Add modify operation to ExtensionFieldValueSet.

During decoding, modifications are frequently made to the extension
values in ExtensionFieldValueSet. In particular for repeated extensions,
these modifications include modifications to copy-on-write data types.
Because the _modify accessor hasn't been standardised yet,
ExtensionFieldValueSet implements get/set only, forcing a modification
of existing data to be done by a get/set pair. This will always emit a
copy-on-write operation.

The result of the above is that decoding repeated extensions is a
quadratic operation, making it diabolically slow. oss-fuzz caught this
behaviour.

This patch adds a non-API modify function to ExtensionFieldValueSet and
uses it on all decoding paths. This code simply encapsulates a somewhat
delicate function invocation that manages to convince the Swift compiler
to use Dictionary's _modify accessor for the underlying operation. The
result is that the oss-fuzz sample runtime decreases from 140s to 5s on
my machine.

This patch will likely have miscellaneous performance improvements
across the board. It adds the oss-fuzz case for regression purposes as
well.

Sources/SwiftProtobuf/BinaryDecoder.swift

@@ -1110,18 +1110,16 @@ internal struct BinaryDecoder: Decoder {
     ) throws {
         assert(!consumed)
         assert(fieldNumber == ext.fieldNumber)
-        var fieldValue = values[fieldNumber]
-        // Message/Group extensions both will call back into the matching
-        // decode methods, so the recursion depth will be tracked there.
-        if fieldValue != nil {
-            try fieldValue!.decodeExtensionField(decoder: &self)
-        } else {
-            fieldValue = try ext._protobuf_newField(decoder: &self)
-        }
-        if consumed {
+
+        try values.modify(index: fieldNumber) { fieldValue in
+            // Message/Group extensions both will call back into the matching
+            // decode methods, so the recursion depth will be tracked there.
             if fieldValue != nil {
-                values[fieldNumber] = fieldValue
+                try fieldValue!.decodeExtensionField(decoder: &self)
             } else {
+                fieldValue = try ext._protobuf_newField(decoder: &self)
+            }
+            if consumed && fieldValue == nil {
                 // Really things should never get here, if the decoder says
                 // the bytes were consumed, then there should have been a
                 // field that consumed them (existing or created). This

Sources/SwiftProtobuf/ExtensionFieldValueSet.swift

@@ -78,6 +78,12 @@ public struct ExtensionFieldValueSet: Hashable {
     set { values[index] = newValue }
   }
 
+  mutating func modify<ReturnType>(index: Int, _ modifier: (inout AnyExtensionField?) throws -> ReturnType) rethrows -> ReturnType {
+    // This internal helper exists to invoke the _modify accessor on Dictionary for the given operation, which can avoid CoWs
+    // during the modification operation.
+    return try modifier(&values[index])
+  }
+
   public var isInitialized: Bool {
     for (_, v) in values {
       if !v.isInitialized {

Sources/SwiftProtobuf/JSONDecoder.swift

@@ -724,18 +724,12 @@ internal struct JSONDecoder: Decoder {
     // Force-unwrap: we can only get here if the extension exists.
     let ext = scanner.extensions[messageType, fieldNumber]!
 
-    var fieldValue = values[fieldNumber]
-    if fieldValue != nil {
-      try fieldValue!.decodeExtensionField(decoder: &self)
-    } else {
-      fieldValue = try ext._protobuf_newField(decoder: &self)
-    }
-    // If the value was `null`, then the 'else' clause will return nil, as there
-    // is nothing to assign. If the api ever supports merging JSON into an
-    // object to update it, then the 'then' clause likely should be update to
-    // support clearing the the value rather that keeping its current value.
-    if fieldValue != nil {
-      values[fieldNumber] = fieldValue
+    try values.modify(index: fieldNumber) { fieldValue in
+      if fieldValue != nil {
+        try fieldValue!.decodeExtensionField(decoder: &self)
+      } else {
+        fieldValue = try ext._protobuf_newField(decoder: &self)
+      }
     }
   }
 }

Sources/SwiftProtobuf/TextFormatDecoder.swift

@@ -708,19 +708,18 @@ internal struct TextFormatDecoder: Decoder {
 
     mutating func decodeExtensionField(values: inout ExtensionFieldValueSet, messageType: Message.Type, fieldNumber: Int) throws {
         if let ext = scanner.extensions?[messageType, fieldNumber] {
-            var fieldValue = values[fieldNumber]
-            if fieldValue != nil {
-                try fieldValue!.decodeExtensionField(decoder: &self)
-            } else {
-                fieldValue = try ext._protobuf_newField(decoder: &self)
-            }
-            if fieldValue != nil {
-                values[fieldNumber] = fieldValue
-            } else {
-                // Really things should never get here, for TextFormat, decoding
-                // the value should always work or throw an error.  This specific
-                // error result is to allow this to be more detectable.
-                throw TextFormatDecodingError.internalExtensionError
+            try values.modify(index: fieldNumber) { fieldValue in
+                if fieldValue != nil {
+                    try fieldValue!.decodeExtensionField(decoder: &self)
+                } else {
+                    fieldValue = try ext._protobuf_newField(decoder: &self)
+                }
+                if fieldValue == nil {
+                    // Really things should never get here, for TextFormat, decoding
+                    // the value should always work or throw an error.  This specific
+                    // error result is to allow this to be more detectable.
+                    throw TextFormatDecodingError.internalExtensionError
+                }
             }
         }
     }