Optimize BSON decoding

bson/src/main/org/bson/ByteBuf.java

@@ -192,7 +192,7 @@ public interface ByteBuf  {
      * @return {@code true} if, and only if, this buffer is backed by an array and is not read-only
      * @since 5.5
      */
-    boolean hasArray();
+    boolean isBackedByArray();
 
     /**
      * Returns the offset of the first byte within the backing byte array of

bson/src/main/org/bson/ByteBufNIO.java

@@ -133,7 +133,7 @@ public byte[] array() {
     }
 
     @Override
-    public boolean hasArray() {
+    public boolean isBackedByArray() {
         return buf.hasArray();
     }
 

bson/src/main/org/bson/io/ByteBufferBsonInput.java

@@ -33,6 +33,13 @@
 public class ByteBufferBsonInput implements BsonInput {
 
     private static final String[] ONE_BYTE_ASCII_STRINGS = new String[Byte.MAX_VALUE + 1];
+    /* A dynamically sized scratch buffer, that is reused across BSON String reads:
+     * 1. Reduces garbage collection by avoiding new byte array creation.
+     * 2. Improves cache utilization through temporal locality.
+     * 3. Avoids JVM allocation and zeroing cost for new memory allocations.
+     */
+    private byte[] scratchBuffer;
+
 
     static {
         for (int b = 0; b < ONE_BYTE_ASCII_STRINGS.length; b++) {
@@ -127,15 +134,12 @@ public String readString() {
 
     @Override
     public String readCString() {
-        int mark = buffer.position();
-        skipCString();
-        int size = buffer.position() - mark;
-        buffer.position(mark);
+        int size = computeCStringLength(buffer.position());
         return readString(size);
     }
 
-    private String readString(final int size) {
-        if (size == 2) {
+    private String readString(final int bsonStringSize) {
+        if (bsonStringSize == 2) {
             byte asciiByte = buffer.get();               // if only one byte in the string, it must be ascii.
             byte nullByte = buffer.get();                // read null terminator
             if (nullByte != 0) {
@@ -146,26 +150,51 @@ private String readString(final int size) {
             }
             return ONE_BYTE_ASCII_STRINGS[asciiByte];  // this will throw if asciiByte is negative
         } else {
-            byte[] bytes = new byte[size - 1];
-            buffer.get(bytes);
-            byte nullByte = buffer.get();
-            if (nullByte != 0) {
-                throw new BsonSerializationException("Found a BSON string that is not null-terminated");
+            if (buffer.isBackedByArray()) {
+                int position = buffer.position();
+                int arrayOffset = buffer.arrayOffset();
+                int newPosition = position + bsonStringSize;
+                buffer.position(newPosition);
+
+                byte[] array = buffer.array();
+                if (array[arrayOffset + newPosition - 1] != 0) {
+                    throw new BsonSerializationException("Found a BSON string that is not null-terminated");
+                }
+                return new String(array, arrayOffset + position, bsonStringSize - 1, StandardCharsets.UTF_8);
+            } else if (scratchBuffer == null || bsonStringSize > scratchBuffer.length) {
+                int scratchBufferSize = bsonStringSize + (bsonStringSize >>> 1); //1.5 times the size
+                scratchBuffer = new byte[scratchBufferSize];
+            }
+
+            buffer.get(scratchBuffer, 0, bsonStringSize);
+            if (scratchBuffer[bsonStringSize - 1] != 0) {
+                throw new BsonSerializationException("BSON string not null-terminated");
             }
-            return new String(bytes, StandardCharsets.UTF_8);
+            return new String(scratchBuffer, 0, bsonStringSize - 1, StandardCharsets.UTF_8);
         }
     }
 
     @Override
     public void skipCString() {
         ensureOpen();
-        boolean checkNext = true;
-        while (checkNext) {
-            if (!buffer.hasRemaining()) {
-                throw new BsonSerializationException("Found a BSON string that is not null-terminated");
+        int pos = buffer.position();
+        int length = computeCStringLength(pos);
+        buffer.position(pos + length);
+    }
+
+    private int computeCStringLength(final int prevPos) {
+        ensureOpen();
+        int pos = buffer.position();
+        int limit = buffer.limit();
+
+        while (pos < limit) {
+            if (buffer.get(pos++) == 0) {
+                return (pos - prevPos);
             }
-            checkNext = buffer.get() != 0;
         }
+
+        buffer.position(pos);
+        throw new BsonSerializationException("Found a BSON string that is not null-terminated");
     }
 
     @Override

driver-core/src/main/com/mongodb/internal/connection/ByteBufferBsonOutput.java

@@ -399,7 +399,7 @@ protected int writeCharacters(final String str, final boolean checkNullTerminati
         int curBufferLimit = curBuffer.limit();
         int remaining = curBufferLimit - curBufferPos;
 
-        if (curBuffer.hasArray()) {
+        if (curBuffer.isBackedByArray()) {
             byte[] dst = curBuffer.array();
             int arrayOffset = curBuffer.arrayOffset();
             if (remaining >= str.length() + 1) {

driver-core/src/main/com/mongodb/internal/connection/CompositeByteBuf.java

@@ -214,7 +214,7 @@ public byte[] array() {
     }
 
     @Override
-    public boolean hasArray() {
+    public boolean isBackedByArray() {
         return false;
     }
 

driver-core/src/main/com/mongodb/internal/connection/ResponseBuffers.java

@@ -53,13 +53,15 @@ <T extends BsonDocument> T getResponseDocument(final int messageId, final Decode
     }
 
     /**
-     * Returns a read-only buffer containing the response body.  Care should be taken to not use the returned buffer after this instance has
+     * Returns a buffer containing the response body.  Care should be taken to not use the returned buffer after this instance has
      * been closed.
      *
-     * @return a read-only buffer containing the response body
+     * NOTE: do not modify this buffer, it is being made writable for performance reasons to avoid redundant copying.
+     *
+     * @return a buffer containing the response body
      */
     public ByteBuf getBodyByteBuffer() {
-        return bodyByteBuffer.asReadOnly();
+        return bodyByteBuffer;
     }
 
     public void reset() {

driver-core/src/main/com/mongodb/internal/connection/netty/NettyByteBuf.java

@@ -125,7 +125,7 @@ public byte[] array() {
     }
 
     @Override
-    public boolean hasArray() {
+    public boolean isBackedByArray() {
         return proxied.hasArray();
     }