Don't use pngcrush on macOS (bazelbuild#2361)

Xcode uses the `copypng` script to post-process PNGs on most platforms,
but not on macOS. This is enabled incorrectly on macOS, which can make
PNGs appear "corrupt" to some libraries on this platform as the use of
Foundation is not guaranteed. This fixes this by making the behavior
match that of `copypng`, replacing `pngcrush` in favor of a simple copy
(via a symlink action) on macOS.

apple/internal/resource_actions/png.bzl

@@ -29,29 +29,41 @@ def copy_png(*, actions, input_file, output_file, platform_prerequisites):
       platform_prerequisites: Struct containing information on the platform being targeted.
     """
 
-    # Xcode uses `xcrun copypng -strip-PNG-text -compress IN OUT`. But pngcrush
-    # is a perl script that doesn't properly handle when the process dies via a
-    # signal, so instead just expand out the comment to skip the script and
-    # directly run Xcode's copy of pngcrush with the same args.
-    apple_support.run(
-        actions = actions,
-        apple_fragment = platform_prerequisites.apple_fragment,
-        arguments = [
-            "pngcrush",
-            # -compress expands to:
-            "-q",
-            "-iphone",
-            "-f",
-            "0",
-            # "-strip-PNG-text",
-            "-rem",
-            "text",
-            input_file.path,
-            output_file.path,
-        ],
-        executable = "/usr/bin/xcrun",
-        inputs = [input_file],
-        mnemonic = "CopyPng",
-        outputs = [output_file],
-        xcode_config = platform_prerequisites.xcode_version_config,
+    # Xcode does not use `pngcrush` on macOS, but allow override using a feature.
+    should_compress_png = (
+        platform_prerequisites.platform_type != apple_common.platform_type.macos or
+        "apple.macos_compress_png_files" in platform_prerequisites.features
     )
+
+    if should_compress_png:
+        # Xcode uses `xcrun copypng -strip-PNG-text -compress IN OUT`. But pngcrush
+        # is a perl script that doesn't properly handle when the process dies via a
+        # signal, so instead just expand out the comment to skip the script and
+        # directly run Xcode's copy of pngcrush with the same args.
+        apple_support.run(
+            actions = actions,
+            apple_fragment = platform_prerequisites.apple_fragment,
+            arguments = [
+                "pngcrush",
+                # -compress expands to:
+                "-q",
+                "-iphone",
+                "-f",
+                "0",
+                # "-strip-PNG-text",
+                "-rem",
+                "text",
+                input_file.path,
+                output_file.path,
+            ],
+            executable = "/usr/bin/xcrun",
+            inputs = [input_file],
+            mnemonic = "CopyPng",
+            outputs = [output_file],
+            xcode_config = platform_prerequisites.xcode_version_config,
+        )
+    else:
+        actions.symlink(
+            target_file = input_file,
+            output = output_file,
+        )

doc/resources.md

@@ -423,8 +423,9 @@ on your use case for the resources.
 *   `.strings` and `.plist` files. string and generic plist (i.e. non Info.plist
     files) files are processed using plutil to convert them into binary format,
     to reduce their size.
-*   `.png` files: PNG files are processed using copypng to optimize them for iOS
-    devices.
+*   `.png` files: PNG files are processed using `copypng` to optimize them for
+    iOS devices. This is disabled by default on macOS, but can be enabled with
+    the `apple.macos_compress_png_files` feature.
 *   `.xib` files: XIB files are processed using `ibtoold`.
 *   `.xcdatamodel` and `.xcmappingmodel` files: These files are processed with
     `momc` and `mapc` respectively.