fix(pipeline): nil-deref in ProcessSingle on multi-Del-per-uid + strip stale READING debug prints

Two bugs surfaced by graphql/e2e/auth's TestOrderAndOffset (cleanup
mutation \`mutation DelTask { deleteTask(filter: {}) }\` crashed the
alpha mid-request, causing the test client to see EOF on POST):

1. posting/index.go ProcessSingle SIGSEGV at line 675.

   GraphQL deleteTask cleanup expands into multiple Del edges per
   entity (one per predicate the entity has — uid, type, list edges,
   etc.). When two Del edges to the same uid land in one transaction's
   batch, the second iteration through ProcessSingle's per-edge loop
   does:

     pl, exists := postings[uid]
     if exists {
         if edge.Op == DEL {
             oldVal = findSingleValueInPostingList(pl)
             if string(edge.Value) == string(oldVal.Value) { ... }
                                              ^^^^^^ nil deref
         }
     }

   findSingleValueInPostingList only returns Set postings; if the
   accumulated list holds only Dels (from the prior iteration), it
   returns nil and we panic dereferencing oldVal.Value.

   Two fixes here:
     - Guard the deref: \`if oldVal != nil && string(...) == ...\`.
     - Move \`var oldVal *pb.Posting\` inside the loop. It was declared
       at function scope, so a stale value from one edge could bleed
       into the nil-guarded branch for a different uid on a later
       iteration. Per-edge scope makes the intent explicit.

2. worker/task.go: two leftover \`fmt.Println("READING SINGLE", ...)\`
   and \`fmt.Println("READING", ...)\` calls in the value-postings
   read path. Same class of debug spew Phase 1B stripped from
   posting/, missed because that sweep didn't include worker/.
   Removed both. Safe — they were unconditional prints on every
   query value read.

The graphql/e2e/auth and graphql/e2e/auth/debug_off packages now both
pass in 30s. \`./posting/\` and \`./worker/\` unit tests still green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: matthewmcneely

posting/index.go

@@ -634,7 +634,6 @@ func (mp *MutationPipeline) ProcessSingle(ctx context.Context, pipeline *Predica
 	dataKey := x.DataKey(pipeline.attr, 0)
 	insertDeleteAllEdge := !(info.index || info.reverse || info.count) // nolint
 
-	var oldVal *pb.Posting
 	for edge := range pipeline.edges {
 		if edge.Op != pb.DirectedEdge_DEL && !schemaExists {
 			return errors.Errorf("runMutation: Unable to find schema for %s", edge.Attr)
@@ -644,6 +643,9 @@ func (mp *MutationPipeline) ProcessSingle(ctx context.Context, pipeline *Predica
 			return err
 		}
 
+		// oldVal is reset per edge so a stale value from a previous iteration
+		// can't bleed into the nil-guarded branch below.
+		var oldVal *pb.Posting
 		uid := edge.Entity
 		pl, exists := postings[uid]
 
@@ -671,8 +673,13 @@ func (mp *MutationPipeline) ProcessSingle(ctx context.Context, pipeline *Predica
 
 		if exists {
 			if edge.Op == pb.DirectedEdge_DEL {
+				// findSingleValueInPostingList returns nil when the
+				// accumulated postings for this uid hold only Del entries
+				// (no Set), which happens when the same uid receives
+				// multiple Del edges in one batch (e.g. GraphQL
+				// deleteTask cleanup deleting many predicates per entity).
 				oldVal = findSingleValueInPostingList(pl)
-				if string(edge.Value) == string(oldVal.Value) {
+				if oldVal != nil && string(edge.Value) == string(oldVal.Value) {
 					setPosting()
 				}
 			} else {

worker/task.go

@@ -487,8 +487,6 @@ func (qs *queryState) handleValuePostings(ctx context.Context, args funcArgs) er
 						Value: p.Value,
 					}
 				}
-				pk, _ := x.Parse(key)
-				fmt.Println("READING SINGLE", pk, vals, pl)
 			} else {
 				pl, err := qs.cache.Get(key)
 				if err != nil {
@@ -508,8 +506,6 @@ func (qs *queryState) handleValuePostings(ctx context.Context, args funcArgs) er
 				}
 
 				vals, fcs, err = retrieveValuesAndFacets(args, pl, facetsTree, listType)
-				pk, _ := x.Parse(key)
-				fmt.Println("READING", pk, vals, fcs, pl.Print())
 
 				switch {
 				case err == posting.ErrNoValue || (err == nil && len(vals) == 0):