Parts of JDK-8293251

Author: GoeLin

src/hotspot/share/utilities/ostream.cpp

@@ -354,6 +354,7 @@ void stringStream::grow(size_t new_capacity) {
 }
 
 void stringStream::write(const char* s, size_t len) {
+  assert(_is_frozen == false, "Modification forbidden");
   assert(_capacity >= _written + 1, "Sanity");
   if (len == 0) {
     return;
@@ -393,6 +394,7 @@ void stringStream::zero_terminate() {
 }
 
 void stringStream::reset() {
+  assert(_is_frozen == false, "Modification forbidden");
   _written = 0; _precount = 0; _position = 0;
   _newlines = 0;
   zero_terminate();

src/hotspot/share/utilities/ostream.hpp

@@ -28,6 +28,7 @@
 #include "memory/allocation.hpp"
 #include "runtime/timer.hpp"
 #include "utilities/globalDefinitions.hpp"
+#include "utilities/macros.hpp"
 
 DEBUG_ONLY(class ResourceMark;)
 
@@ -191,6 +192,7 @@ class ttyUnlocker: StackObj {
 // for writing to strings; buffer will expand automatically.
 // Buffer will always be zero-terminated.
 class stringStream : public outputStream {
+  DEBUG_ONLY(bool _is_frozen = false);
   char*  _buffer;
   size_t _written;  // Number of characters written, excluding termin. zero
   size_t _capacity;
@@ -215,9 +217,18 @@ class stringStream : public outputStream {
   // Return number of characters written into buffer, excluding terminating zero and
   // subject to truncation in static buffer mode.
   size_t      size() const { return _written; }
+  // Returns internal buffer containing the accumulated string.
+  // Returned buffer is only guaranteed to be valid as long as stream is not modified
   const char* base() const { return _buffer; }
+  // Freezes stringStream (no further modifications possible) and returns pointer to it.
+  // No-op if stream is frozen already.
+  // Returns the internal buffer containing the accumulated string.
+  const char* freeze() NOT_DEBUG(const) {
+    DEBUG_ONLY(_is_frozen = true);
+    return _buffer;
+  };
   void  reset();
-  // copy to a resource, or C-heap, array as requested
+  // Copy to a resource, or C-heap, array as requested
   char* as_string(bool c_heap = false) const;
 };