diff --git a/compiler/src/dotty/tools/dotc/profile/FileUtils.scala b/compiler/src/dotty/tools/dotc/profile/FileUtils.scala
index 97c2417d236b..4aec428c05bf 100644
--- a/compiler/src/dotty/tools/dotc/profile/FileUtils.scala
+++ b/compiler/src/dotty/tools/dotc/profile/FileUtils.scala
@@ -27,6 +27,7 @@ import java.util.concurrent.atomic.AtomicBoolean
 import scala.concurrent.duration.Duration
 import scala.concurrent.{Await, Promise}
 import scala.util.{Failure, Success}
+import scala.annotation.internal.sharable
 
 object FileUtils {
   def newAsyncBufferedWriter(path: Path, charset: Charset = StandardCharsets.UTF_8.nn, options: Array[OpenOption] = NO_OPTIONS, threadsafe: Boolean = false): LineWriter = {
@@ -72,8 +73,8 @@ object FileUtils {
   }
 
   private object AsyncBufferedWriter {
-    private val Close = CharBuffer.allocate(0)
-    private val Flush = CharBuffer.allocate(0)
+    @sharable private val Close = CharBuffer.allocate(0)
+    @sharable private val Flush = CharBuffer.allocate(0)
   }
   private class AsyncBufferedWriter(val underlying: Writer, bufferSize : Int = 4096) extends LineWriter {
     private var current: CharBuffer = allocate
diff --git a/compiler/test/dotty/tools/dotc/profile/ChromeTraceTest.scala b/compiler/test/dotty/tools/dotc/profile/ChromeTraceTest.scala
index 143c6b37dad5..1cd3bc498f38 100644
--- a/compiler/test/dotty/tools/dotc/profile/ChromeTraceTest.scala
+++ b/compiler/test/dotty/tools/dotc/profile/ChromeTraceTest.scala
@@ -10,41 +10,41 @@ import java.util.concurrent.locks.LockSupport
 import scala.concurrent.duration.*
 
 class ChromeTraceTest:
-  private def testTraceOutputs(generator: ChromeTrace => Unit)(checkContent: String => Unit): Unit = {
+  private def testTraceOutputs(generator: ChromeTrace => Unit)(checkContent: PartialFunction[List[String], Unit]): Unit = {
     val outfile = Files.createTempFile("trace-", ".json").nn
     val tracer = new ChromeTrace(outfile)
     try generator(tracer)
     finally tracer.close()
-    val content = scala.io.Source.fromFile(outfile.toFile().nn).mkString
-    checkContent(content)
+    val contentLines = scala.io.Source.fromFile(outfile.toFile().nn).getLines().toList
+    checkContent.applyOrElse(contentLines, content => fail(s"Invalid output lines: ${content.mkString(System.lineSeparator())}"))
   }
 
   @Test def traceCounterEvent(): Unit = testTraceOutputs{ tracer =>
     tracer.traceCounterEvent("foo", "counter1", 42, processWide = true)
     tracer.traceCounterEvent("bar", "counter2", 21, processWide = false)
   }{
-    case s"""{"traceEvents":[
-{"cat":"scalac","name":"foo","ph":"C","tid":"${tid1}","pid":"${pid1}","ts":${ts1},"args":{"counter1":42}}
-,{"cat":"scalac","name":"bar","ph":"C","tid":"${tid2}","pid":"${pid2}","ts":${ts2},"args":{"counter2":21}}
-]}""" =>
-    assertEquals(tid1, tid2)
-    assertTrue(tid1.toIntOption.isDefined)
-    assertNotEquals(pid1, pid2)
-    assertTrue(pid1.toIntOption.isDefined)
-    assertEquals(s"$pid1-$tid1", pid2)
-    assertTrue(ts1.toLong < ts2.toLong)
+    case """{"traceEvents":[""" ::
+        s"""{"cat":"scalac","name":"foo","ph":"C","tid":"${tid1}","pid":"${pid1}","ts":${ts1},"args":{"counter1":42}}""" ::
+        s""",{"cat":"scalac","name":"bar","ph":"C","tid":"${tid2}","pid":"${pid2}","ts":${ts2},"args":{"counter2":21}}""" ::
+           "]}" :: Nil =>
+      assertEquals(tid1, tid2)
+      assertTrue(tid1.toIntOption.isDefined)
+      assertNotEquals(pid1, pid2)
+      assertTrue(pid1.toIntOption.isDefined)
+      assertEquals(s"$pid1-$tid1", pid2)
+      assertTrue(ts1.toLong < ts2.toLong)
   }
 
