From 121a075bc2558003990377843393ca27d784f50f Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Wed, 2 Dec 2020 20:47:06 +0300
Subject: Fix buildscript assertion failure on redirecting command stderr to
 stdout

---
 libbuild2/script/run.cxx            | 14 +++++++++++++-
 tests/recipe/buildscript/testscript | 23 +++++++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/libbuild2/script/run.cxx b/libbuild2/script/run.cxx
index 182696b..90ddc4d 100644
--- a/libbuild2/script/run.cxx
+++ b/libbuild2/script/run.cxx
@@ -1552,8 +1552,20 @@ namespace build2
       // Merge standard streams.
       //
       bool mo (out != nullptr && out->type == redirect_type::merge);
-      if (mo || err.type == redirect_type::merge)
+      bool me (err.type == redirect_type::merge);
+
+      if (mo || me)
       {
+        // Note that while the parser verifies that there is no stdout/stderr
+        // mutual redirects specified on the command line, we can still end up
+        // with mutual redirects here since one of such redirects can be
+        // provided as a default by the script environment implementation
+        // which the parser is not aware of at the time of parsing the command
+        // line.
+        //
+        if (mo && me)
+          fail (ll) << "stdout and stderr redirected to each other";
+
         auto_fd& self  (mo ? ofd.out : efd);
         auto_fd& other (mo ? efd : ofd.out);
 
diff --git a/tests/recipe/buildscript/testscript b/tests/recipe/buildscript/testscript
index d1adc36..2603f62 100644
--- a/tests/recipe/buildscript/testscript
+++ b/tests/recipe/buildscript/testscript
@@ -83,6 +83,29 @@
     $* clean 2>-
   }
 
+  : mutual-redirects
+  :
+  {
+    echo 'bar' >=bar;
+
+    cat <<EOI >=buildfile;
+      foo: bar
+      % [diag=cp]
+      {{
+        echo 'copying' 2>&1
+        cp $path($<) $path($>)
+      }}
+      EOI
+
+    $* 2>>~%EOE% != 0;
+      cp file{foo}
+      buildfile:4:3: error: stdout and stderr redirected to each other
+      %.+
+      EOE
+
+    $* clean 2>-
+  }
+
   : untracked-var
   :
   {
-- 
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