Restart presentation compilers if memory is low

I have noted during long editing sessions (lasting several days,
typically) that memory can get full because the Dotty compiler has some
space leaks. The leaks looks really hard to fix, and we don't know
yet whether it's at all possible. To mitigate the leaks, this commit
makes the language server watch available memory, and, if it
is low (i.e. free memory after a GC < 10% of maximal available)
restart all interactive drivers. This will free all memory of
the compiler(s) except the shared nametable.

There's a stressTest option in `Memory.scala`, which, when turned on,
causes a restart every 10 editing actions. I verified that the compiler
stays functional and reasonably responsive in that mode.

compiler/src/dotty/tools/dotc/Run.scala

@@ -147,7 +147,9 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
   }
 
   protected def compileUnits()(implicit ctx: Context) = Stats.maybeMonitored {
-    ctx.checkSingleThreaded()
+    if (!ctx.mode.is(Mode.Interactive)) // IDEs might have multi-threaded access, accesses are synchronized
+      ctx.checkSingleThreaded()
+
     compiling = true
 
     // If testing pickler, make sure to stop after pickling phase:

compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala

@@ -24,7 +24,7 @@ import reporting._, reporting.diagnostic.MessageContainer
 import util._
 
 /** A Driver subclass designed to be used from IDEs */
-class InteractiveDriver(settings: List[String]) extends Driver {
+class InteractiveDriver(val settings: List[String]) extends Driver {
   import tpd._
   import InteractiveDriver._
 
@@ -212,7 +212,17 @@ class InteractiveDriver(settings: List[String]) extends Driver {
     cleanupTree(tree)
   }
 
-  def run(uri: URI, sourceCode: String): List[MessageContainer] = {
+  private def toSource(uri: URI, sourceCode: String): SourceFile = {
+    val virtualFile = new VirtualFile(uri.toString, Paths.get(uri).toString)
+    val writer = new BufferedWriter(new OutputStreamWriter(virtualFile.output, "UTF-8"))
+    writer.write(sourceCode)
+    writer.close()
+    new SourceFile(virtualFile, Codec.UTF8)
+  }
+
+  def run(uri: URI, sourceCode: String): List[MessageContainer] = run(uri, toSource(uri, sourceCode))
+
+  def run(uri: URI, source: SourceFile): List[MessageContainer] = {
     val previousCtx = myCtx
     try {
       val reporter =
@@ -223,11 +233,6 @@ class InteractiveDriver(settings: List[String]) extends Driver {
 
       implicit val ctx = myCtx
 
-      val virtualFile = new VirtualFile(uri.toString, Paths.get(uri).toString)
-      val writer = new BufferedWriter(new OutputStreamWriter(virtualFile.output, "UTF-8"))
-      writer.write(sourceCode)
-      writer.close()
-      val source = new SourceFile(virtualFile, Codec.UTF8)
       myOpenedFiles(uri) = source
 
       run.compileSources(List(source))

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -25,6 +25,7 @@ import reporting._, reporting.diagnostic.MessageContainer
 import util._
 import interactive._, interactive.InteractiveDriver._
 import Interactive.Include
+import config.Printers.interactiv
 
 import languageserver.config.ProjectConfig
 
@@ -68,12 +69,40 @@ class DottyLanguageServer extends LanguageServer
         val classpathFlags = List("-classpath", (config.classDirectory +: config.dependencyClasspath).mkString(File.pathSeparator))
         val sourcepathFlags = List("-sourcepath", config.sourceDirectories.mkString(File.pathSeparator), "-scansource")
         val settings = defaultFlags ++ config.compilerArguments.toList ++ classpathFlags ++ sourcepathFlags
-        myDrivers.put(config, new InteractiveDriver(settings))
+        myDrivers(config) = new InteractiveDriver(settings)
       }
     }
     myDrivers
   }
 
