defer opening logging output files until an actual write

tasks/standard/Streams.scala

@@ -77,7 +77,7 @@ object Streams
 				make(a, sid)(f => new BufferedInputStream(new FileInputStream(f)))
 
 			def text(sid: String = default): PrintWriter =
-				make(a, sid)(f => new PrintWriter(new BufferedWriter(new OutputStreamWriter(new FileOutputStream(f), IO.defaultCharset))) )
+				make(a, sid)(f => new PrintWriter(new DeferredWriter(new BufferedWriter(new OutputStreamWriter(new FileOutputStream(f), IO.defaultCharset)))) )
 
 			def binary(sid: String = default): BufferedOutputStream =
 				make(a, sid)(f => new BufferedOutputStream(new FileOutputStream(f)))

util/io/DeferredWriter.scala

@@ -0,0 +1,29 @@
+package sbt
+
+	import java.io.Writer
+
+final class DeferredWriter(make: => Writer) extends Writer
+{
+	private[this] var opened = false
+	private[this] var delegate0: Writer = _
+	private[this] def delegate: Writer = synchronized {
+		if(delegate0 eq null) {
+			delegate0 = make
+			opened = true
+		}
+		delegate0
+	}
+	override def close() = synchronized {
+		if(opened) delegate0.close()
+	}
+
+	override def append(c: Char): Writer = delegate.append(c)
+	override def append(csq: CharSequence): Writer = delegate.append(csq)
+	override def append(csq: CharSequence, start: Int, end: Int): Writer = delegate.append(csq, start, end)
+	override def flush() = delegate.flush()
+	override def write(cbuf: Array[Char]) = delegate.write(cbuf)
+	override def write(cbuf: Array[Char], off: Int, len: Int): Unit = delegate.write(cbuf, off, len)
+	override def write(c: Int): Unit = delegate.write(c)
+	override def write(s: String): Unit = delegate.write(s)
+	override def write(s: String, off: Int, len: Int): Unit = delegate.write(s, off, len)
+}