Reduce SourceGraph queue synchronization contention. (#307)

Author: ileitch

Sources/PeripheryKit/Indexer/SourceGraph.swift

@@ -127,14 +127,18 @@ public final class SourceGraph {
 
     func add(_ reference: Reference) {
         mutationQueue.sync {
-            _ = allReferences.insert(reference)
+            addUnsafe(reference)
+        }
+    }
 
-            if allReferencesByUsr[reference.usr] == nil {
-                allReferencesByUsr[reference.usr] = []
-            }
+    func addUnsafe(_ reference: Reference) {
+        _ = allReferences.insert(reference)
 
-            allReferencesByUsr[reference.usr]?.insert(reference)
+        if allReferencesByUsr[reference.usr] == nil {
+            allReferencesByUsr[reference.usr] = []
         }
+
+        allReferencesByUsr[reference.usr]?.insert(reference)
     }
 
     func add(_ reference: Reference, from declaration: Declaration) {

Sources/PeripheryKit/Indexer/SwiftIndexer.swift

@@ -622,7 +622,6 @@ public final class SwiftIndexer {
                         reference.name = baseFunc.name
                         reference.isRelated = true
 
-                        graph.add(reference)
                         self.referencedDeclsByUsr[occurrenceUsr, default: []].insert(reference)
                         refs.append(reference)
                     }
@@ -631,7 +630,9 @@ public final class SwiftIndexer {
                 return true
             }
 
-            refs.forEach { graph.add($0) }
+            graph.mutating {
+                refs.forEach { graph.addUnsafe($0) }
+            }
         }
 
         private func parseReference(
@@ -679,7 +680,9 @@ public final class SwiftIndexer {
                 danglingReferences.append(ref)
             }
 
-            refs.forEach { graph.add($0) }
+            graph.mutating {
+                refs.forEach { graph.addUnsafe($0) }
+            }
         }
 
         private func transformLocation(_ input: IndexStoreOccurrence.Location) -> SourceLocation? {