feat(grader): redirect solution's stderr to file during communication…

… with communicator (#502)

judgels-backends/judgels-grader-engines/src/integTest/resources/engines/interactive/source/binsearch-OK.cpp

@@ -13,7 +13,6 @@ int main()
         fflush(stdout);
 
         fprintf(stderr, "debug");
-        fflush(stderr);
 
         scanf("%s", response);
 
@@ -22,4 +21,4 @@ int main()
         else
             hi = mid-1;
     }
-}
+}

judgels-backends/judgels-grader-engines/src/integTest/resources/engines/interactive/source/trigger-solution-sigpipe.cpp

@@ -14,7 +14,6 @@ int main()
         fflush(stdout);
 
         fprintf(stderr, "debug");
-        fflush(stderr);
 
         scanf("%s", response);
 

judgels-backends/judgels-grader-engines/src/main/java/judgels/gabriel/helpers/communicator/Communicator.java

@@ -29,6 +29,7 @@
 
 public class Communicator {
     private static final String COMMUNICATION_OUTPUT_FILENAME = "_communication.out";
+    private static final String SOLUTION_ERROR_FILENAME = "_solution.err";
 
     private final SingleSourceFileCompiler compiler;
     private final TestCaseVerdictParser verdictParser;
@@ -126,6 +127,13 @@ public EvaluationResult communicate(File input) throws EvaluationException {
             }
         }
 
+        solutionSandbox.resetRedirections();
+
+        // By default, stderr is not buffered.
+        // Writing to the unbuffered stderr sometimes interferes with the interaction.
+        // We redirect the stderr to a file so that it is buffered.
+        solutionSandbox.redirectStandardError(SOLUTION_ERROR_FILENAME);
+
         try {
             FileUtils.cleanDirectory(communicationDir);
         } catch (IOException e) {