Infer dependencies from download strategies and URLs

Closes Homebrew/homebrew#20849.
Closes Homebrew/homebrew#22871.

Library/Homebrew/dependency_collector.rb

@@ -57,6 +57,8 @@ class DependencyCollector
     case spec
     when String
       parse_string_spec(spec, tags)
+    when Resource
+      resource_dep(spec, tags)
     when Symbol
       parse_symbol_spec(spec, tags)
     when Requirement, Dependency
@@ -138,4 +140,35 @@ class DependencyCollector
       Dependency.new(spec.to_s, tags)
     end
   end
+
+  def resource_dep(spec, tags)
+    tags << :build
+    strategy = spec.download_strategy
+
+    case
+    when strategy <= CurlDownloadStrategy
+      parse_url_spec(spec.url, tags)
+    when strategy <= GitDownloadStrategy
+      GitDependency.new(tags)
+    when strategy <= MercurialDownloadStrategy
+      MercurialDependency.new(tags)
+    when strategy <= FossilDownloadStrategy
+      Dependency.new("fossil", tags)
+    when strategy <= BazaarDownloadStrategy
+      Dependency.new("bazaar", tags)
+    when strategy < AbstractDownloadStrategy
+      # allow unknown strategies to pass through
+    else
+      raise TypeError,
+        "#{strategy.inspect} is not an AbstractDownloadStrategy subclass"
+    end
+  end
+
+  def parse_url_spec(url, tags)
+    case File.extname(url)
+    when '.xz'  then Dependency.new('xz', tags)
+    when '.rar' then Dependency.new('unrar', tags)
+    when '.7z'  then Dependency.new('p7zip', tags)
+    end
+  end
 end

Library/Homebrew/download_strategy.rb

@@ -126,17 +126,14 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
       with_system_path { safe_system 'tar', 'xf', tarball_path }
       chdir
     when :xz
-      raise "You must install XZutils: brew install xz" unless File.executable? xzpath
       with_system_path { safe_system "#{xzpath} -dc \"#{tarball_path}\" | tar xf -" }
       chdir
     when :pkg
       safe_system '/usr/sbin/pkgutil', '--expand', tarball_path, basename_without_params
       chdir
     when :rar
-      raise "You must install unrar: brew install unrar" unless which "unrar"
       quiet_safe_system 'unrar', 'x', {:quiet_flag => '-inul'}, tarball_path
     when :p7zip
-      raise "You must install 7zip: brew install p7zip" unless which "7zr"
       safe_system '7zr', 'x', tarball_path
     else
       FileUtils.cp tarball_path, basename_without_params
@@ -416,8 +413,6 @@ class GitDownloadStrategy < AbstractDownloadStrategy
   end
 
   def fetch
-    raise "You must: brew install git" unless which "git"
-
     ohai "Cloning #@url"
 
     if @clone.exist? && repo_valid?
@@ -634,8 +629,6 @@ class MercurialDownloadStrategy < AbstractDownloadStrategy
   end
 
   def fetch
-    raise "You must: brew install mercurial" unless hgpath
-
     ohai "Cloning #{@url}"
 
     unless @clone.exist?
@@ -684,8 +677,6 @@ class BazaarDownloadStrategy < AbstractDownloadStrategy
   end
 
   def fetch
-    raise "You must: brew install bazaar" unless bzrpath
-
     ohai "Cloning #{@url}"
     unless @clone.exist?
       url=@url.sub(%r[^bzr://], '')
@@ -737,8 +728,6 @@ class FossilDownloadStrategy < AbstractDownloadStrategy
   end
 
   def fetch
-    raise "You must: brew install fossil" unless fossilpath
-
     ohai "Cloning #{@url}"
     unless @clone.exist?
       url=@url.sub(%r[^fossil://], '')

Library/Homebrew/requirements.rb

@@ -93,3 +93,9 @@ class MercurialDependency < Requirement
 
   satisfy { which('hg') }
 end
+
+class GitDependency < Requirement
+  fatal true
+  default_formula 'git'
+  satisfy { which('git') }
+end

Library/Homebrew/software_spec.rb

@@ -15,7 +15,7 @@ class SoftwareSpec
   def_delegators :@resource, :stage, :fetch
   def_delegators :@resource, :download_strategy, :verify_download_integrity
   def_delegators :@resource, :checksum, :mirrors, :specs, :using, :downloader
-  def_delegators :@resource, :url, :version, :mirror, *Checksum::TYPES
+  def_delegators :@resource, :version, :mirror, *Checksum::TYPES
 
   def initialize
     @resource = Resource.new
@@ -30,6 +30,12 @@ class SoftwareSpec
     resources.each_value { |r| r.owner = self }
   end
 
+  def url val=nil, specs={}
+    return @resource.url if val.nil?
+    @resource.url(val, specs)
+    dependency_collector.add(@resource)
+  end
+
   def resource? name
     resources.has_key?(name)
   end

Library/Homebrew/test/test_dependency_collector.rb

@@ -135,4 +135,28 @@ class DependencyCollectorTests < Test::Unit::TestCase
     @d.add(spec)
     assert_equal copy, spec
   end
+
+  def test_resource_dep_git_url
+    resource = Resource.new
+    resource.url("git://github.com/foo/bar.git")
+    assert_instance_of GitDependency, @d.add(resource)
+  end
+
+  def test_resource_dep_gzip_url
+    resource = Resource.new
+    resource.url("http://foo.com/bar.tar.gz")
+    assert_nil @d.add(resource)
+  end
+
+  def test_resource_dep_xz_url
+    resource = Resource.new
+    resource.url("http://foo.com/bar.tar.xz")
+    assert_equal Dependency.new("xz", [:build]), @d.add(resource)
+  end
+
+  def test_resource_dep_raises_for_unknown_classes
+    resource = Resource.new
+    resource.url "foo", :using => Class.new
+    assert_raises(TypeError) { @d.add(resource) }
+  end
 end