Handle very long socket file paths on UNIX

Fixes #3930

main/src/main/scala/sbt/internal/CommandExchange.scala

@@ -9,6 +9,7 @@ package sbt
 package internal
 
 import java.io.IOException
+import java.nio.file.Files
 import java.util.concurrent.ConcurrentLinkedQueue
 import java.util.concurrent.atomic._
 import scala.collection.mutable.ListBuffer
@@ -23,6 +24,7 @@ import BasicKeys.{
   logLevel
 }
 import java.net.Socket
+import org.scalasbt.ipcsocket.UnixDomainSocketLibrary
 import sjsonnew.JsonFormat
 import sjsonnew.shaded.scalajson.ast.unsafe._
 import scala.concurrent.Await
@@ -32,7 +34,7 @@ import sbt.io.syntax._
 import sbt.io.{ Hash, IO }
 import sbt.internal.server._
 import sbt.internal.langserver.{ LogMessageParams, MessageType }
-import sbt.internal.util.{ StringEvent, ObjectEvent, MainAppender }
+import sbt.internal.util.{ StringEvent, ObjectEvent, MainAppender, Util }
 import sbt.internal.util.codec.JValueFormats
 import sbt.protocol.{ EventMessage, ExecStatusEvent }
 import sbt.util.{ Level, Logger, LogExchange }
@@ -140,7 +142,18 @@ private[sbt] final class CommandExchange {
       val portfile = s.baseDir / "project" / "target" / "active.json"
       val h = Hash.halfHashString(IO.toURI(portfile).toString)
       val tokenfile = BuildPaths.getGlobalBase(s) / "server" / h / "token.json"
-      val socketfile = BuildPaths.getGlobalBase(s) / "server" / h / "sock"
+      val socketfile = {
+        val socketfile = BuildPaths.getGlobalBase(s) / "server" / h / "sock"
+        connectionType match {
+          case ConnectionType.Local if !Util.isWindows =>
+            val maxSocketLength = new UnixDomainSocketLibrary.SockaddrUn().sunPath.length - 1
+            if (socketfile.absolutePath.length > maxSocketLength)
+              Files.createTempFile("sbt-server", ".sock").toFile // assuming this is short enough..
+            else
+              socketfile
+          case _ => socketfile
+        }
+      }
       val pipeName = "sbt-server-" + h
       val connection = ServerConnection(
         connectionType,