Add backtracking tests

Verify that transitive constraints are properly popped during a
backtrack

Author: carolynvs-msft

gps/solve_bimodal_test.go

@@ -152,6 +152,58 @@ var bimodalFixtures = map[string]bimodalFixture{
 			"baz 1.0.0",
 		),
 	},
+	"backtrack drops transitive constraint": {
+		ds: []depspec{
+			dsp(mkDepspec("root 1.0.0", "foo ^1.0.0"),
+				pkg("root", "foo"),
+			),
+			dsp(mkDepspec("foo 1.0.1", "bar =1.0.1", "baz =1.0.1"), // This transitive constraint should not be applied in the final solution
+				pkg("foo", "bar"),
+			),
+			dsp(mkDepspec("foo 1.0.0", "bar =1.0.0"),
+				pkg("foo", "bar"),
+			),
+			dsp(mkDepspec("bar 1.0.1", "baz =1.0.0-notexist"), // This should trigger a backtrack
+				pkg("bar", "baz"),
+			),
+			dsp(mkDepspec("bar 1.0.0", "baz =1.0.0"), // This will conflict with the transitive constraint if it's not dropped during backtracking
+				pkg("bar", "baz"),
+			),
+			dsp(mkDepspec("baz 1.0.1"), pkg("baz")),
+			dsp(mkDepspec("baz 1.0.0"), pkg("baz")),
+		},
+		r: mksolution(
+			"foo 1.0.0",
+			"bar 1.0.0",
+			"baz 1.0.0",
+		),
+	},
+	"backtrack adds transitive constraint": {
+		ds: []depspec{
+			dsp(mkDepspec("root 1.0.0", "foo ^1.0.0"),
+				pkg("root", "foo"),
+			),
+			dsp(mkDepspec("foo 1.0.1", "bar =1.0.1"),
+				pkg("foo", "bar"),
+			),
+			dsp(mkDepspec("foo 1.0.0", "bar =1.0.0", "baz =1.0.0"), // This transitive constraint should be applied in the final solution
+				pkg("foo", "bar"),
+			),
+			dsp(mkDepspec("bar 1.0.1", "baz =1.0.0-notexist"), // This should trigger a backtrack
+				pkg("bar", "baz"),
+			),
+			dsp(mkDepspec("bar 1.0.0", "baz ^1.0.0"), // Normally this would pick baz 1.0.1 but the transitive constraint should force an older version
+				pkg("bar", "baz"),
+			),
+			dsp(mkDepspec("baz 1.0.1"), pkg("baz")),
+			dsp(mkDepspec("baz 1.0.0"), pkg("baz")),
+		},
+		r: mksolution(
+			"foo 1.0.0",
+			"bar 1.0.0",
+			"baz 1.0.0",
+		),
+	},
 	// Constraints apply only if the project that declares them has a
 	// reachable import
 	"constraints activated by import": {