expand output buffer when no data are processed (#10)

When no data are consumed and processed, the codec would expect the
output buffer will be expanded in the next call. This is necessary when
the codec want to write a epilogue to the output but it has not enough
size to store the data.

src/stream.jl

@@ -392,6 +392,9 @@ function fillbuffer(stream::TranscodingStream)
         if stream.state.code == :error
             handle_error(stream)
         end
+        if stream.state.code == :ok && Δin == Δout == 0
+            makemargin!(buffer1, max(16, marginsize(buffer1) * 2))
+        end
     end
     return nfilled
 end
@@ -443,6 +446,8 @@ function process_to_write(stream::TranscodingStream)
     buffer2.total += Δout
     if stream.state.code == :error
         handle_error(stream)
+    elseif stream.state.code == :ok && Δin == Δout == 0
+        makemargin!(buffer2, max(16, marginsize(buffer2) * 2))
     end
     makemargin!(buffer1, 0)
     return Δin

test/runtests.jl

@@ -187,6 +187,29 @@ struct InvalidCodec <: TranscodingStreams.Codec end
     @test_throws MethodError read(TranscodingStream(InvalidCodec(), IOBuffer()))
 end
 
+struct QuadrupleCodec <: TranscodingStreams.Codec end
+
+function TranscodingStreams.process(
+        codec  :: QuadrupleCodec,
+        input  :: TranscodingStreams.Memory,
+        output :: TranscodingStreams.Memory,
+        error  :: TranscodingStreams.Error)
+    i = j = 0
+    while i + 1 ≤ endof(input) && j + 4 ≤ endof(output)
+        b = input[i+1]
+        i += 1
+        output[j+1] = output[j+2] = output[j+3] = output[j+4] = b
+        j += 4
+    end
+    return i, j, input.size == 0 ? (:end) : (:ok)
+end
+
+@testset "QuadrupleCodec" begin
+    @test transcode(QuadrupleCodec(), b"") == b""
+    @test transcode(QuadrupleCodec(), b"a") == b"aaaa"
+    @test transcode(QuadrupleCodec(), b"ab") == b"aaaabbbb"
+end
+
 for pkg in ["CodecZlib", "CodecBzip2", "CodecXz", "CodecZstd"]
     Pkg.test(pkg)
 end