fix(bootstrap): keep snapshot progress bar live during ranged download

[scarmuega]

Problem

Follow-up to #1025 (ranged ring-buffer snapshot download). During a bootstrap the progress bar looked frozen even though the download was clearly progressing.

Root cause

In the ranged downloader, progress was advanced once per completed 64 MiB chunk. So the bar sat motionless for seconds at a time while a chunk downloaded, then jumped — the opposite of the old single-stream path, which incremented on every small read and therefore looked smooth.

Fix

• Increment per read (256 KiB granularity) as bytes arrive within each chunk, instead of once per finished chunk.
• Roll back a failed attempt's bytes before retrying, so a retried chunk re-downloads its range without the bar over-counting past the total.
• Enable a steady tick (120 ms) so the bar keeps redrawing even while the downloader is legitimately paused on window backpressure (waiting for the extractor to free a slot).

Verification

• cargo build + cargo clippy clean.
• Live-endpoint e2e test (ranged_matches_direct_fetch, --ignored) passes, including its progress.position() == total assertion — confirming per-read increments sum exactly to the total with no double-count.

🤖 Generated with Claude Code

Summary by CodeRabbit

Bug Fixes

• Download progress bars now continuously update in real-time as data streams during ranged requests, replacing batch updates after completion.
• Progress displays remain responsive during snapshot downloads, preventing the UI from appearing frozen during system backpressure handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d3bbe3fa-05fd-4b3b-92a7-868b0ba72165

📥 Commits

Reviewing files that changed from the base of the PR and between 0452ec2 and e6bda99.

📒 Files selected for processing (2)

• src/bin/dolos/bootstrap/ranged.rs
• src/bin/dolos/bootstrap/snapshot.rs

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📝 Walkthrough

Walkthrough

The ranged download helper try_download_chunk is updated to accept a ProgressBar, increment it per received byte-chunk during streaming, and roll back the progress position by the bytes written on failure before retrying. download_chunk removes its post-success increment and passes the progress handle down. In snapshot.rs, enable_steady_tick(120ms) is added to keep the progress bar redrawing during extractor backpressure.

Changes

Ranged download progress bar improvements

Layer / File(s)	Summary
Streaming progress accounting with retry rollback src/bin/dolos/bootstrap/ranged.rs	try_download_chunk now accepts a ProgressBar, increments it per received chunk, and rolls back (saturating_sub(written)) on error before returning failure. download_chunk's retry loop passes the progress handle down and removes its prior success-only increment. Doc comments updated to describe rollback semantics.
Steady tick for backpressure pauses src/bin/dolos/bootstrap/snapshot.rs	fetch_snapshot_ranged calls progress.enable_steady_tick(Duration::from_millis(120)) so the progress bar continues redrawing while the download thread blocks on extractor backpressure.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• txpipe/dolos#1025: Introduced the ranged ring-buffer downloader logic in ranged.rs, including the original download_chunk/try_download_chunk structure that this PR modifies for per-chunk progress accounting.

Poem

🐇 Hoppity-hop through the byte stream I go,
Watching the progress bar put on a show!
Each chunk ticks the bar, no more sudden jump,
On failure I roll back — no misleading lump.
Steady tick keeps the spinner alive,
Even when backpressure says "don't you arrive!"
A rabbit-approved download, smooth as a dive. 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: keeping the snapshot progress bar responsive during ranged downloads by adding steady tick mechanism and per-read increments.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/bootstrap-progress-bar

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