=htp #19099 fix encoding of empty stings

akka-http-tests/src/test/scala/akka/http/scaladsl/coding/CodecSpecSupport.scala

@@ -22,9 +22,11 @@ trait CodecSpecSupport extends Matchers with BeforeAndAfterAll { self: Suite ⇒
     i
   }
 
+  lazy val emptyTextBytes = ByteString(emptyText, "UTF8")
   lazy val smallTextBytes = ByteString(smallText, "UTF8")
   lazy val largeTextBytes = ByteString(largeText, "UTF8")
 
+  val emptyText = ""
   val smallText = "Yeah!"
   val largeText =
     """Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore

akka-http-tests/src/test/scala/akka/http/scaladsl/coding/CoderSpec.scala

@@ -32,6 +32,18 @@ abstract class CoderSpec extends WordSpec with CodecSpecSupport with Inspectors
   def extraTests(): Unit = {}
 
   s"The ${Coder.encoding.value} codec" should {
+    "produce valid data on immediate finish" in {
+      streamDecode(Coder.newCompressor.finish()) should readAs(emptyText)
+    }
+    "properly encode an empty string" in {
+      streamDecode(ourEncode(emptyTextBytes)) should readAs(emptyText)
+    }
+    "properly decode an empty string" in {
+      ourDecode(streamEncode(emptyTextBytes)) should readAs(emptyText)
+    }
+    "properly round-trip encode/decode an empty string" in {
+      ourDecode(ourEncode(emptyTextBytes)) should readAs(emptyText)
+    }
     "properly encode a small string" in {
       streamDecode(ourEncode(smallTextBytes)) should readAs(smallText)
     }

akka-http/src/main/scala/akka/http/scaladsl/coding/Deflate.scala

@@ -55,7 +55,7 @@ class DeflateCompressor extends Compressor {
     res
   }
 
-  private def newTempBuffer(size: Int = 65536): Array[Byte] =
+  private def newTempBuffer(size: Int = 65536): Array[Byte] = {
     // The default size is somewhat arbitrary, we'd like to guess a better value but Deflater/zlib
     // is buffering in an unpredictable manner.
     // `compress` will only return any data if the buffered compressed data has some size in
@@ -63,10 +63,15 @@ class DeflateCompressor extends Compressor {
     // `flush` and `finish` will return any size depending on the previous input.
     // This value will hopefully provide a good compromise between memory churn and
     // excessive fragmentation of ByteStrings.
-    new Array[Byte](size)
+    // We also make sure that buffer size stays within a reasonable range, to avoid
+    // draining deflator with too small buffer.
+    new Array[Byte](math.max(size, MinBufferSize))
+  }
 }
 
 private[http] object DeflateCompressor {
+  val MinBufferSize = 1024
+
   // TODO: remove reflective call once Java 6 support is dropped
   /**
    * Compatibility mode: reflectively call deflate(..., flushMode) if available or use a hack otherwise