   @Test def traceDurationEvent(): Unit = testTraceOutputs{ tracer =>
     tracer.traceDurationEvent(name = "name1", startNanos = 1000L, durationNanos = 2500L,  tid = "this-thread")
     tracer.traceDurationEvent(name = "name2", startNanos = 1000L, durationNanos = 5000L,  tid = "this-thread", pidSuffix = "pidSuffix")
   }{
-    case s"""{"traceEvents":[
-{"cat":"scalac","name":"name1","ph":"X","tid":"this-thread","pid":"${pid1}","ts":1,"dur":2}
-,{"cat":"scalac","name":"name2","ph":"X","tid":"this-thread","pid":"${pid2}","ts":1,"dur":5}
-]}""" =>
-    assertTrue(pid1.toIntOption.isDefined)
-    assertEquals(s"$pid1-pidSuffix", pid2)
+    case """{"traceEvents":[""" ::
+        s"""{"cat":"scalac","name":"name1","ph":"X","tid":"this-thread","pid":"${pid1}","ts":1,"dur":2}""" ::
+        s""",{"cat":"scalac","name":"name2","ph":"X","tid":"this-thread","pid":"${pid2}","ts":1,"dur":5}""" ::
+           "]}" :: Nil =>
+      assertTrue(pid1.toIntOption.isDefined)
+      assertEquals(s"$pid1-pidSuffix", pid2)
   }
 
   @Test def traceDurationEvents(): Unit = {
@@ -58,33 +58,33 @@ class ChromeTraceTest:
     LockSupport.parkNanos(8.millis.toNanos)
     tracer.traceDurationEventEnd(cat = "test1", name = "event1", colour = "RED", pidSuffix = "pid-suffix")
   }{
-    case s"""{"traceEvents":[
-{"cat":"test1","name":"event1","ph":"B","pid":"${pid1}","tid":"${tid1}","ts":${ts1}}
-,{"cat":"test2","name":"event2","ph":"B","pid":"${pid2}","tid":"${tid2}","ts":${ts2},"cname":"RED"}
-,{"cat":"test2","name":"event2","ph":"E","pid":"${pid3}","tid":"${tid3}","ts":${ts3}}
-,{"cat":"test1","name":"event1","ph":"E","pid":"${pid4}","tid":"${tid4}","ts":${ts4},"cname":"RED"}
-]}""" =>
-    val traceEnd = System.nanoTime()
-    assertTrue(tid1.toIntOption.isDefined)
-    assertEquals(pid1, pid3)
-    assertTrue(pid1.endsWith(s"-$tid1"))
-    assertEquals(pid2, pid4)
-    assertTrue(pid2.endsWith("-pid-suffix"))
-    List(tid1, tid2, tid3).foreach: tid =>
-      assertEquals(tid4, tid)
-    List(pid1, pid2, pid3, pid4).foreach: pid =>
-      assertTrue(pid.takeWhile(_ != '-').toIntOption.isDefined)
+    case """{"traceEvents":[""" ::
+        s"""{"cat":"test1","name":"event1","ph":"B","pid":"${pid1}","tid":"${tid1}","ts":${ts1}}""" ::
+        s""",{"cat":"test2","name":"event2","ph":"B","pid":"${pid2}","tid":"${tid2}","ts":${ts2},"cname":"RED"}""" ::
+        s""",{"cat":"test2","name":"event2","ph":"E","pid":"${pid3}","tid":"${tid3}","ts":${ts3}}""" ::
+        s""",{"cat":"test1","name":"event1","ph":"E","pid":"${pid4}","tid":"${tid4}","ts":${ts4},"cname":"RED"}""" ::
+           "]}" :: Nil =>
+      val traceEnd = System.nanoTime()
+      assertTrue(tid1.toIntOption.isDefined)
+      assertEquals(pid1, pid3)
+      assertTrue(pid1.endsWith(s"-$tid1"))
+      assertEquals(pid2, pid4)
+      assertTrue(pid2.endsWith("-pid-suffix"))
+      List(tid1, tid2, tid3).foreach: tid =>
+        assertEquals(tid4, tid)
+      List(pid1, pid2, pid3, pid4).foreach: pid =>
+        assertTrue(pid.takeWhile(_ != '-').toIntOption.isDefined)
 
-    List(ts1, ts2, ts3, ts4).map(_.toLong) match {
-      case all @ List(ts1, ts2, ts3, ts4) =>
-        all.foreach: ts =>
-          // Timestamps are presented using Epoch microsecondos
-          assertTrue(ts >= testStart / 1000)
-          assertTrue(ts <= traceEnd / 1000)
-        assertTrue(ts2 >= ts1 + 2.millis.toMicros)
-        assertTrue(ts3 >= ts2 + 4.millis.toMicros)
-        assertTrue(ts4 >= ts3 + 8.millis.toMicros)
-      case _ => fail("unreachable")
-    }
+      List(ts1, ts2, ts3, ts4).map(_.toLong) match {
+        case all @ List(ts1, ts2, ts3, ts4) =>
+          all.foreach: ts =>
+            // Timestamps are presented using Epoch microsecondos
+            assertTrue(ts >= testStart / 1000)
+            assertTrue(ts <= traceEnd / 1000)
+          assertTrue(ts2 >= ts1 + 2.millis.toMicros)
+          assertTrue(ts3 >= ts2 + 4.millis.toMicros)
+          assertTrue(ts4 >= ts3 + 8.millis.toMicros)
+        case _ => fail("unreachable")
+      }
   }
 }