Don't fallback to tempdir when required directories exist.

When home directory is not writable, but the required .gem and .bundle
are, we should use them instead of falling back to use tempdirs.

This creates a workaround for more restrictive setups using Omnibus
Docker or any hardened setup, to overcome the annoyances introduced by rubygems#4951.

lib/bundler.rb

@@ -159,12 +159,13 @@ def ruby_scope
     def user_home
       @user_home ||= begin
         home = Bundler.rubygems.user_home
+        user_dirs = home ? %w[.bundle .gem].map {|path| File.join(home, path) } : []
 
         warning = if home.nil?
           "Your home directory is not set."
         elsif !File.directory?(home)
           "`#{home}` is not a directory."
-        elsif !File.writable?(home)
+        elsif !File.writable?(home) && (user_dirs.any? {|path| !File.directory?(path) || !File.writable?(path) } || user_dirs.empty?)
           "`#{home}` is not writable."
         end
 

spec/bundler/bundler_spec.rb

@@ -199,6 +199,60 @@
         allow(File).to receive(:writable?).with(path).and_return true
         expect(Bundler.user_home).to eq(Pathname(path))
       end
+
+      context "is not a directory" do
+        it "should issue a warning and return a temporary user home" do
+          path = "/home/oggy"
+          allow(Bundler.rubygems).to receive(:user_home).and_return(path)
+          allow(File).to receive(:directory?).with(path).and_return false
+          allow(Etc).to receive(:getlogin).and_return("USER")
+          allow(Dir).to receive(:tmpdir).and_return("/TMP")
+          allow(FileTest).to receive(:exist?).with("/TMP/bundler/home").and_return(true)
+          expect(FileUtils).to receive(:mkpath).with("/TMP/bundler/home/USER")
+          message = <<~EOF
+            `/home/oggy` is not a directory.
+            Bundler will use `/TMP/bundler/home/USER' as your home directory temporarily.
+          EOF
+          expect(Bundler.ui).to receive(:warn).with(message)
+          expect(Bundler.user_home).to eq(Pathname("/TMP/bundler/home/USER"))
+        end
+      end
+
+      context "is not writable" do
+        let(:path) { "/home/oggy" }
+        let(:dotbundle) { "/home/oggy/.bundle" }
+        let(:dotgem) { "/home/oggy/.gem" }
+
+        it "should issue a warning and return a temporary user home" do
+          allow(Bundler.rubygems).to receive(:user_home).and_return(path)
+          allow(File).to receive(:directory?).with(path).and_return true
+          allow(File).to receive(:writable?).with(path).and_return false
+          allow(File).to receive(:directory?).with(dotbundle).and_return false
+          allow(Etc).to receive(:getlogin).and_return("USER")
+          allow(Dir).to receive(:tmpdir).and_return("/TMP")
+          allow(FileTest).to receive(:exist?).with("/TMP/bundler/home").and_return(true)
+          expect(FileUtils).to receive(:mkpath).with("/TMP/bundler/home/USER")
+          message = <<~EOF
+            `/home/oggy` is not writable.
+            Bundler will use `/TMP/bundler/home/USER' as your home directory temporarily.
+          EOF
+          expect(Bundler.ui).to receive(:warn).with(message)
+          expect(Bundler.user_home).to eq(Pathname("/TMP/bundler/home/USER"))
+        end
+
+        context ".gem and .bundle exists and have correct permissions" do
+          it "should return the user home" do
+            allow(Bundler.rubygems).to receive(:user_home).and_return(path)
+            allow(File).to receive(:directory?).with(path).and_return true
+            allow(File).to receive(:writable?).with(path).and_return false
+            allow(File).to receive(:directory?).with(dotbundle).and_return true
+            allow(File).to receive(:writable?).with(dotbundle).and_return true
+            allow(File).to receive(:directory?).with(dotgem).and_return true
+            allow(File).to receive(:writable?).with(dotgem).and_return true
+            expect(Bundler.user_home).to eq(Pathname(path))
+          end
+        end
+      end
     end
 
     context "home directory is not set" do