uninstall: refuse when dependants still installed

Closes Homebrew#934.

Library/Homebrew/cmd/uninstall.rb

@@ -16,6 +16,15 @@ def uninstall
 
     if !ARGV.force?
       ARGV.kegs.each do |keg|
+        dependants = keg.installed_dependants
+        if dependants.any?
+          dependants_output = dependants.map { |k| "#{k.name} #{k.version}" }.join(", ")
+          conjugation = dependants.count == 1 ? "is" : "are"
+          ofail "Refusing to uninstall #{keg} because it is required by #{dependants_output}, which #{conjugation} currently installed."
+          puts "Remove it anyway with `brew uninstall --force #{keg.name}`."
+          next
+        end
+
         keg.lock do
           puts "Uninstalling #{keg}... (#{keg.abv})"
           keg.unlink

Library/Homebrew/formula.rb

@@ -1337,6 +1337,13 @@ def self.each
     end
   end
 
+  # Clear caches of .racks and .installed.
+  # @private
+  def self.clear_cache
+    @racks = nil
+    @installed = nil
+  end
+
   # An array of all racks currently installed.
   # @private
   def self.racks

Library/Homebrew/keg.rb

@@ -292,6 +292,21 @@ def version
     PkgVersion.parse(path.basename.to_s)
   end
 
+  def formula
+    Formulary.from_keg(self)
+  end
+
+  def installed_dependants
+    Formula.installed.flat_map(&:installed_kegs).select do |keg|
+      Tab.for_keg(keg).runtime_dependencies.any? do |dep|
+        # Resolve formula rather than directly comparing names
+        # in case of conflicts between formulae from different taps.
+        dep_formula = Formulary.factory(dep["full_name"])
+        dep_formula == formula && dep["version"] == version.to_s
+      end
+    end
+  end
+
   def find(*args, &block)
     path.find(*args, &block)
   end

Library/Homebrew/test/test_keg.rb

@@ -5,15 +5,22 @@
 class LinkTests < Homebrew::TestCase
   include FileUtils
 
-  def setup
-    keg = HOMEBREW_CELLAR.join("foo", "1.0")
-    keg.join("bin").mkpath
+  def setup_test_keg(name, version)
+    path = HOMEBREW_CELLAR.join(name, version)
+    path.join("bin").mkpath
 
     %w[hiworld helloworld goodbye_cruel_world].each do |file|
-      touch keg.join("bin", file)
+      touch path.join("bin", file)
     end
 
-    @keg = Keg.new(keg)
+    keg = Keg.new(path)
+    @kegs ||= []
+    @kegs << keg
+    keg
+  end
+
+  def setup
+    @keg = setup_test_keg("foo", "1.0")
     @dst = HOMEBREW_PREFIX.join("bin", "helloworld")
     @nonexistent = Pathname.new("/some/nonexistent/path")
 
@@ -27,8 +34,10 @@ def setup
   end
 
   def teardown
-    @keg.unlink
-    @keg.uninstall
+    @kegs.each do |keg|
+      keg.unlink
+      keg.uninstall
+    end
 
     $stdout = @old_stdout
 
@@ -305,3 +314,49 @@ def test_removes_broken_symlinks_that_conflict_with_directories
     keg.uninstall
   end
 end
+
+class InstalledDependantsTests < LinkTests
+  def stub_formula_name(name)
+    stub_formula_loader formula(name) { url "foo-1.0" }
+  end
+
+  def setup_test_keg(name, version)
+    stub_formula_name(name)
+    keg = super
+    Formula.clear_cache
+    keg
+  end
+
+  def setup
+    super
+    @dependant = setup_test_keg("bar", "1.0")
+  end
+
+  def dependencies(deps)
+    tab = Tab.for_keg(@dependant)
+    tab.tabfile = @dependant.join("INSTALL_RECEIPT.json")
+    tab.runtime_dependencies = deps
+    tab.write
+  end
+
+  def test_no_dependencies
+    dependencies []
+    assert_empty @keg.installed_dependants
+  end
+
+  def test_same_name_different_version
+    dependencies [{ "full_name" => "foo", "version" => "1.1" }]
+    assert_empty @keg.installed_dependants
+  end
+
+  def test_different_name_same_version
+    stub_formula_name("baz")
+    dependencies [{ "full_name" => "baz", "version" => @keg.version.to_s }]
+    assert_empty @keg.installed_dependants
+  end
+
+  def test_same_name_and_version
+    dependencies [{ "full_name" => "foo", "version" => "1.0" }]
+    assert_equal [@dependant], @keg.installed_dependants
+  end
+end