Emit all bridge methods non-final (perhaps affecting serialization compat)

[lrytz]

Scala 3 does this too.

This change was at some point in Scala 2 as well, but then reverted (#7980, reverted in #8037).

The revert was for two PRs

• Emit mixin forwarders as bridges to avoid needing generic signatures #7843, which marked mixin forwarders as bridges
• Don't emit mixin forwarders as final #7980, which made all bridges non-final

It seems both changes remained in Scala 3, mixin forwarders are marked synthetic bridge, and bridges are always non-final. @smarter suggested (#8037 (review)) that we could keep all bridges non-final and just revert the first change, which is done here.

This change can affect serialization compatibility between 2.13.8 and 2.13.9 (#9976 (comment)).

Fixes scala/bug#12532

[SethTisue]

Travis-CI build is actually green; not sure why the status didn't update, here.

[dwijnand]

No strong feelings. A fix is a fix. Happy to land this?

[SethTisue]

I see I marked this as "release notes", but I'm not sure what I was thinking at the time. So I'm removing the label again, but @lrytz let me know if you think it's actually note-able.

[lrytz]

The risk is changes to synthetic serialVersionUIDs

[SethTisue]

I'm curious — a little, anyway — why the final or non-final status of a bridge method would affect serialization?

Sounds like the PR description is going to need some user-facing wording on that?

[lrytz]

The computed SerialVersionUID is highly sensitive to changes that are actually safe in terms of serialization compatibility, such as changing the flags of a method.

Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.1).
Type in expressions for evaluation. Or try :help.

scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

class C extends Serializable { def f = 1; val x = 1 }

// Exiting paste mode, now interpreting.


scala> def serialize(obj: AnyRef): Array[Byte] = {
     |   import java.io._
     |   val buffer = new ByteArrayOutputStream
     |   val out = new ObjectOutputStream(buffer)
     |   out.writeObject(obj)
     |   buffer.toByteArray
     | }
def serialize(obj: AnyRef): Array[Byte]

scala> val a = serialize(new C())
val a: Array[Byte] = Array(-84, -19, 0, 5, 115, 114, 0, 1, 67, -76, -97, -7, -88, -63, -61, 55, 29, 2, 0, 1, 73, 0, 1, 120, 120, 112, 0, 0, 0, 1)

scala> def deserialize(a: Array[Byte]): AnyRef = {
     |   import java.io._
     |   val in = new ObjectInputStream(new ByteArrayInputStream(a))
     |   in.readObject
     | }
def deserialize(a: Array[Byte]): AnyRef

scala> deserialize(a)
val res0: AnyRef = C@56ed2baf

Make f final changes the SVUID:

Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.1).
Type in expressions for evaluation. Or try :help.

scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

class C extends Serializable { final def f = 1; val x = 1 }

// Exiting paste mode, now interpreting.


scala> def deserialize(a: Array[Byte]): AnyRef = {
     |   import java.io._
     |   val in = new ObjectInputStream(new ByteArrayInputStream(a))
     |   in.readObject
     | }
def deserialize(a: Array[Byte]): AnyRef

scala> deserialize(Array(-84, -19, 0, 5, 115, 114, 0, 1, 67, -76, -97, -7, -88, -63, -61, 55, 29, 2, 0, 1, 73, 0, 1, 120, 120, 112, 0, 0, 0, 1))
java.io.InvalidClassException: C; local class incompatible: stream classdesc serialVersionUID = -5431348122384910563, local class serialVersionUID = -4765401118249298305
  at java.base/java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:728)
  at java.base/java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:2060)
  at java.base/java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1907)
  at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2209)
  at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1742)
  at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:514)
  at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:472)
  at deserialize(<console>:4)
  ... 32 elided

Deserialization works when the SVUID matches:

Welcome to Scala 2.13.8 (OpenJDK 64-Bit Server VM, Java 17.0.1).
Type in expressions for evaluation. Or try :help.

scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

@SerialVersionUID(-5431348122384910563L) class C extends Serializable { final def f = 1; val x = 1 }

// Exiting paste mode, now interpreting.


scala> def deserialize(a: Array[Byte]): AnyRef = {
     |   import java.io._
     |   val in = new ObjectInputStream(new ByteArrayInputStream(a))
     |   in.readObject
     | }
def deserialize(a: Array[Byte]): AnyRef

scala> deserialize(Array(-84, -19, 0, 5, 115, 114, 0, 1, 67, -76, -97, -7, -88, -63, -61, 55, 29, 2, 0, 1, 73, 0, 1, 120, 120, 112, 0, 0, 0, 1))
val res0: AnyRef = C@44864ebe

scala> res0.asInstanceOf[C].x
val res2: Int = 1

[SethTisue]

@lrytz can you either improve the PR description, or just hit "merge" if you feel that's unnecessary?

[lrytz]

Done 👍