core: skip resource validation when count is unknown

The approach here is a little hacky, since this edge case applies only to
validate and all of the other evaluateResourceCountExpression callers
don't care about it: we overload the "count" return value as a flag to
allow NodeValidatableResource to allow it to detect this situation and
silently ignore errors in this case.

terraform/eval_count.go

@@ -22,8 +22,13 @@ import (
 // If the result is -1, this indicates that the given expression is nil and so
 // the "count" behavior should not be enabled for this resource at all.
 //
-// If error diagnostics are returned then the result is undefined and must
-// not be used.
+// If error diagnostics are returned then the result is always the meaningless
+// placeholder value -1, except in one case: if the count expression evaluates
+// to an unknown number value then the result is zero, allowing this situation
+// to be treated by the caller as special if needed. For example, an early
+// graph walk may wish to just silently skip resources with unknown counts
+// to allow them to be dealt with in a later graph walk where more information
+// is available.
 func evaluateResourceCountExpression(expr hcl.Expression, ctx EvalContext) (int, tfdiags.Diagnostics) {
 	if expr == nil {
 		return -1, nil
@@ -60,7 +65,11 @@ func evaluateResourceCountExpression(expr hcl.Expression, ctx EvalContext) (int,
 			Detail:   `The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to first apply only the resources that the count depends on.`,
 			Subject:  expr.Range().Ptr(),
 		})
-		return -1, diags
+		// We return zero+errors in this one case to allow callers to handle
+		// an unknown count as special. This is rarely necessary, but is used
+		// by the validate walk in particular so that it can just skip
+		// validation in this case, assuming a later walk will take care of it.
+		return 0, diags
 	}
 
 	err := gocty.FromCtyValue(countVal, &count)

terraform/node_resource_validate.go

@@ -31,10 +31,19 @@ func (n *NodeValidatableResource) DynamicExpand(ctx EvalContext) (*Graph, error)
 	count, countDiags := evaluateResourceCountExpression(n.Config.Count, ctx)
 	diags = diags.Append(countDiags)
 	if countDiags.HasErrors() {
-		log.Printf("[TRACE] %T %s: count expression has errors", n, n.Name())
-		return nil, diags.Err()
-	}
-	if count >= 0 {
+		if count != 0 {
+			log.Printf("[TRACE] %T %s: count expression has errors", n, n.Name())
+			return nil, diags.Err()
+		}
+
+		// evaluateResourceCountExpression returns zero+errors only in the
+		// case where the count value successfully evaluated to an unknown
+		// number. We don't treat this as an error here because counts that
+		// are computed during validate can become known during the plan
+		// walk, if they refer to data resources, and so we'll just defer
+		// our validation steps to the plan phase in that case.
+		log.Printf("[TRACE] %T %s: count expression value not yet known, so deferring validation until the plan walk", n, n.Name())
+	} else if count >= 0 {
 		log.Printf("[TRACE] %T %s: count expression evaluates to %d", n, n.Name(), count)
 	} else {
 		log.Printf("[TRACE] %T %s: no count argument present", n, n.Name())