Function presign_get 

pub fn presign_get(
    ctx: &Ctx,
    attachment: &Attachment,
    disposition: Disposition,
    expires_in: u64,
) -> Result<String>

Presigns a GET of an attachment on R2’s S3 API, valid expires_in seconds (at most 7 days).

The browser downloads straight from R2 (https://<account>.r2.cloudflarestorage.com/...), not through the Worker. R2 answers with the same safe Content-Type and Content-Disposition as serve (the URL carries response-content-type and response-content-disposition). Anyone with the URL can download the file until it expires: authorize before presigning and keep lifetimes short. serve_redirect answers a redirect to it.

Reads the R2_ACCOUNT_ID and R2_BUCKET variables and the R2_ACCESS_KEY_ID and R2_SECRET_ACCESS_KEY secrets. Signing is local: no R2 operation, microseconds of CPU; the download is one class B operation. In ocre dev the URL points to the real bucket, not the local simulation.

Errors

Error::Internal (500) when one of the four settings is missing (the message names them and how to set them), or expires_in is 0 or over 7 days.

Examples

use axum::{Json, extract::{Path, State}};
use ocre::storage::{self, Attachment, Disposition};
use ocre::{Ctx, OptionExt, Result, params};

// GET /api/documents/{id}/download_url: a link valid 5 minutes.
async fn download_url(State(ctx): State<Ctx>, Path(id): Path<i64>) -> Result<Json<String>> {
    let sql = "SELECT file_key AS key, file_filename AS filename, file_content_type AS content_type, \
               file_size AS size FROM documents WHERE id = ?1 AND file_key IS NOT NULL";
    let file: Attachment = ctx.db()?.first(sql, params![id]).await?.or_404()?;
    Ok(Json(storage::presign_get(&ctx, &file, Disposition::Download, 300)?))
}