independent of what system an application was built against. Often these preferences are provided for +> debugging purposes only; in some cases the preferences can be used to globally modify the behavior +> of an application by registering the values (do it somewhere very early, with `-[NSUserDefaults registerDefaults:]`). + +It's interesting that they mention this at all because, as far as I can tell, none of these values are documented. |