comment that, because it's a bit non-obvious

1 files changed, 9 insertions, 0 deletions

diff --git a/src/main/scala/org/perl8/test/ExternalTest.scala b/src/main/scala/org/perl8/test/ExternalTest.scala
index 5c3be81..533570c 100644
--- a/src/main/scala/org/perl8/test/ExternalTest.scala
+++ b/src/main/scala/org/perl8/test/ExternalTest.scala
@@ -10,12 +10,21 @@ import scala.annotation.tailrec
 class ExternalTest (cmdLine: Seq[String]) extends Test {
   def run: Int = {
     val processBuilder = new ProcessBuilder(cmdLine: _*)
+
+    // Ensure that if stdout and stderr are both pointing to the same place (a
+    // terminal or file or something) that they remain synchronized. This is
+    // only possible if the endpoint they are streaming to is the same place
+    // underneath, with no extra processing in between. (This is safe because
+    // stdout is typically line-buffered, and we only ever output a line at a
+    // time when writing TAP, so in theory, buffering of the underlying file
+    // descriptor shouldn't make a difference here)
     if (Console.out eq System.out) {
       processBuilder.redirectOutput(ProcessBuilder.Redirect.INHERIT)
     }
     if (Console.err eq System.err) {
       processBuilder.redirectError(ProcessBuilder.Redirect.INHERIT)
     }
+
     val process = processBuilder.start
 
     val streams = Seq(