+  /** Restart all presentation compiler drivers, copying open files over */
+  private def restart() = thisServer.synchronized {
+    interactiv.println("restarting presentation compiler")
+    val driverConfigs = for ((config, driver) <- myDrivers.toList) yield
+      (config, new InteractiveDriver(driver.settings), driver.openedFiles)
+    for ((config, driver, _) <- driverConfigs)
+      myDrivers(config) = driver
+    System.gc()
+    for ((_, driver, opened) <- driverConfigs; (uri, source) <- opened)
+      driver.run(uri, source)
+    if (Memory.isCritical())
+      println(s"WARNING: Insufficient memory to run Scala language server on these projects.")
+  }
+
+  private def checkMemory() =
+    if (Memory.isCritical())
+      CompletableFutures.computeAsync { _ => restart(); new Object() }
+        // new Object() necessary or we get a BootstrapMethodError:
+        //
+        // Caused by: java.lang.invoke.LambdaConversionException: Type mismatch for lambda expected return: void is not convertible to class java.lang.Object
+        // at java.lang.invoke.AbstractValidatingLambdaMetafactory.validateMetafactoryArgs(AbstractValidatingLambdaMetafactory.java:286)
+        // at java.lang.invoke.LambdaMetafactory.metafactory(LambdaMetafactory.java:303)
+        // at java.lang.invoke.CallSite.makeSite(CallSite.java:302)
+        // ... 11 more
+        //
+        // This looks like a problem with Dottys code generation for void-returning closures passed
+        // to Java methods. (or SAM functions in general?)
+
   /** The driver instance responsible for compiling `uri` */
   def driverFor(uri: URI): InteractiveDriver = {
     val matchingConfig =
@@ -102,10 +131,11 @@ class DottyLanguageServer extends LanguageServer
   }
 
   private[this] def computeAsync[R](fun: CancelChecker => R): CompletableFuture[R] =
-    CompletableFutures.computeAsync({(cancelToken: CancelChecker) =>
+    CompletableFutures.computeAsync { cancelToken =>
       // We do not support any concurrent use of the compiler currently.
       thisServer.synchronized {
         cancelToken.checkCanceled()
+        checkMemory()
         try {
           fun(cancelToken)
         } catch {
@@ -114,7 +144,7 @@ class DottyLanguageServer extends LanguageServer
             throw ex
         }
       }
-    })
+    }
 
   override def initialize(params: InitializeParams) = computeAsync { cancelToken =>
     rootUri = params.getRootUri
@@ -150,6 +180,7 @@ class DottyLanguageServer extends LanguageServer
   }
 
   override def didOpen(params: DidOpenTextDocumentParams): Unit = thisServer.synchronized {
+    checkMemory()
     val document = params.getTextDocument
     val uri = new URI(document.getUri)
     val driver = driverFor(uri)
@@ -163,6 +194,7 @@ class DottyLanguageServer extends LanguageServer
   }
 
   override def didChange(params: DidChangeTextDocumentParams): Unit = thisServer.synchronized {
+    checkMemory()
     val document = params.getTextDocument
     val uri = new URI(document.getUri)
     val driver = driverFor(uri)

language-server/src/dotty/tools/languageserver/Memory.scala

@@ -0,0 +1,47 @@
+package dotty.tools
+package languageserver
+
+object Memory {
+
+  /** Memory is judged to be critical if after a GC the amount of used memory
+   *  divided by total available memory exceeds this threshold.
+   */
+  val UsedThreshold = 0.9
+
+  /** If total available memory is unknown, memory is judged to be critical if
+   *  after a GC free memory divided by used memory is under this threshold.
+   */
+  val FreeThreshold = 0.1
+
+  /** Turn this flag on to stress test restart capability in compiler.
+   *  It will restart the presentation compiler after every 10 editing actions
+   */
+  private final val stressTest = false
+  private var stressTestCounter = 0
+
+  /** Is memory critically low? */
+  def isCritical(): Boolean = {
+    if (stressTest) {
+      stressTestCounter += 1
+      if (stressTestCounter % 10 == 0) return true
+    }
+    val runtime = Runtime.getRuntime
+    def total = runtime.totalMemory
+    def maximal = runtime.maxMemory
+    def free = runtime.freeMemory
+    def used = total - free
+    def usedIsCloseToMax =
+      if (maximal == Long.MaxValue) free.toDouble / used < FreeThreshold
+      else used.toDouble / maximal > UsedThreshold
+    usedIsCloseToMax && { runtime.gc(); usedIsCloseToMax }
+  }
+
+  def stats(): String = {
+    final val M = 2 << 20
+    val runtime = Runtime.getRuntime
+    def total = runtime.totalMemory / M
+    def maximal = runtime.maxMemory / M
+    def free = runtime.freeMemory / M
+    s"total used memory: $total MB, free: $free MB, maximal available = $maximal MB"
+  }
+}