keg: installed dependencies of unknown formulae

Previously, trying to resolve the dependencies of a keg would raise an
exception if the formulae for any of the dependencies could not be found
(e.g. if it had been moved to another tap).

This commit updates the dependency finding logic to catch these
exceptions, and fall back to comparing names and taps of formulae, which
should give the correct behaviour.

Fixes #1586.

Library/Homebrew/keg.rb

@@ -139,6 +139,10 @@ class Keg
     raise NotAKegError, "#{path} is not inside a keg"
   end
 
+  def self.all
+    Formula.racks.flat_map(&:subdirs).map { |d| new(d) }
+  end
+
   attr_reader :path, :name, :linked_keg_record, :opt_record
   protected :path
 
@@ -353,14 +357,21 @@ class Keg
   end
 
   def installed_dependents
-    Formula.installed.flat_map(&:installed_kegs).select do |keg|
+    my_tab = Tab.for_keg(self)
+    Keg.all.select do |keg|
       tab = Tab.for_keg(keg)
       next if tab.runtime_dependencies.nil? # no dependency information saved.
       tab.runtime_dependencies.any? do |dep|
         # Resolve formula rather than directly comparing names
         # in case of conflicts between formulae from different taps.
+        begin
           dep_formula = Formulary.factory(dep["full_name"])
-        dep_formula == to_formula && dep["version"] == version.to_s
+          next false unless dep_formula == to_formula
+        rescue FormulaUnavailableError
+          next false unless my_tab["full_name"] = dep["full_name"]
+        end
+
+        dep["version"] == version.to_s
       end
     end
   end

Library/Homebrew/test/keg_test.rb

@@ -47,6 +47,11 @@ class LinkTestCase < Homebrew::TestCase
 end
 
 class LinkTests < LinkTestCase
+  def test_all
+    Formula.clear_racks_cache
+    assert_equal [@keg], Keg.all
+  end
+
   def test_empty_installation
     %w[.DS_Store INSTALL_RECEIPT.json LICENSE.txt].each do |file|
       touch @keg/file
@@ -355,10 +360,11 @@ class InstalledDependantsTests < LinkTestCase
   # from a file path or URL.
   def test_unknown_formula
     Formulary.unstub(:loader_for)
-    dependencies []
+    alter_tab { |t| t.source["tap"] = "some/tap" }
+    dependencies [{ "full_name" => "some/tap/bar", "version" => "1.0" }]
     alter_tab { |t| t.source["path"] = nil }
-    assert_empty @keg.installed_dependents
-    assert_nil Keg.find_some_installed_dependents([@keg])
+    assert_equal [@dependent], @keg.installed_dependents
+    assert_equal [[@keg], ["bar 1.0"]], Keg.find_some_installed_dependents([@keg])
   end
 
   def test_no_dependencies_anywhere