add ImmutableMap to prevent BM persist where unsupported

hail/src/main/scala/is/hail/backend/local/LocalBackend.scala

@@ -99,7 +99,7 @@ class LocalBackend(
  ExecutionCache.fromFlags(flags, fs, tmpdir)
  },
  new IrMetadata(),
- mutable.Map.empty,
+ ImmutableMap.empty,
  )(f)
  }
 

hail/src/main/scala/is/hail/backend/service/ServiceBackend.scala

@@ -388,7 +388,7 @@ class ServiceBackend(
  flags,
  serviceBackendContext,
  new IrMetadata(),
- mutable.Map.empty,
+ ImmutableMap.empty,
  )(f)
  }
 

hail/src/main/scala/is/hail/backend/spark/SparkBackend.scala

@@ -362,7 +362,7 @@ class SparkBackend(
  ExecutionCache.forTesting
  },
  new IrMetadata(),
- null,
+ ImmutableMap.empty,
  )
 
  override def withExecuteContext[T](f: ExecuteContext => T)(implicit E: Enclosing): T =

hail/src/main/scala/is/hail/expr/ir/BlockMatrixIR.scala

@@ -1,4 +1,5 @@
 package is.hail.expr.ir
+
 import is.hail.annotations.NDArray
 import is.hail.backend.ExecuteContext
 import is.hail.expr.Nat

hail/src/main/scala/is/hail/expr/ir/BlockMatrixWriter.scala

@@ -1,4 +1,5 @@
 package is.hail.expr.ir
+
 import is.hail.annotations.Region
 import is.hail.asm4s._
 import is.hail.backend.ExecuteContext

hail/src/main/scala/is/hail/utils/ImmutableMap.scala

@@ -0,0 +1,25 @@
+package is.hail.utils
+
+import scala.collection.mutable
+
+case class ImmutableMap[K, V](m: Map[K, V]) extends mutable.AbstractMap[K, V] {
+ override def +=(kv: (K, V)): ImmutableMap.this.type =
+ throw new UnsupportedOperationException()
+
+ override def -=(key: K): ImmutableMap.this.type =
+ throw new UnsupportedOperationException()
+
+ override def get(key: K): Option[V] =
+ m.get(key)
+
+ override def iterator: Iterator[(K, V)] =
+ m.iterator
+
+ override def toMap[T, U](implicit ev: (K, V) <:< (T, U)): Map[T, U] =
+ m.toMap
+}
+
+object ImmutableMap {
+ def empty[K, V]: ImmutableMap[K, V] =
+ ImmutableMap(Map.empty)
+}

hail/src/main/scala/is/hail/utils/richUtils/RichMap.scala

@@ -1,5 +1,9 @@
 package is.hail.utils.richUtils
 
+import is.hail.utils.ImmutableMap
+
+import scala.collection.mutable
+
 class RichMap[K, V](val m: Map[K, V]) extends AnyVal {
  def force =
  m.map(identity) // needed to make serializable: https://issues.scala-lang.org/browse/SI-7005
@@ -9,4 +13,7 @@ class RichMap[K, V](val m: Map[K, V]) extends AnyVal {
 
  def isTrivial(implicit eq: K =:= V): Boolean =
  m.forall { case (k, v) => k == v }
+
+ def immutableMutableMap: mutable.Map[K, V] =
+ ImmutableMap(m)
 }