Use fixed size buffer in ReadJsonFromInputStream

It is inefficient to be constantly allocating and filling an
ArrayBuffer. The buffer is only used for reading headers that we mostly
discard anyway. My assumption is that since we only care about content
length, it's fine to put a fixed limit on the buffer size.

main-command/src/main/scala/sbt/internal/util/ReadJsonFromInputStream.scala

@@ -10,7 +10,6 @@ package sbt.internal.util
 import java.io.InputStream
 import java.nio.channels.ClosedChannelException
 import java.util.concurrent.atomic.AtomicBoolean
-import scala.collection.mutable
 import scala.util.Try
 
 private[sbt] object ReadJsonFromInputStream {
@@ -22,10 +21,19 @@ private[sbt] object ReadJsonFromInputStream {
     val newline = '\n'.toInt
     val carriageReturn = '\r'.toInt
     val contentLength = "Content-Length: "
-    var bytes = new mutable.ArrayBuffer[Byte]
+    var index = 0
+    /*
+     * This is the buffer into which we copy headers. The value of 128 bytes is
+     * somewhat arbitrary but chosen to ensure that there is enough space
+     * for any reasonable header. Any header exceeding 128 bytes will
+     * be truncated. The only header we care about at the moment is
+     * content-length, so this should be fine. If we ever start doing anything
+     * with headers, we may need to adjust this buffer size.
+     */
+    val headerBuffer = new Array[Byte](128)
     def getLine(): String = {
-      val line = new String(bytes.toArray, "UTF-8")
-      bytes = new mutable.ArrayBuffer[Byte]
+      val line = new String(headerBuffer, 0, index, "UTF-8")
+      index = 0
       onHeader.foreach(oh => oh(line))
       line
     }
@@ -51,7 +59,10 @@ private[sbt] object ReadJsonFromInputStream {
                     case `carriageReturn` => onCarriageReturn = true
                     case c =>
                       if (c == newline) getLine()
-                      else bytes += c.toByte
+                      else {
+                        if (index < headerBuffer.length) headerBuffer(index) = c.toByte
+                        index += 1
+                      }
                       onCarriageReturn = false
                       consecutiveLineEndings = 0
                   }
@@ -74,7 +85,8 @@ private[sbt] object ReadJsonFromInputStream {
         case `carriageReturn` => onCarriageReturn = true
         case c =>
           onCarriageReturn = false
-          bytes += c.toByte
+          if (index < headerBuffer.length) headerBuffer(index) = c.toByte
+          index += 1
 
       }
     } while (content.isEmpty && running.get)