Add object lock to avoid race conditions between reads and resets

src/tools/android/java/com/google/devtools/build/android/ResourceProcessorBusyBox.java

@@ -27,7 +27,11 @@
 import com.google.devtools.common.options.OptionsParsingException;
 import com.google.devtools.common.options.ShellQuotedParamsFilePreProcessor;
 
-import java.io.*;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.PrintStream;
+import java.io.PrintWriter;
 import java.nio.file.FileSystems;
 import java.time.Duration;
 import java.util.Arrays;
@@ -140,6 +144,8 @@ void call(String[] args) throws Exception {
 
   private static final Logger logger = Logger.getLogger(ResourceProcessorBusyBox.class.getName());
   private static final Properties properties = loadSiteCustomizations();
+  // Static object locked used when reading/resetting the shared byte buffer
+  private static final Object LOCK = new Object();
 
   /** Converter for the Tool enum. */
   public static final class ToolConverter extends EnumConverter<Tool> {
@@ -220,11 +226,14 @@ private static int processRequest(List<String> args, PrintWriter pw, ByteArrayOu
       e.printStackTrace(pw);
       exitCode = 1;
     } finally {
-      // Write the captured buffer to the work response
-      String captured = buf.toString().trim();
-      buf.reset();
-      if (!captured.isEmpty()) {
-        pw.print(captured);
+      // Write the captured buffer to the work response. We synchronize to avoid race conditions
+      // while reading from and calling reset on the shared ByteArrayOutputStream.
+      synchronized (LOCK) {
+        String captured = buf.toString().trim();
+        buf.reset();
+        if (!captured.isEmpty()) {
+          pw.print(captured);
+        }
       }
     }