[nrf fromtree] storage/stream_flash: Add API to query buffered data size

include/zephyr/storage/stream_flash.h

@@ -106,6 +106,15 @@ int stream_flash_init(struct stream_flash_ctx *ctx, const struct device *fdev,
  */
 size_t stream_flash_bytes_written(const struct stream_flash_ctx *ctx);
 
+/**
+ * @brief Read number of bytes buffered for the next flash write.
+ *
+ * @param ctx context
+ *
+ * @return Number of payload bytes buffered for the next flash write.
+ */
+size_t stream_flash_bytes_buffered(const struct stream_flash_ctx *ctx);
+
 /**
  * @brief Process input buffers to be written to flash device in single blocks.
  * Will store remainder between calls.

subsys/storage/stream/stream_flash.c

@@ -307,6 +307,11 @@ size_t stream_flash_bytes_written(const struct stream_flash_ctx *ctx)
 	return ctx->bytes_written;
 }
 
+size_t stream_flash_bytes_buffered(const struct stream_flash_ctx *ctx)
+{
+	return ctx->buf_bytes;
+}
+
 #ifdef CONFIG_STREAM_FLASH_INSPECT
 struct _inspect_flash {
 	size_t buf_len;

tests/subsys/storage/stream/stream_flash/src/main.c

@@ -276,6 +276,42 @@ ZTEST(lib_stream_flash, test_stream_flash_bytes_written)
 	VERIFY_WRITTEN(BUF_LEN, BUF_LEN);
 }
 
+ZTEST(lib_stream_flash, test_stream_flash_bytes_buffered)
+{
+	int rc;
+	size_t buffered;
+
+	init_target();
+
+	/* Initially no bytes should be buffered */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 0, "expected no buffered bytes");
+
+	/* Write partial buffer */
+	rc = stream_flash_buffered_write(&ctx, write_buf, BUF_LEN - 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* Verify buffered bytes */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, BUF_LEN - 128, "expected buffered bytes");
+
+	/* Write remaining buffer */
+	rc = stream_flash_buffered_write(&ctx, write_buf, 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* After auto-flush, no bytes should be buffered */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 0, "expected no buffered bytes");
+
+	/* Write more than buffer size to trigger auto-flush */
+	rc = stream_flash_buffered_write(&ctx, write_buf, BUF_LEN + 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* Verify buffered bytes */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 128, "expected remaining buffered bytes after auto-flush");
+}
+
 ZTEST(lib_stream_flash, test_stream_flash_buf_size_greater_than_page_size)
 {
 	int rc;