internal/core/adt: allow selectively turning off sharing

Some bugs only manifest itself when a combination of
sharing and not sharing of nodes occurs.

It is hard to write tests for these bugs, as the sharing
algorithm is continuously evolving: a construct that
disables sharing right now, might not do so in the future.

We add a __no_sharing directive that, when unified
with another value, will guarantee that this value is not
shared.

Issue #3601

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Ib3d2c5783761583072e46107fbd8ba84827e6f70
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205230
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mpvl

cue/testdata/eval/sharing.txtar

@@ -9,17 +9,39 @@ issue3062: ok1: {
 	X: x: A
 	A: "a"
 }
+
+// Test debug facilities to turn of sharing.
+debug: {
+	sharingOn: {
+		a: b
+		b: c: 1
+	}
+	sharingOff: t1: {
+		a: b & __no_sharing
+		b: c: 1
+	}
+	sharingOff: t2: {
+		a: b
+		a: __no_sharing
+		b: c: 1
+	}
+	sharingOff: t3: {
+		a: __no_sharing
+		a: b
+		b: c: 1
+	}
+}
 -- out/eval/stats --
 Leaks:  0
-Freed:  11
-Reused: 4
+Freed:  33
+Reused: 26
 Allocs: 7
-Retain: 3
+Retain: 7
 
-Unifications: 11
-Conjuncts:    18
-Disjuncts:    12
--- out/eval --
+Unifications: 33
+Conjuncts:    49
+Disjuncts:    38
+-- out/evalalpha --
 (struct){
   issue3062: (struct){
     ok1: (struct){
@@ -36,6 +58,186 @@ Disjuncts:    12
       A: (string){ "a" }
     }
   }
+  debug: (struct){
+    sharingOn: (struct){
+      a: ~(debug.sharingOn.b)
+      b: (struct){
+        c: (int){ 1 }
+      }
+    }
+    sharingOff: (struct){
+      t1: (struct){
+        a: (struct){
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+      t2: (struct){
+        a: (struct){
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+      t3: (struct){
+        a: (struct){
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+    }
+  }
+}
+-- diff/-out/evalalpha<==>+out/eval --
+diff old new
+--- old
++++ new
+@@ -1,9 +1,4 @@
+-Errors:
+-no sharing
+-
+-Result:
+-(_|_){
+-  // [eval]
++(struct){
+   issue3062: (struct){
+     ok1: (struct){
+       #S: (string){ "a" }
+@@ -19,42 +14,32 @@
+       A: (string){ "a" }
+     }
+   }
+-  debug: (_|_){
+-    // [eval]
++  debug: (struct){
+     sharingOn: (struct){
+-      a: (struct){
+-        c: (int){ 1 }
+-      }
++      a: ~(debug.sharingOn.b)
+       b: (struct){
+         c: (int){ 1 }
+       }
+     }
+-    sharingOff: (_|_){
+-      // [eval]
+-      t1: (_|_){
+-        // [eval]
+-        a: (_|_){
+-          // [eval] no sharing
+-          c: (int){ 1 }
+-        }
+-        b: (struct){
+-          c: (int){ 1 }
+-        }
+-      }
+-      t2: (_|_){
+-        // [eval]
+-        a: (_|_){
+-          // [eval] no sharing
+-          c: (int){ 1 }
+-        }
+-        b: (struct){
+-          c: (int){ 1 }
+-        }
+-      }
+-      t3: (_|_){
+-        // [eval]
+-        a: (_|_){
+-          // [eval] no sharing
++    sharingOff: (struct){
++      t1: (struct){
++        a: (struct){
++          c: (int){ 1 }
++        }
++        b: (struct){
++          c: (int){ 1 }
++        }
++      }
++      t2: (struct){
++        a: (struct){
++          c: (int){ 1 }
++        }
++        b: (struct){
++          c: (int){ 1 }
++        }
++      }
++      t3: (struct){
++        a: (struct){
+           c: (int){ 1 }
+         }
+         b: (struct){
+-- out/eval --
+Errors:
+no sharing
+
+Result:
+(_|_){
+  // [eval]
+  issue3062: (struct){
+    ok1: (struct){
+      #S: (string){ "a" }
+      #o: (#struct){
+        x: (string){ "a" }
+      }
+      o: (#struct){
+        x: (string){ "a" }
+      }
+      X: (struct){
+        x: (string){ "a" }
+      }
+      A: (string){ "a" }
+    }
+  }
+  debug: (_|_){
+    // [eval]
+    sharingOn: (struct){
+      a: (struct){
+        c: (int){ 1 }
+      }
+      b: (struct){
+        c: (int){ 1 }
+      }
+    }
+    sharingOff: (_|_){
+      // [eval]
+      t1: (_|_){
+        // [eval]
+        a: (_|_){
+          // [eval] no sharing
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+      t2: (_|_){
+        // [eval]
+        a: (_|_){
+          // [eval] no sharing
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+      t3: (_|_){
+        // [eval]
+        a: (_|_){
+          // [eval] no sharing
+          c: (int){ 1 }
+        }
+        b: (struct){
+          c: (int){ 1 }
+        }
+      }
+    }
+  }
 }
 -- out/compile --
 --- in.cue
@@ -54,4 +256,38 @@ Disjuncts:    12
       A: "a"
     }
   }
+  debug: {
+    sharingOn: {
+      a: 〈0;b〉
+      b: {
+        c: 1
+      }
+    }
+    sharingOff: {
+      t1: {
+        a: (〈0;b〉 & _|_(no sharing))
+        b: {
+          c: 1
+        }
+      }
+    }
+    sharingOff: {
+      t2: {
+        a: 〈0;b〉
+        a: _|_(no sharing)
+        b: {
+          c: 1
+        }
+      }
+    }
+    sharingOff: {
+      t3: {
+        a: _|_(no sharing)
+        a: 〈0;b〉
+        b: {
+          c: 1
+        }
+      }
+    }
+  }
 }

internal/core/adt/conjunct.go

@@ -18,6 +18,8 @@ import (
 	"fmt"
 
 	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/cue/token"
 )
 
 // This file contains functionality for processing conjuncts to insert the
@@ -554,6 +556,12 @@ func (n *nodeContext) addNotify2(v *Vertex, c CloseInfo) []receiver {
 
 // Literal conjuncts
 
+// NoSharingSentinel is a sentinel value that is used to disable sharing of
+// nodes. We make this an error to make it clear that we discard the value.
+var NoShareSentinel = &Bottom{
+	Err: errors.Newf(token.NoPos, "no sharing"),
+}
+
 func (n *nodeContext) insertValueConjunct(env *Environment, v Value, id CloseInfo) {
 	n.updateCyclicStatusV3(id)
 
@@ -613,6 +621,10 @@ func (n *nodeContext) insertValueConjunct(env *Environment, v Value, id CloseInf
 		return
 
 	case *Bottom:
+		if x == NoShareSentinel {
+			n.unshare()
+			return
+		}
 		id.cc.hasNonTop = true
 		n.addBottom(x)
 		return

internal/core/compile/predeclared.go

@@ -60,6 +60,9 @@ func predeclared(n *ast.Ident) adt.Expr {
 		return quoBuiltin
 	case "rem", "__rem":
 		return remBuiltin
+
+	case "__no_sharing":
+		return adt.NoShareSentinel
 	}
 
 	if r, ok := predefinedRanges[n.Name]; ok {