Description
Description
The source code of the standard library includes object initialization cycles, including a cycle between Predef and collection.immutable.Vector, which could lead to deadlock when two threads initialize them simultaneously.
Test
deadlock.scala
object Test extends App {
val createPredef = new Runnable {
def run = {
val _ = Predef;
}
}
val createVector = new Runnable {
def run = {
val _ = scala.collection.immutable.Vector;
}
}
val t1 = new Thread(createPredef)
val t2 = new Thread(createVector)
t1.start()
t2.start()
t1.join()
t2.join()
}
Reproduction steps
Scala version: (2.13.15)
git clone https://github.com/scala/scala.git
cd scala
git checkout 2.13.x
sbt publishLocal
scala-cli -S 2.13.15-bin-SNAPSHOT deadlock.scala --main-class Test
Problem
The test deadlocks due to cyclic initialization between Predef and collection.immutable.Vector. Specifically, Predef triggers initialization of Vector through
|├── object Predef extends LowPriorityImplicits { [ Predef.scala:106 ]
|│ ^
|├── scala.`package` // to force scala package object to be seen. [ Predef.scala:151 ]
|│ ^^^^^^^^^^^^^^^
|├── package object scala { [ package.scala:19 ]
|│ ^
|└── val Vector = scala.collection.immutable.Vector [ package.scala:101 ]
|
and the constructor of Vector contains another call to Predef.augmentString, as shown in Vector.tasty
private[this] val defaultApplyPreferredMaxLength: scala.Int = try scala.Predef.augmentString(java.lang.System.getProperty("scala.collection.immutable.Vector.defaultApplyPreferredMaxLength", "250")).toInt catch {
case _: java.lang.SecurityException =>
250
}
The warning is given by the global initialization checker that is under development to fix initialization warning in standard tasty library (scala/scala3#18882), and this issue is linked to a PR with a fix to manually inline the call to Predef.augmentString (scala/scala#10793). Our current conclusion is that such calls must be inlined to ensure soundness, which may happen in released build.
To reproduce the warning using the global initialization checker, apply the following patch to Dotty main branch:
diff --git a/compiler/src/dotty/tools/dotc/transform/init/Objects.scala b/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
index bfa684eef8..3bb7b23095 100644
--- a/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
+++ b/compiler/src/dotty/tools/dotc/transform/init/Objects.scala
@@ -864,8 +864,10 @@ class Objects(using Context @constructorOnly):
Bottom
case Bottom =>
- if field.isStaticObject then ObjectRef(field.moduleClass.asClass)
- else Bottom
+ if field.isStaticObject then
+ accessObject(field.moduleClass.asClass)
+ else
+ Bottom
case ValueSet(values) =>
values.map(ref => select(ref, field, receiver)).join
and in sbt:
sbt> set ThisBuild/Build.scala2Library := Build.Scala2LibraryTasty
sbt> scala3-compiler-bootstrapped/scalac -d out -Ysafe-init-global tests/init-global/pos/deadlock2.scala
The checker will report the following warning:
-- Warning: out/bootstrap/scala2-library-bootstrapped/scala-3.5.1-RC1-bin-SNAPSHOT-nonbootstrapped/src_managed/main/scala-library-src/scala/collection/immutable/Vector.scala:34:7
34 |object Vector extends StrictOptimizedSeqFactory[Vector] {
| ^
|Cyclic initialization: object Vector -> object Predef -> package object scala -> object Vector. Calling trace:
|├── object Predef extends LowPriorityImplicits { [ Predef.scala:106 ]
|│ ^
|├── scala.`package` // to force scala package object to be seen. [ Predef.scala:151 ]
|│ ^^^^^^^^^^^^^^^
|├── object Vector extends StrictOptimizedSeqFactory[Vector] { [ Vector.scala:34 ]
|│ ^
|├── try System.getProperty("scala.collection.immutable.Vector.defaultApplyPreferredMaxLength", [ Vector.scala:84 ]
|│ ^
|├── package object scala { [ package.scala:19 ]
|│ ^
|└── val Vector = scala.collection.immutable.Vector [ package.scala:101 ]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1 warning found