cmd/cue/cmd: prevent infinite loops while building dependency graph

The existing implementatin resulted in an infinite loop in two cases:

1. In the v.Equals(after) call when the 'after' dependencies were cyclic
2. In the task.Walk function when task fields cyclically referenced
other tasks

This commit solves both cases by
- stepping out of the Walk lambda when it had already been evaluated
with the given parameter; and
- removing the special case for 'after' dependencies as it is no longer
necessary.

Relevent tests are included.

Updates #245

Change-Id: I914ff9aad96eb43ae89964000dea60ccd44c38a2
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4910
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/custom.go

@@ -283,13 +283,23 @@ func executeTasks(cmd *Command, typ, command string, inst *cue.Instance) (err er
 			exitIfErr(cmd, inst, err, true)
 		}
 
+		visited := make(map[string]bool)
 		task.Walk(func(v cue.Value) bool {
 			if v == task {
 				return true
 			}
-			if after.Err() == nil && v.Equals(after) {
-				return false
+
+			// Prevent inifinite walks
+			_, vPath := v.Reference()
+			if vPath != nil {
+				vPath := string(keyForReference(vPath...))
+				_, isVisited := visited[vPath]
+				if isVisited {
+					return false
+				}
+				visited[vPath] = true
 			}
+
 			for _, r := range appendReferences(nil, cr.root, v) {
 				if dep := cr.findTask(r); dep != nil && t != dep {
 					// TODO(string): consider adding dependencies

cmd/cue/cmd/testdata/script/cmd_dep_cycle.txt

@@ -0,0 +1,33 @@
+! cue cmd cycle
+cmp stderr expect-stderr
+! cue cmd aftercycle
+cmp stderr expect-stderr
+cue cmd interlockedTasks
+cmp stdout interlocked-stdout
+
+-- expect-stderr --
+cyclic dependency in tasks
+-- interlocked-stdout --
+v
+v
+-- after_tool.cue --
+package home
+
+import (
+	"tool/cli"
+)
+
+command: interlockedTasks: {
+  t1: cli.Print & { text: ref.value, ref: t2, value: "v" }
+  t2: cli.Print & { text: ref.value, ref: t1, value: "v" }
+}
+
+command: aftercycle: {
+	t1: cli.Print & { $after: t2, text: "t1" }
+	t2: cli.Print & { $after: t1, text: "t2" }
+}
+
+command: cycle: {
+	t1: cli.Print & { text: t2.text }
+	t2: cli.Print & { text: t1.text }
+}