internal/core/adt: ensure shared nodes are evaluated as per request

CL https://cuelang.org/cl/1211563 prevented values
from finalizing when finalization was not requested.
This prevented premature evaluation, but, as it turned
out, it was too aggressive and could lead values to be
less evaluated than was requested by the call to unify.

This ensures evaluation is started for shared values.

Issue #3835

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I59199e268cd2a3dd71d5ca5f915411050c9d3021
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211930
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: mpvl

cue/testdata/eval/sharing.txtar

@@ -412,10 +412,7 @@ Result:
   }
   issue3835: (struct){
     foo: (struct){
-      out: (_|_){
-        // [cycle] issue3835.foo.out: cycle with field: orgBotUser:
-        //     ./issue3835.cue:8:22
-      }
+      out: (string){ "prefix: global value" }
     }
     _global: (string){ "global value" }
   }
@@ -704,19 +701,7 @@ diff old new
        b: (struct){
          c: (int){ 1 }
        }
-@@ -283,7 +237,10 @@
-   }
-   issue3835: (struct){
-     foo: (struct){
--      out: (string){ "prefix: global value" }
-+      out: (_|_){
-+        // [cycle] issue3835.foo.out: cycle with field: orgBotUser:
-+        //     ./issue3835.cue:8:22
-+      }
-     }
-     _global: (string){ "global value" }
-   }
-@@ -312,7 +269,7 @@
+@@ -312,7 +266,7 @@
            // [eval] shareCycle.t2.Y.x: conflicting values int and {x:(#X & Y)} (mismatched types int and struct):
            //     ./sharecycle.cue:8:6
            //     ./sharecycle.cue:9:5

internal/core/adt/unify.go

@@ -348,17 +348,23 @@ func (v *Vertex) unify(c *OpContext, needs condition, mode runMode, checkTypos b
 
 	n.finalizeDisjunctions()
 
-	if n.completed&(subFieldsProcessed) == 0 {
-		return false
-	}
-
 	w = v.DerefValue() // Dereference anything, including shared nodes.
 	if w != v {
 		// Clear value fields that are now referred to in the dereferenced
 		// value (w).
 		v.ChildErrors = nil
 		v.Arcs = nil
 
+		if n.completed&(subFieldsProcessed) == 0 {
+			// Ensure the shared node is processed to the requested level. This is
+			// typically needed for scalar values.
+			if w.status == unprocessed {
+				w.unify(c, needs, mode, false)
+			}
+
+			return n.meets(needs)
+		}
+
 		// Set control fields that are referenced without dereferencing.
 		if w.ClosedRecursive {
 			v.ClosedRecursive = true
@@ -368,6 +374,7 @@ func (v *Vertex) unify(c *OpContext, needs condition, mode runMode, checkTypos b
 		if w.HasEllipsis {
 			v.HasEllipsis = true
 		}
+
 		v.status = w.status
 
 		n.finalizeSharing()
@@ -386,6 +393,10 @@ func (v *Vertex) unify(c *OpContext, needs condition, mode runMode, checkTypos b
 		return true
 	}
 
+	if n.completed&(subFieldsProcessed) == 0 {
+		return n.meets(needs)
+	}
+
 	// TODO: adding this is wrong, but it should not cause the snippet below
 	// to hang. Investigate.
 	// v.Closed = v.cc.isClosed