[libc++] Avoid conflating stderr and stdout in the DSL

This is a workaround until https://reviews.llvm.org/D81892 is merged
and the internal Lit shell stops conflating error output with normal
output. Without this, any program that writes to stderr will trip up
the programOutput function, because it will pick up the '# command stderr:'
string and think it's part of the command's stdout.

rdar://81056048

Differential Revision: https://reviews.llvm.org/D107912

Author: ldionne

libcxx/test/libcxx/selftest/dsl/dsl.sh.py

@@ -196,6 +196,7 @@ def test_caching_is_not_too_aggressive(self):
         #include <cstdio>
         int main(int, char**) {
             std::printf("MACRO=%u\\n", MACRO);
+            return 0;
         }
         """
         compileFlagsIndex = findIndex(self.config.substitutions, lambda x: x[0] == '%{compile_flags}')
@@ -209,6 +210,19 @@ def test_caching_is_not_too_aggressive(self):
         output2 = dsl.programOutput(self.config, source)
         self.assertEqual(output2, "MACRO=2\n")
 
+    def test_program_stderr_is_not_conflated_with_stdout(self):
+        # Run a program that produces stdout output and stderr output too, making
+        # sure the stderr output does not pollute the stdout output.
+        source = """
+        #include <cstdio>
+        int main(int, char**) {
+            std::fprintf(stdout, "STDOUT-OUTPUT");
+            std::fprintf(stderr, "STDERR-OUTPUT");
+            return 0;
+        }
+        """
+        self.assertEqual(dsl.programOutput(self.config, source), "STDOUT-OUTPUT")
+
 
 class TestHasLocale(SetupConfigs):
     """

libcxx/utils/libcxx/test/dsl.py

@@ -64,9 +64,20 @@ def _executeScriptInternal(test, commands):
     if not os.path.exists(d):
       os.makedirs(d)
   res = lit.TestRunner.executeScriptInternal(test, litConfig, tmpBase, parsedCommands, execDir)
-  if isinstance(res, lit.Test.Result):
-    res = ('', '', 127, None)
-  return res
+  if isinstance(res, lit.Test.Result): # Handle failure to parse the Lit test
+    res = ('', res.output, 127, None)
+  (out, err, exitCode, timeoutInfo) = res
+
+  # TODO: As a temporary workaround until https://reviews.llvm.org/D81892 lands, manually
+  #       split any stderr output that is included in stdout. It shouldn't be there, but
+  #       the Lit internal shell conflates stderr and stdout.
+  conflatedErrorOutput = re.search("(# command stderr:.+$)", out, flags=re.DOTALL)
+  if conflatedErrorOutput:
+    conflatedErrorOutput = conflatedErrorOutput.group(0)
+    out = out[:-len(conflatedErrorOutput)]
+    err += conflatedErrorOutput
+
+  return (out, err, exitCode, timeoutInfo)
 
 def _makeConfigTest(config, testPrefix=''):
   sourceRoot = os.path.join(config.test_exec_root, '__config_src__')
@@ -122,7 +133,7 @@ def programOutput(config, program, args=None, testPrefix=''):
       if exitCode != 0:
         return None
 
-      actualOut = re.search("command output:\n(.+)\n$", out, flags=re.DOTALL)
+      actualOut = re.search("# command output:\n(.+)\n$", out, flags=re.DOTALL)
       actualOut = actualOut.group(1) if actualOut else ""
       return actualOut