backend: create endpoint for downloading data created by cron

This solution raises two concerns for the future once we have a
lot of data collected:
- it will be a while until the data will be tarred - creating delay before
  actual download (after cca 200 feedbacks relly noticable delay :/)
  look at example with only 300 (tiny!!) feedbacks:
  [root@backend persistent]# time tar -zcf results.tar.gz results/
  real m9.473s
  user m9.328s
  sys m0.483s
- downloading takes also some time

-> thus blocking the whole worker during this. IIRC we have 8 workers thus
8 downloads and API is unresponsive.

Solution:
- how to solve the delay before download:
  fedora-copr#64
- the issue above is the slowest, once that will be resolved and people
  still complain, do this:
  fedora-copr#65

backend/api.py

@@ -2,11 +2,14 @@
 import logging
 import os
 from base64 import b64decode
+from datetime import datetime
 from http import HTTPStatus
+from pathlib import Path
+from typing import Iterator
 
-from fastapi import FastAPI, Request
+from fastapi import FastAPI, Request, Depends
 from fastapi.exceptions import RequestValidationError
-from fastapi.responses import HTMLResponse, JSONResponse
+from fastapi.responses import HTMLResponse, JSONResponse, StreamingResponse
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 from pydantic import BaseModel
@@ -33,6 +36,7 @@
     FeedbackSchema,
     schema_inp_to_out,
 )
+from backend.spells import make_tar, get_temporary_dir
 from backend.store import Storator3000
 
 logger = logging.getLogger(__name__)
@@ -263,3 +267,35 @@ def frontend_review_latest() -> FeedbackSchema:
     with open(feedback_file) as file:
         content = json.loads(file.read())
         return FeedbackSchema(**content)
+
+
+def _make_tpm_tar_file_from_results() -> Iterator[Path]:
+    results = os.environ.get("FEEDBACK_DIR")
+    if results is None:
+        raise HTTPException(status_code=HTTPStatus.NOT_FOUND, detail="No data found")
+
+    with get_temporary_dir() as tmp_dir:
+        tar_path = make_tar(
+            f"results-{int(datetime.now().timestamp())}.tar.gz", Path(results), tmp_dir
+        )
+        try:
+            yield tar_path
+        finally:
+            os.unlink(tar_path)
+
+
+@app.get("/download", response_class=StreamingResponse)
+def download_results(_tar_path=Depends(_make_tpm_tar_file_from_results)):
+    def iter_large_file(file_name: Path):
+        with open(file_name, mode="rb") as file:
+            yield from file
+
+    return StreamingResponse(
+        iter_large_file(_tar_path),
+        media_type="application/x-tar",
+        # TODO: https://github.com/fedora-copr/log-detective-website/issues/63
+        headers={
+            "Content-Disposition": f"attachment; filename={_tar_path.name}",  # noqa: E702
+            "Content-Length": str(_tar_path.stat().st_size),
+        },
+    )

backend/spells.py

@@ -3,6 +3,7 @@
 """
 
 import shutil
+import tarfile
 import tempfile
 from contextlib import contextmanager
 from pathlib import Path
@@ -16,3 +17,22 @@ def get_temporary_dir() -> Iterator[Path]:
         yield temp_dir
     finally:
         shutil.rmtree(temp_dir)
+
+
+def make_tar(name: str, source: Path, destination: Path) -> Path:
+    """
+    Make tar from source path.
+
+    Args:
+        name: Name of the tar file
+        source: Source to be tarred
+        destination: Folder where to put tar file
+
+    Returns:
+        Path where to find a tar file.
+    """
+    tar_path = destination / name
+    with tarfile.open(tar_path, "w:gz") as tar_f:
+        tar_f.add(source, arcname=name)
+
+    return tar_path