use a local tailwindcss binary by setting TAILWINDCSS_INSTALL_DIR

this will work for both the vanilla "ruby" platform as well as
override the behavior of the native platform gem.

Author: flavorjones

README.md

@@ -12,6 +12,17 @@ With Rails 7 you can generate a new application preconfigured with Tailwind by u
 This gem wraps [the standalone executable version](https://tailwindcss.com/blog/standalone-cli) of the Tailwind CSS v3 framework. These executables are platform specific, so there are actually separate underlying gems per platform, but the correct gem will automatically be picked for your platform. Supported platforms are Linux x64, macOS arm64, macOS x64, and Windows x64. (Note that due to this setup, you must install the actual gems – you can't pin your gem to the github repo.)
 
 
+### Using a local installation of `tailwindcss`
+
+If you are not able to use the vendored upstream executables (for example, if you're on a BSD platform), you can force `tailwindcss-rails` to use a local installation of the `tailwindcss` executable by setting an environment variable named `TAILWINDCSS_INSTALL_DIR` to the directory containing the executable.
+
+For example, if you've installed `tailwindcss` so that the executable is found at `/nix/store/asdfasdf-tailwindcss-3.3.0/bin/tailwindcss`, then you should set your environment variable like so:
+
+``` sh
+TAILWINDCSS_INSTALL_DIR=/nix/store/asdfasdf-tailwindcss-3.3.0/bin
+```
+
+
 ## Developing with Tailwindcss
 
 ### Configuration

lib/tailwindcss/commands.rb

@@ -2,6 +2,8 @@
 
 module Tailwindcss
   module Commands
+    DEFAULT_DIR = File.expand_path(File.join(__dir__, "..", "..", "exe"))
+
     # raised when the host platform is not supported by upstream tailwindcss's binary releases
     class UnsupportedPlatformException < StandardError
     end
@@ -10,26 +12,41 @@ class UnsupportedPlatformException < StandardError
     class ExecutableNotFoundException < StandardError
     end
 
+    # raised when TAILWINDCSS_INSTALL_DIR does not exist
+    class DirectoryNotFoundException < StandardError
+    end
+
     class << self
       def platform
         [:cpu, :os].map { |m| Gem::Platform.local.send(m) }.join("-")
       end
 
-      def executable(
-        exe_path: File.expand_path(File.join(__dir__, "..", "..", "exe"))
-      )
-        if Tailwindcss::Upstream::NATIVE_PLATFORMS.keys.none? { |p| Gem::Platform.match(Gem::Platform.new(p)) }
-          raise UnsupportedPlatformException, <<~MESSAGE
-            tailwindcss-rails does not support the #{platform} platform
-            Please install tailwindcss following instructions at https://tailwindcss.com/docs/installation
-          MESSAGE
-        end
-
-        exe_path = Dir.glob(File.expand_path(File.join(exe_path, "*", "tailwindcss"))).find do |f|
-          Gem::Platform.match(Gem::Platform.new(File.basename(File.dirname(f))))
+      def executable(exe_path: DEFAULT_DIR)
+        tailwindcss_install_dir = ENV["TAILWINDCSS_INSTALL_DIR"]
+        if tailwindcss_install_dir.present?
+          if File.directory?(tailwindcss_install_dir)
+            warn "NOTE: using TAILWINDCSS_INSTALL_DIR to find tailwindcss executable: #{tailwindcss_install_dir}"
+            exe_path = tailwindcss_install_dir
+            exe_file = File.expand_path(File.join(tailwindcss_install_dir, "tailwindcss"))
+          else
+            raise DirectoryNotFoundException, <<~MESSAGE
+              TAILWINDCSS_INSTALL_DIR is set to #{tailwindcss_install_dir}, but that directory does not exist.
+            MESSAGE
+          end
+        else
+          if Tailwindcss::Upstream::NATIVE_PLATFORMS.keys.none? { |p| Gem::Platform.match(Gem::Platform.new(p)) }
+            raise UnsupportedPlatformException, <<~MESSAGE
+              tailwindcss-rails does not support the #{platform} platform
+              Please install tailwindcss following instructions at https://tailwindcss.com/docs/installation
+            MESSAGE
+          end
+
+          exe_file = Dir.glob(File.expand_path(File.join(exe_path, "*", "tailwindcss"))).find do |f|
+            Gem::Platform.match(Gem::Platform.new(File.basename(File.dirname(f))))
+          end
         end
 
-        if exe_path.nil?
+        unless exe_file.present? && File.exist?(exe_file) && File.executable?(exe_file)
           raise ExecutableNotFoundException, <<~MESSAGE
             Cannot find the tailwindcss executable for #{platform} in #{exe_path}
 
