Implement Ruby gem building with Bazel

This adds 2 new targets to Ruby Bazel build. Each target copies all
necessary files for gem it needs to build and then packages into a gem
file that is stored in Bazel output directory. For now there is no
way to publish gem to RubyGems, but this should be easy to implement.
Please see the following table to understand what commands to use and
what gem files each command generates.

| command                             | output                              |
| bazel build //rb:selenium-devtools  | bazel-bin/rb/selenium-devtools.gem  |
| bazel build //rb:selenium-webdriver | bazel-bin/rb/selenium-webdriver.gem |

Author: p0deje

rb/BUILD.bazel

@@ -1,4 +1,5 @@
 load("//common:defs.bzl", "copy_file")
+load("//rb:defs.bzl", "rb_gem")
 load(
     "@coinbase_rules_ruby//ruby:defs.bzl",
     "rb_binary",
@@ -46,6 +47,36 @@ copy_file(
     out = "selenium_server_deploy.jar",
 )
 
+genrule(
+    name = "license",
+    srcs = ["//:license"],
+    outs = [
+        "LICENSE",
+        "NOTICE",
+    ],
+    cmd = "cp $(locations //:license) $(@D)",
+)
+
+rb_gem(
+    name = "selenium-webdriver",
+    gemspec = "selenium-webdriver.gemspec",
+    deps = [
+        ":bidi",
+        ":chrome",
+        ":common",
+        ":edge",
+        ":firefox",
+        ":ie",
+        ":safari",
+    ],
+)
+
+rb_gem(
+    name = "selenium-devtools",
+    gemspec = "selenium-devtools.gemspec",
+    deps = [":devtools"],
+)
+
 rb_library(
     name = "bidi",
     srcs = glob([
@@ -88,7 +119,7 @@ rb_library(
         ":find-elements",
         ":get-attribute",
         ":is-displayed",
-        "//:license",
+        ":license",
     ],
     deps = [
         "@bundle//:childprocess",
@@ -146,17 +177,13 @@ rb_library(
 rb_library(
     name = "devtools",
     srcs = glob([
-        "selenium-devtools.gemspec",
+        "Gemfile",
         "lib/selenium/devtools.rb",
         "lib/selenium/devtools/version.rb",
-        "Gemfile",
-        "CHANGES",
-        "README.md",
+        "selenium-devtools.gemspec",
     ]),
-    data = ["//:license"] + [":cdp-" + n for n in CDP_VERSIONS],
-    deps = [
-        "@bundle//:websocket",
-    ],
+    data = [":license"] + [":cdp-" + n for n in CDP_VERSIONS],
+    deps = ["@bundle//:websocket"],
 )
 
 [genrule(
@@ -170,9 +197,7 @@ rb_library(
         "lib/selenium/devtools/" + n + ".rb",
     ],
     cmd = "ruby $(location :cdp-generate) $(location //common/devtools/chromium/" + n + ":browser_protocol) $(location //common/devtools/chromium/" + n + ":js_protocol) $(OUTS) " + n,
-    tools = [
-        ":cdp-generate",
-    ],
+    tools = [":cdp-generate"],
 ) for n in CDP_VERSIONS]
 
 rb_binary(
@@ -542,9 +567,7 @@ rb_binary(
         "rb/README.md",
     ],
     main = "@bundle//:bin/yard",
-    deps = [
-        "@bundle//:yard",
-    ],
+    deps = ["@bundle//:yard"],
 )
 
 rb_binary(

rb/NOTICE

@@ -0,0 +1,3 @@
+load("//rb/private:rb_gem.bzl", _rb_gem = "rb_gem")
+
+rb_gem = _rb_gem

rb/defs.bzl

@@ -0,0 +1,6 @@
+package(default_visibility = ["//rb/private:__pkg__"])
+
+exports_files(
+    ["gem_builder.tpl"],
+    visibility = ["//visibility:public"],
+)

rb/private/BUILD.bazel

@@ -0,0 +1,34 @@
+#!/usr/bin/env ruby
+
+require 'fileutils'
+require 'rubygems/package'
+require 'tmpdir'
+
+gemspec = File.read('{gemspec}')
+gem_file = File.expand_path('{gem_filename}', '{bazel_out_dir}')
+
+def copy_srcs(from:, to:)
+  Dir.chdir(from) do
+    Dir.glob('**/*').each do |src|
+      dst = File.join(to, src)
+      if File.directory?(src)
+        FileUtils.mkdir_p(dst)
+      else
+        FileUtils.cp(src, dst)
+      end
+    end
+  end
+end
+
+Dir.mktmpdir do |tmpdir|
+  # First copy file that live in gem directory.
+  copy_srcs(from: File.dirname('{gemspec}'), to: tmpdir)
+  # Then copy all the files that generated by target dependencies.
+  copy_srcs(from: '{bazel_out_dir}', to: tmpdir)
+
+  Dir.chdir(tmpdir) do
+    spec = eval(gemspec)
+    file = Gem::Package.build(spec)
+    FileUtils.mv(file, gem_file)
+  end
+end

rb/private/gem_builder.tpl

@@ -0,0 +1,53 @@
+def _rb_gem_impl(ctx):
+    gem_builder = ctx.actions.declare_file("gem_builder.rb")
+
+    inputs = []
+    for dep in ctx.attr.deps:
+        inputs.extend(dep.files.to_list())
+        inputs.extend(dep[DefaultInfo].data_runfiles.files.to_list())
+
+    ctx.actions.expand_template(
+        template = ctx.file._gem_builder_template,
+        output = gem_builder,
+        substitutions = {
+            "{bazel_out_dir}": ctx.outputs.gem.dirname,
+            "{gem_filename}": ctx.label.name + ".gem",
+            "{gemspec}": ctx.file.gemspec.path,
+        },
+    )
+
+    ctx.actions.run(
+        inputs = inputs,
+        executable = gem_builder,
+        outputs = [ctx.outputs.gem],
+        execution_requirements = {
+            "no-sandbox": "1", # allow to traverse directory symlinks
+        },
+    )
+
+rb_gem = rule(
+    _rb_gem_impl,
+    attrs = {
+        "srcs": attr.label_list(
+            allow_files = True,
+        ),
+        "gemspec": attr.label(
+            allow_single_file = True,
+            mandatory = True,
+        ),
+        "deps": attr.label_list(
+            allow_files = True,
+        ),
+        "data": attr.label_list(
+            allow_empty = True,
+            allow_files = True,
+        ),
+        "_gem_builder_template": attr.label(
+            allow_single_file = True,
+            default = Label("//rb/private:gem_builder.tpl"),
+        ),
+    },
+    outputs = {
+        "gem": "%{name}.gem",
+    },
+)