[api] Reuse overflow buffer storage instead of allocating per stalled write (#19093)

tests/components/api/test_api_buffer.cpp

@@ -0,0 +1,65 @@
+#include <gtest/gtest.h>
+
+#include <cstdint>
+#include <cstring>
+
+#include "esphome/components/api/api_buffer.h"
+
+namespace esphome::api::testing {
+
+// Pointer plus two 16 bit sizes
+static_assert(sizeof(APIBuffer) <= 2 * sizeof(void *));
+
+TEST(APIBuffer, RefusesSizesAbove16Bits) {
+  APIBuffer buf;
+  ASSERT_TRUE(buf.resize(16));
+  EXPECT_FALSE(buf.reserve(UINT16_MAX + 1));
+  EXPECT_EQ(buf.size(), 16u);
+  EXPECT_EQ(buf.capacity(), 16u);
+  EXPECT_TRUE(buf.reserve(UINT16_MAX));
+  EXPECT_EQ(buf.capacity(), UINT16_MAX);
+}
+
+static const uint8_t BYTES[] = {1, 2, 3, 4, 5, 6};
+
+TEST(APIBuffer, AppendReturnsTheNewBytes) {
+  APIBuffer buf;
+  ASSERT_TRUE(buf.reserve(8));
+  uint8_t *first = buf.append(3);
+  ASSERT_NE(first, nullptr);
+  std::memcpy(first, BYTES, 3);
+  EXPECT_EQ(buf.size(), 3u);
+  EXPECT_EQ(buf.capacity(), 8u);
+
+  // Grows through realloc and keeps what was there
+  uint8_t *second = buf.append(6);
+  ASSERT_EQ(second, buf.data() + 3);
+  std::memcpy(second, BYTES + 3, 3);
+  EXPECT_EQ(buf.size(), 9u);
+  EXPECT_EQ(buf.capacity(), 9u);
+  EXPECT_EQ(std::memcmp(buf.data(), BYTES, 6), 0);
+}
+
+TEST(APIBuffer, DropFrontSlidesTheRestDown) {
+  APIBuffer buf;
+  uint8_t *bytes = buf.append(6);
+  ASSERT_NE(bytes, nullptr);
+  std::memcpy(bytes, BYTES, 6);
+
+  buf.drop_front(2);
+  EXPECT_EQ(buf.size(), 4u);
+  EXPECT_EQ(buf.capacity(), 6u);
+  EXPECT_EQ(std::memcmp(buf.data(), BYTES + 2, 4), 0);
+
+  // Growing afterwards keeps the slid bytes
+  ASSERT_TRUE(buf.reserve(64));
+  EXPECT_EQ(buf.size(), 4u);
+  EXPECT_EQ(std::memcmp(buf.data(), BYTES + 2, 4), 0);
+
+  // Dropping everything leaves an empty buffer with its capacity
+  buf.drop_front(4);
+  EXPECT_EQ(buf.size(), 0u);
+  EXPECT_EQ(buf.capacity(), 64u);
+}
+
+}  // namespace esphome::api::testing