Line buffer stderr.

Ideally, stdout would be line buffered, too, except that we sometimes pump large amounts of binary data through it, so line buffering would result in performance loss.

--
MOS_MIGRATED_REVID=135237450
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/LineBufferedOutputStream.java b/src/main/java/com/google/devtools/build/lib/runtime/LineBufferedOutputStream.java
new file mode 100644
index 0000000..3d0c44f
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/runtime/LineBufferedOutputStream.java
@@ -0,0 +1,88 @@
+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.runtime;
+
+import java.io.IOException;
+import java.io.OutputStream;
+
+/**
+ * A decorator output stream that does line buffering.
+ */
+public class LineBufferedOutputStream extends OutputStream {
+  private static final int DEFAULT_BUFFER_SIZE = 1024;
+
+  private final OutputStream wrapped;
+  private final byte[] buffer;
+  private int pos;
+
+  public LineBufferedOutputStream(OutputStream wrapped) {
+    this(wrapped, DEFAULT_BUFFER_SIZE);
+  }
+
+  public LineBufferedOutputStream(OutputStream wrapped, int bufferSize) {
+    this.wrapped = wrapped;
+    this.buffer = new byte[bufferSize];
+    this.pos = 0;
+  }
+
+  @Override
+  public synchronized void write(byte[] b, int off, int inlen) throws IOException {
+    if (inlen > buffer.length * 2) {
+      // Do not buffer large writes
+      if (pos > 0) {
+        wrapped.write(buffer, 0, pos);
+        pos = 0;
+      }
+      wrapped.write(b, off, inlen);
+      return;
+    }
+
+    int next = off;
+    while (next < off + inlen) {
+      buffer[pos++] = b[next];
+      if (b[next] == '\n') {
+        wrapped.write(buffer, 0, pos);
+        pos = 0;
+      }
+
+      if (pos == buffer.length) {
+        wrapped.write(buffer, 0, pos);
+        pos = 0;
+      }
+
+      next++;
+    }
+  }
+
+  @Override
+  public void write(int byteAsInt) throws IOException {
+    byte b = (byte) byteAsInt; // make sure we work with bytes in comparisons
+    write(new byte[] {b}, 0, 1);
+  }
+
+  @Override
+  public synchronized void flush() throws IOException {
+    if (pos != 0) {
+      wrapped.write(buffer, 0, pos);
+      pos = 0;
+    }
+    wrapped.flush();
+  }
+
+  @Override
+  public synchronized void close() throws IOException {
+    flush();
+    wrapped.close();
+  }
+}