[api] Extract overflow buffer from frame helper into APIOverflowBuffer (#14871)

esphome/components/api/api_overflow_buffer.cpp

@@ -0,0 +1,73 @@
+#include "api_overflow_buffer.h"
+#ifdef USE_API
+#include <cstring>
+
+namespace esphome::api {
+
+APIOverflowBuffer::~APIOverflowBuffer() {
+  for (auto *entry : this->queue_) {
+    if (entry != nullptr)
+      Entry::destroy(entry);
+  }
+}
+
+ssize_t APIOverflowBuffer::try_drain(socket::Socket *socket) {
+  while (this->count_ > 0) {
+    Entry *front = this->queue_[this->head_];
+
+    ssize_t sent = socket->write(front->current_data(), front->remaining());
+
+    if (sent <= 0) {
+      // -1 = error (caller checks errno for EWOULDBLOCK vs hard error)
+      // 0 = nothing sent (treat as no progress)
+      return sent;
+    }
+
+    if (static_cast<uint16_t>(sent) < front->remaining()) {
+      // Partially sent, update offset and stop
+      front->offset += static_cast<uint16_t>(sent);
+      return sent;
+    }
+
+    // Entry fully sent — free it and advance
+    Entry::destroy(front);
+    this->queue_[this->head_] = nullptr;
+    this->head_ = (this->head_ + 1) % API_MAX_SEND_QUEUE;
+    this->count_--;
+  }
+
+  return 0;  // All drained
+}
+
+bool APIOverflowBuffer::enqueue_iov(const struct iovec *iov, int iovcnt, uint16_t total_len, uint16_t skip) {
+  if (this->count_ >= API_MAX_SEND_QUEUE)
+    return false;
+
+  uint16_t buffer_size = total_len - skip;
+  // NOLINTNEXTLINE(cppcoreguidelines-owning-memory)
+  auto *entry = new Entry{new uint8_t[buffer_size], buffer_size, 0};
+  this->queue_[this->tail_] = entry;
+
+  uint16_t to_skip = skip;
+  uint16_t write_pos = 0;
+
+  for (int i = 0; i < iovcnt; i++) {
+    if (to_skip >= iov[i].iov_len) {
+      to_skip -= static_cast<uint16_t>(iov[i].iov_len);
+    } else {
+      const uint8_t *src = reinterpret_cast<uint8_t *>(iov[i].iov_base) + to_skip;
+      uint16_t len = static_cast<uint16_t>(iov[i].iov_len) - to_skip;
+      std::memcpy(entry->data + write_pos, src, len);
+      write_pos += len;
+      to_skip = 0;
+    }
+  }
+
+  this->tail_ = (this->tail_ + 1) % API_MAX_SEND_QUEUE;
+  this->count_++;
+  return true;
+}
+
+}  // namespace esphome::api
+
+#endif  // USE_API