Fix three reliability issues: request hangs, infinite retries, malformed PDFs

[chen3feng]

Summary

Three independent robustness fixes found while translating real-world PDFs with the deepseek/openai services. Each one could make a run hang forever or crash outright. All three are validated end-to-end (a 114-page and a 47-page paper now translate cleanly; the 47-page one previously hung 100% of the time on its last page).

Changes

1. translator.py — add a request timeout to the OpenAI client

openai.OpenAI(...) was created without a timeout, so a single request that silently stalls (network blip / unresponsive endpoint) blocks the entire document indefinitely (0% CPU, no error, no progress). Added timeout (default 120s, override via PDF2ZH_OPENAI_TIMEOUT) and max_retries=2, so a stalled request fails fast and is retried instead of hanging forever.

2. converter.py — bound paragraph translation retries

worker() used @retry(wait=wait_fixed(1)) with no stop condition, so a permanently-failing paragraph retried every second forever and the whole run got stuck. Added stop_after_attempt (default 5, override via PDF2ZH_TRANSLATE_RETRIES) and a retry_error_callback that falls back to the source text on exhaustion, so the document still completes instead of stalling.

3. high_level.py — auto-repair malformed input PDFs

Some structurally broken PDFs make MuPDF raise during the initial doc_en.save() (cannot parse object (N 0 R)), aborting before translation even starts. Caught that failure and repaired the input with pikepdf (already a dependency, already imported in this module) before retrying.

Notes

• All three are opt-out-safe: behavior is unchanged on healthy inputs, and the two new knobs are environment variables with sensible defaults.
• No new dependencies (pikepdf was already required).

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