Small file upload optimization

If the entire object is uploaded during a single `Write` call, then
skip the multipart upload and just do a single non-buffered upload.

src/S3Commands.cc

@@ -476,7 +476,7 @@ bool AmazonRequest::SendS3Request(const std::string_view payload,
 
 AmazonS3Upload::~AmazonS3Upload() {}
 
-bool AmazonS3Upload::SendRequest(const std::string &payload) {
+bool AmazonS3Upload::SendRequest(const std::string_view &payload) {
 	httpVerb = "PUT";
 	return SendS3Request(payload, payload.size(), true);
 }

src/S3Commands.hh

@@ -158,7 +158,7 @@ class AmazonS3Upload final : public AmazonRequest {
 
 	virtual ~AmazonS3Upload();
 
-	bool SendRequest(const std::string &payload);
+	bool SendRequest(const std::string_view &payload);
 
   protected:
 	std::string path;

src/S3File.cc

@@ -224,6 +224,25 @@ ssize_t S3File::Write(const void *buffer, off_t offset, size_t size) {
 	}
 	std::lock_guard lk(*write_mutex);
 
+	// Small object optimization -- if this is the full object, upload
+	// it immediately.
+	if (!m_write_offset && m_object_size == size) {
+		AmazonS3Upload upload(m_ai, m_object, m_log);
+		m_write_lk.reset();
+		if (!upload.SendRequest(
+				std::string_view(static_cast<const char *>(buffer), size))) {
+			m_log.Log(LogMask::Warning, "Write",
+					  "Failed to create small object");
+			return -EIO;
+		} else {
+			m_write_offset += size;
+			m_log.Log(LogMask::Debug, "Write",
+					  "Creation of small object succeeded",
+					  std::to_string(size).c_str());
+			return size;
+		}
+	}
+
 	if (offset != m_write_offset) {
 		m_log.Emsg(
 			"Write",