resource: allow on_os blocks

Library/Homebrew/extend/os/linux/resource.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class Resource
+  undef on_linux
+
+  def on_linux(&_block)
+    yield
+  end
+end

Library/Homebrew/extend/os/mac/resource.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class Resource
+  undef on_macos
+
+  def on_macos(&_block)
+    yield
+  end
+end

Library/Homebrew/extend/os/resource.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+if OS.mac?
+  require "extend/os/mac/resource"
+elsif OS.linux?
+  require "extend/os/linux/resource"
+end

Library/Homebrew/formula.rb

@@ -2474,13 +2474,13 @@ class Formula
       specs.each { |spec| spec.uses_from_macos(dep, bounds) }
     end
 
-    # Block executed only executed on macOS. No-op on Linux.
+    # Block only executed on macOS. No-op on Linux.
     # <pre>on_macos do
     #   depends_on "mac_only_dep"
     # end</pre>
     def on_macos(&_block); end
 
-    # Block executed only executed on Linux. No-op on macOS.
+    # Block only executed on Linux. No-op on macOS.
     # <pre>on_linux do
     #   depends_on "linux_only_dep"
     # end</pre>

Library/Homebrew/resource.rb

@@ -178,6 +178,18 @@ class Resource
     patches << p
   end
 
+  # Block only executed on macOS. No-op on Linux.
+  # <pre>on_macos do
+  #   url "mac_only_url"
+  # end</pre>
+  def on_macos(&_block); end
+
+  # Block only executed on Linux. No-op on macOS.
+  # <pre>on_linux do
+  #   url "linux_only_url"
+  # end</pre>
+  def on_linux(&_block); end
+
   protected
 
   def mktemp(prefix)
@@ -252,3 +264,5 @@ class ResourceStageContext
     "<#{self.class}: resource=#{resource} staging=#{staging}>"
   end
 end
+
+require "extend/os/resource"

Library/Homebrew/test/os/linux/formula_spec.rb

@@ -110,4 +110,23 @@ describe Formula do
       expect(f.patchlist.second.url).to eq("patch_linux")
     end
   end
+
+  describe "#on_linux" do
+    it "uses on_linux within a resource block" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        resource "test_resource" do
+          on_linux do
+            url "on_linux"
+          end
+        end
+      end
+      expect(f.resources.length).to eq(1)
+      expect(f.resources.first.url).to eq("on_linux")
+    end
+  end
 end

Library/Homebrew/test/os/mac/formula_spec.rb

@@ -117,4 +117,23 @@ describe Formula do
       expect(f.patchlist.second.url).to eq("patch_macos")
     end
   end
+
+  describe "#on_macos" do
+    it "uses on_macos within a resource block" do
+      f = formula do
+        homepage "https://brew.sh"
+
+        url "https://brew.sh/test-0.1.tbz"
+        sha256 TEST_SHA256
+
+        resource "test_resource" do
+          on_macos do
+            url "resource_macos"
+          end
+        end
+      end
+      expect(f.resources.length).to eq(1)
+      expect(f.resources.first.url).to eq("resource_macos")
+    end
+  end
 end