@@ -52,7 +69,7 @@ def executable(
           MESSAGE
         end
 
-        exe_path
+        exe_file
       end
 
       def compile_command(debug: false, **kwargs)

test/lib/tailwindcss/commands_test.rb

@@ -2,22 +2,32 @@
 require "minitest/mock"
 
 class Tailwindcss::CommandsTest < ActiveSupport::TestCase
-  test ".platform is a string containing just the cpu and os (not the version)" do
-    expected = "#{Gem::Platform.local.cpu}-#{Gem::Platform.local.os}"
-    assert_equal(expected, Tailwindcss::Commands.platform)
-  end
-
   def mock_exe_directory(platform)
     Dir.mktmpdir do |dir|
       FileUtils.mkdir(File.join(dir, platform))
       path = File.join(dir, platform, "tailwindcss")
       FileUtils.touch(path)
+      FileUtils.chmod("ugo+x", path)
       Gem::Platform.stub(:match, true) do
         yield(dir, path)
       end
     end
   end
 
+  def mock_local_tailwindcss_install
+    Dir.mktmpdir do |dir|
+      path = File.join(dir, "tailwindcss")
+      FileUtils.touch(path)
+      FileUtils.chmod("ugo+x", path)
+      yield(dir, path)
+    end
+  end
+
+  test ".platform is a string containing just the cpu and os (not the version)" do
+    expected = "#{Gem::Platform.local.cpu}-#{Gem::Platform.local.os}"
+    assert_equal(expected, Tailwindcss::Commands.platform)
+  end
+
   test ".executable returns the absolute path to the binary" do
     mock_exe_directory("sparc-solaris2.8") do |dir, executable|
       expected = File.expand_path(File.join(dir, "sparc-solaris2.8", "tailwindcss"))
@@ -42,6 +52,66 @@ def mock_exe_directory(platform)
     end
   end
 
+  test ".executable returns the executable in TAILWINDCSS_INSTALL_DIR when no packaged binary exists" do
+    mock_local_tailwindcss_install do |local_install_dir, expected|
+      result = nil
+      begin
+        ENV["TAILWINDCSS_INSTALL_DIR"] = local_install_dir
+        assert_output(nil, /using TAILWINDCSS_INSTALL_DIR/) do
+          result = Tailwindcss::Commands.executable(exe_path: "/does/not/exist")
+        end
+      ensure
+        ENV["TAILWINDCSS_INSTALL_DIR"] = nil
+      end
+      assert_equal(expected, result)
+    end
+  end
+
+  test ".executable returns the executable in TAILWINDCSS_INSTALL_DIR when we're not on a supported platform" do
+    Gem::Platform.stub(:match, false) do # nothing is supported
+      mock_local_tailwindcss_install do |local_install_dir, expected|
+        result = nil
+        begin
+          ENV["TAILWINDCSS_INSTALL_DIR"] = local_install_dir
+          assert_output(nil, /using TAILWINDCSS_INSTALL_DIR/) do
+            result = Tailwindcss::Commands.executable
+          end
+        ensure
+          ENV["TAILWINDCSS_INSTALL_DIR"] = nil
+        end
+        assert_equal(expected, result)
+      end
+    end
+  end
+
+  test ".executable returns the executable in TAILWINDCSS_INSTALL_DIR even when a packaged binary exists" do
+    mock_exe_directory("sparc-solaris2.8") do |dir, _executable|
+      mock_local_tailwindcss_install do |local_install_dir, expected|
+        result = nil
+        begin
+          ENV["TAILWINDCSS_INSTALL_DIR"] = local_install_dir
+          assert_output(nil, /using TAILWINDCSS_INSTALL_DIR/) do
+            result = Tailwindcss::Commands.executable(exe_path: dir)
+          end
+        ensure
+          ENV["TAILWINDCSS_INSTALL_DIR"] = nil
+        end
+        assert_equal(expected, result)
+      end
+    end
+  end
+
+  test ".executable raises ExecutableNotFoundException is TAILWINDCSS_INSTALL_DIR is set to a nonexistent dir" do
+    begin
+      ENV["TAILWINDCSS_INSTALL_DIR"] = "/does/not/exist"
+      assert_raises(Tailwindcss::Commands::DirectoryNotFoundException) do
+        Tailwindcss::Commands.executable
+      end
+    ensure
+      ENV["TAILWINDCSS_INSTALL_DIR"] = nil
+    end
+  end
+
   test ".compile_command" do
     mock_exe_directory("sparc-solaris2.8") do |dir, executable|
       Rails.stub(:root, File) do # Rails.root won't work in this test suite