Don't try to pop dep when it's the root

Project-level cycles are fine, but we can't apply the same logic to
the root project as non-root projects when cleaning up the selection
while backtracking. Fixes #176.

solve_bimodal_test.go

@@ -285,6 +285,33 @@ var bimodalFixtures = map[string]bimodalFixture{
 			"a 1.0.0",
 		),
 	},
+	"project cycle involving root with backtracking": {
+		ds: []depspec{
+			dsp(mkDepspec("root 0.0.0", "a ~1.0.0"),
+				pkg("root", "a", "b"),
+				pkg("root/foo"),
+			),
+			dsp(mkDepspec("a 1.0.0"),
+				pkg("a", "root/foo"),
+			),
+			dsp(mkDepspec("a 1.0.1"),
+				pkg("a", "root/foo"),
+			),
+			dsp(mkDepspec("b 1.0.0", "a 1.0.0"),
+				pkg("b", "a"),
+			),
+			dsp(mkDepspec("b 1.0.1", "a 1.0.0"),
+				pkg("b", "a"),
+			),
+			dsp(mkDepspec("b 1.0.2", "a 1.0.0"),
+				pkg("b", "a"),
+			),
+		},
+		r: mksolution(
+			"a 1.0.0",
+			"b 1.0.2",
+		),
+	},
 	"project cycle not involving root": {
 		ds: []depspec{
 			dsp(mkDepspec("root 0.0.0", "a ~1.0.0"),

solver.go

@@ -1152,6 +1152,12 @@ func (s *solver) unselectLast() (atomWithPackages, bool) {
 	}
 
 	for _, dep := range deps {
+		// Skip popping if the dep is the root project, which can occur if
+		// there's a project-level import cycle. (This occurs frequently with
+		// e.g. kubernetes and docker)
+		if s.rd.isRoot(dep.Ident.ProjectRoot) {
+			continue
+		}
 		s.sel.popDep(dep.Ident)
 
 		// if no parents/importers, remove from unselected queue