Remove buffer copy when writing base64 JSON value (dotnet#97687)

Author: habbes

src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteProperties.Bytes.cs

@@ -236,7 +236,7 @@ private void WriteBase64Minimized(ReadOnlySpan<char> escapedPropertyName, ReadOn
 
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodedLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }
@@ -272,7 +272,7 @@ private void WriteBase64Minimized(ReadOnlySpan<byte> escapedPropertyName, ReadOn
 
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodedLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }
@@ -322,7 +322,7 @@ private void WriteBase64Indented(ReadOnlySpan<char> escapedPropertyName, ReadOnl
 
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodedLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }
@@ -373,7 +373,7 @@ private void WriteBase64Indented(ReadOnlySpan<byte> escapedPropertyName, ReadOnl
 
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodedLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }

src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Bytes.cs

@@ -79,7 +79,7 @@ private void WriteBase64Minimized(ReadOnlySpan<byte> bytes)
             }
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodingLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }
@@ -130,7 +130,7 @@ private void WriteBase64Indented(ReadOnlySpan<byte> bytes)
 
             output[BytesPending++] = JsonConstants.Quote;
 
-            Base64EncodeAndWrite(bytes, output, encodingLength);
+            Base64EncodeAndWrite(bytes, output);
 
             output[BytesPending++] = JsonConstants.Quote;
         }

src/libraries/System.Text.Json/src/System/Text/Json/Writer/Utf8JsonWriter.WriteValues.Helpers.cs

@@ -35,29 +35,13 @@ private void ValidateWritingValue()
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private void Base64EncodeAndWrite(ReadOnlySpan<byte> bytes, Span<byte> output, int encodingLength)
+        private void Base64EncodeAndWrite(ReadOnlySpan<byte> bytes, Span<byte> output)
         {
-            byte[]? outputText = null;
-
-            Span<byte> encodedBytes = encodingLength <= JsonConstants.StackallocByteThreshold ?
-                stackalloc byte[JsonConstants.StackallocByteThreshold] :
-                (outputText = ArrayPool<byte>.Shared.Rent(encodingLength));
-
-            OperationStatus status = Base64.EncodeToUtf8(bytes, encodedBytes, out int consumed, out int written);
+            Span<byte> destination = output.Slice(BytesPending);
+            OperationStatus status = Base64.EncodeToUtf8(bytes, destination, out int consumed, out int written);
             Debug.Assert(status == OperationStatus.Done);
             Debug.Assert(consumed == bytes.Length);
-
-            encodedBytes = encodedBytes.Slice(0, written);
-            Span<byte> destination = output.Slice(BytesPending);
-
-            Debug.Assert(destination.Length >= written);
-            encodedBytes.Slice(0, written).CopyTo(destination);
             BytesPending += written;
-
-            if (outputText != null)
-            {
-                ArrayPool<byte>.Shared.Return(outputText);
-            }
         }
     }
 }