fix(async): chiamate bloccanti in asyncio.to_thread (#36 punto 1)

kaleido (pio.to_image), WeasyPrint (write_pdf), pdfplumber (extract_text_from_pdf)
e la scrittura del CSV di produzione giravano sincrone dentro handler async:
un PDF pesante o un run grande fermava il worker per tutte le altre richieste.
Ora avvolte in await asyncio.to_thread(...) in report_service.py,
ai_service.py e production_export_service.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/services/ai_service.py

@@ -1,4 +1,5 @@
 """AI service for parsing technical sheets via OpenRouter."""
+import asyncio
 import json
 import logging
 from io import BytesIO
@@ -36,7 +37,9 @@ def extract_text_from_pdf(pdf_bytes: bytes) -> str:
 
 
 async def parse_technical_sheet(pdf_bytes: bytes) -> list[dict]:
-    text = extract_text_from_pdf(pdf_bytes)
+    # pdfplumber parses synchronously and can take a while on a big PDF: off
+    # the event loop, or one upload stalls every other request being served.
+    text = await asyncio.to_thread(extract_text_from_pdf, pdf_bytes)
     if not text.strip():
         return []
 

src/backend/services/production_export_service.py

@@ -9,6 +9,7 @@ The CSV uses the delimiters configured in system_settings, the same ones the man
 export honours, so a shop floor set to the Italian convention (';' and ',') gets
 files that open correctly in their Excel.
 """
+import asyncio
 import csv
 import io
 import logging
@@ -192,7 +193,11 @@ async def export_run_statistics(db: AsyncSession, run: ProductionRun) -> str | N
 
     stamp = (run.closed_at or datetime.now()).strftime("%Y%m%d_%H%M%S")
     filename = f"production_{run.id}_{stamp}.csv"
-    (directory / filename).write_text(content, encoding="utf-8-sig")
+    # A large run's file can take a moment to write: off the event loop, so
+    # closing one run does not stall every other request being served.
+    await asyncio.to_thread(
+        (directory / filename).write_text, content, encoding="utf-8-sig"
+    )
 
     # utf-8-sig: the BOM is what makes Excel read accented characters correctly,
     # and these files are opened in Excel on the shop floor.

src/backend/services/report_service.py

@@ -1,4 +1,5 @@
 """Report generation service — PDF reports via Jinja2 + Plotly/Kaleido + WeasyPrint."""
+import asyncio
 import base64
 from datetime import datetime
 from pathlib import Path
@@ -204,6 +205,11 @@ def _generate_control_chart_svg(chart_data, subtask_info: dict | None) -> str:
     return svg_bytes.decode("utf-8")
 
 
+def _render_pdf(html_content: str) -> bytes:
+    """Blocking WeasyPrint render — always call via asyncio.to_thread."""
+    return HTML(string=html_content).write_pdf()
+
+
 def _generate_histogram_svg(hist_data, subtask_info: dict | None) -> str:
     """Generate histogram SVG using Plotly + Kaleido."""
     if not hist_data.bins:
@@ -300,9 +306,13 @@ async def generate_spc_report(
     )
     histogram = compute_histogram(values)
 
-    # 3. Generate SVG charts
-    control_chart_svg = _generate_control_chart_svg(control_chart, subtask_info)
-    histogram_svg = _generate_histogram_svg(histogram, subtask_info)
+    # 3. Generate SVG charts (kaleido shells out and blocks: off the event loop)
+    control_chart_svg = await asyncio.to_thread(
+        _generate_control_chart_svg, control_chart, subtask_info
+    )
+    histogram_svg = await asyncio.to_thread(
+        _generate_histogram_svg, histogram, subtask_info
+    )
 
     # 4. Get company info and recipe info
     company = await _get_company_info(db)
@@ -336,8 +346,8 @@ async def generate_spc_report(
         n_measurements=len(measurements),
     )
 
-    # 7. Convert HTML to PDF
-    pdf_bytes = HTML(string=html_content).write_pdf()
+    # 7. Convert HTML to PDF (WeasyPrint blocks: off the event loop)
+    pdf_bytes = await asyncio.to_thread(_render_pdf, html_content)
     return pdf_bytes
 
 
@@ -424,6 +434,6 @@ async def generate_measurement_report(
         n_measurements=len(measurements),
     )
 
-    # 8. Convert to PDF
-    pdf_bytes = HTML(string=html_content).write_pdf()
+    # 8. Convert to PDF (WeasyPrint blocks: off the event loop)
+    pdf_bytes = await asyncio.to_thread(_render_pdf, html_content)
     return pdf_bytes