nix: fix PkgExists panic for some missing packages (#249)

Author: gcurtis

nix/nix.go

@@ -59,9 +59,6 @@ func parseInfo(pkg string, data []byte) *Info {
 	if err != nil {
 		panic(err)
 	}
-	if len(results) != 1 {
-		panic(fmt.Sprintf("unexpected number of results: %d", len(results)))
-	}
 	for _, result := range results {
 		pkgInfo := &Info{
 			NixName: pkg,

nix/nix_test.go

@@ -0,0 +1,14 @@
+package nix
+
+import "testing"
+
+func TestPkgExists(t *testing.T) {
+	// nix-env returns an empty JSON object instead of an error for some
+	// missing packages, which was leading to a panic. "rust" happens to be
+	// one of those packages.
+	pkg := "rust"
+	exists := PkgExists(pkg)
+	if exists {
+		t.Errorf("got PkgExists(%q) = true, want false.", pkg)
+	}
+}