Allow backjumping over pkg-only selections

There had been a canary panic in this case, but that was just incorrect.
It's perfectly legitimate for a project marked for retrying to have
induced some pkg-only selections.

Fixes #408.

Author: sdboyer

gps/solve_bimodal_test.go

@@ -232,6 +232,49 @@ var bimodalFixtures = map[string]bimodalFixture{
 			"c 1.0.0",
 		),
 	},
+	"backjump through pkg-only selection": {
+		ds: []depspec{
+			dsp(mkDepspec("root 0.0.0"),
+				pkg("root", "root/foo"),
+				pkg("root/foo", "a", "b"),
+			),
+			dsp(mkDepspec("a 1.0.0"),
+				pkg("a", "c"),
+			),
+			// Include two versions of b to ensure that a is visited first
+			dsp(mkDepspec("b 0.9.0", "d ^1.0.0"),
+				pkg("b", "c/other", "d"),
+			),
+			dsp(mkDepspec("b 1.0.0", "d ^1.2.0"),
+				pkg("b", "c/other", "d"),
+			),
+			// Three versions of c so it's last
+			dsp(mkDepspec("c 1.0.0", "d ^1.0.0"),
+				pkg("c", "d"),
+				pkg("c/other"),
+			),
+			//dsp(mkDepspec("c 1.0.1", "d 1.0.0"),
+			//pkg("c", "d"),
+			//pkg("c/other"),
+			//),
+			//dsp(mkDepspec("c 1.0.2", "d 1.0.0"),
+			//pkg("c", "d"),
+			//pkg("c/other"),
+			//),
+			dsp(mkDepspec("d 1.0.0"),
+				pkg("d"),
+			),
+			dsp(mkDepspec("d 1.1.0"),
+				pkg("d"),
+			),
+		},
+		r: mksolution(
+			"a 1.0.0",
+			"b 0.9.0",
+			mklp("c 1.0.0", ".", "other"),
+			"d 1.1.0",
+		),
+	},
 	// Import jump is in a dep subpkg, and points to a transitive dep
 	"transitive subpkg bm-add": {
 		ds: []depspec{

gps/solver.go

@@ -951,10 +951,13 @@ func (s *solver) backtrack() bool {
 		// Grab the last versionQueue off the list of queues
 		q := s.vqs[len(s.vqs)-1]
 
-		// Walk back to the next project
-		awp, proj := s.unselectLast()
-		if !proj {
-			panic("canary - *should* be impossible to have a pkg-only selection here")
+		// Walk back to the next project. This may entail walking through some
+		// package-only selections.
+		var proj bool
+		var awp atomWithPackages
+		for !proj {
+			awp, proj = s.unselectLast()
+			s.traceBacktrack(awp.bmi(), !proj)
 		}
 
 		if !q.id.eq(awp.a.id) {