Release the buffer in StringBuilder::toString() and toAtomicString().

third_party/WebKit/Source/wtf/text/StringBuilder.cpp

Index: third_party/WebKit/Source/wtf/text/StringBuilder.cpp
diff --git a/third_party/WebKit/Source/wtf/text/StringBuilder.cpp b/third_party/WebKit/Source/wtf/text/StringBuilder.cpp
index f8e36ae59db04902603ee5a0c615bd644e67ef88..ce023c882ba17320144c83a5e2f2fc6ecb4ab628 100644
--- a/third_party/WebKit/Source/wtf/text/StringBuilder.cpp
+++ b/third_party/WebKit/Source/wtf/text/StringBuilder.cpp
@@ -42,6 +42,7 @@ String StringBuilder::toString()
             m_string = String(characters8(), m_length);
         else
             m_string = String(characters16(), m_length);
+        clearBuffer();
     }
     return m_string;
 }
@@ -55,6 +56,7 @@ AtomicString StringBuilder::toAtomicString()
             m_string = AtomicString(characters8(), m_length);
         else
             m_string = AtomicString(characters16(), m_length);
+        clearBuffer();
     }
     return AtomicString(m_string);
 }
@@ -79,14 +81,19 @@ void StringBuilder::swap(StringBuilder& builder)
     std::swap(m_is8Bit, builder.m_is8Bit);
 }
 
-void StringBuilder::clear()
+void StringBuilder::clearBuffer()
 {
-    m_string = String();
     if (m_is8Bit)
         delete m_buffer8;
     else
         delete m_buffer16;
     m_buffer = nullptr;
+}
+
+void StringBuilder::clear()

[haraken, 2016/09/14 00:05:41]

I'm just curious but would you help me understand the difference between clear()
and clearBuffer()?

[esprehn, 2016/09/14 00:14:31]

clear() resets the entire state of the builder, like it's brand new.

clearBuffer() just resets the buffer used to make the string, leaving behind
state like m_string, m_length and m_is8Bit. That's why clear() needs to call
clearBuffer() to reset all of the state.

We only want to reset the storage when you call toString(), not the state
related to the length or the char type so if you did append() after calling
toString() we allocate the right kind of buffer (LChar vs UChar), and so calling
length() or is8Bit() after calling toString() still works.

[haraken, 2016/09/14 00:21:05]

Ah, got it. Thanks.

(I was misunderstanding that this CL is making StringBuilder entirely useless
once StringBuilder::toString is called. Maybe this is something we want to
consider though.)

[esprehn, 2016/09/14 00:25:39]

Yeah we could consider that. I don't understand the various code that keeps
StringBuilder members enough to know if that's doable. We should audit those. :)

+{
+    clearBuffer();
+    m_string = String();
     m_length = 0;
     m_is8Bit = true;
 }
@@ -111,14 +118,14 @@ void StringBuilder::reserveCapacity(unsigned newCapacity)
 void StringBuilder::resize(unsigned newSize)
 {
     DCHECK_LE(newSize, m_length);
+    m_string = m_string.left(newSize);
     m_length = newSize;
-    m_string = String();
-    if (!hasBuffer())
-        return;
-    if (m_is8Bit)
-        m_buffer8->resize(newSize);
-    else
-        m_buffer16->resize(newSize);
+    if (hasBuffer()) {
+        if (m_is8Bit)
+            m_buffer8->resize(newSize);
+        else
+            m_buffer16->resize(newSize);
+    }
 }
 
 void StringBuilder::createBuffer8(unsigned addedSize)
@@ -137,9 +144,8 @@ void StringBuilder::createBuffer8(unsigned addedSize)
     // This allows doing append(string); append('\0') without extra mallocs.
     m_buffer8->reserveInitialCapacity(m_length + std::max(addedSize, initialBufferSize()));
     m_length = 0;
-    // Must keep a ref to the string since append will clear it.
-    String string(m_string);
-    append(string);
+    append(m_string);
+    m_string = String();
 }
 
 void StringBuilder::createBuffer16(unsigned addedSize)
@@ -160,9 +166,8 @@ void StringBuilder::createBuffer16(unsigned addedSize)
         append(buffer8.data(), length);
         return;
     }
-    // Must keep a ref to the string since append will clear it.
-    String string(m_string);
-    append(string);
+    append(m_string);
+    m_string = String();
 }
 
 void StringBuilder::append(const UChar* characters, unsigned length)
@@ -179,7 +184,6 @@ void StringBuilder::append(const UChar* characters, unsigned length)
     }
 
     ensureBuffer16(length);
-    m_string = String();
     m_buffer16->append(characters, length);
     m_length += length;
 }
@@ -192,14 +196,12 @@ void StringBuilder::append(const LChar* characters, unsigned length)
 
     if (m_is8Bit) {
         ensureBuffer8(length);
-        m_string = String();
         m_buffer8->append(characters, length);
         m_length += length;
         return;
     }
 
     ensureBuffer16(length);
-    m_string = String();
     m_buffer16->append(characters, length);
     m_length += length;
 }