dfs: improve read path downloader reuse

[Trifocals3537]

📌 Description

• Improves DFS read-path behavior by reusing active downloaders across rapid sequential, nearby, and contiguous reads instead of creating and stopping downloaders too aggressively.

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Target Branch Check (IMPORTANT)

• I confirm this PR is targeting the correct branch

Expected target:

• beta (for features)

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Changes Made

• Reuses active DFS downloaders when reads fall within or near an assigned downloader range.
• Preserves adaptive chunk state across contiguous downloader activity.
• Delays downloader shutdown across short close/reopen gaps to reduce churn from Plex, ffprobe, and seek behavior.
• Adds tracing for downloader lifecycle, upstream fetches, and read amplification.
• Adds focused unit tests for downloader reuse, adaptive state handling, read-ahead behavior, delayed stop handling, and watchdog behavior.

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Testing

• Tested locally

Steps:

1. Rebased branch onto upstream/beta.
2. Ran focused DFS VFS tests: go test ./pkg/mount/dfs/vfs -v
3. Performed live Plex playback testing with remux/media files, including start playback, short playback, forward seek, and backward seek behavior.

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Risks / Notes

• This change is focused on the DFS read path and downloader lifecycle behavior.
• Full go test ./... currently has unrelated failures on upstream/beta in:
  • internal/utils
  • pkg/usenet/fs/reader
• No migrations or config changes.

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Screenshots (if applicable)

N/A

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Checklist

• Code builds successfully
• No console/log errors
• Reviewed my own code
• Target branch is correct

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR improves DFS VFS download behavior by adding shared adaptive chunking across contiguous downloader lifecycles, enhancing downloader reuse heuristics, and introducing a grace period before stopping downloaders when the final file handle closes.

Changes:

• Add shared “adaptive state” so sequential reads don’t re-ramp chunk sizes when a nearby downloader restarts.
• Expand downloader reuse matching and add detailed trace logging for reuse, read-ahead expansion, and upstream fetches.
• Delay stopping downloaders after last handle close via a grace period; add tests for the new behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
pkg/mount/dfs/vfs/downloaders_test.go	Adds tests for adaptive state inheritance/reset decisions, downloader reuse matching, and grace-period stop/cancel behavior.
pkg/mount/dfs/vfs/downloaders.go	Implements shared adaptive state, extends downloader matching to include assigned range, and adds multiple trace events around downloader lifecycle/read-ahead/upstream fetch.
pkg/mount/dfs/vfs/cache.go	Adds delayed downloader shutdown with a grace-period timer, cancellation on reopen/close, and trace logging for cache misses.

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[Trifocals3537]

Quick update on this PR:

I rebased the branch onto the latest upstream/beta and resolved the conflict in downloaders.go.

The main thing I checked during the conflict resolution was making sure we kept both sets of behavior:

• the newer priority downloader behavior from beta
• the read-path reuse/adaptive downloader improvements from this branch

I also added one small safety fix during the merge: priority/probe downloaders now avoid updating the shared adaptive state, so short probe reads should not accidentally reset or ramp down normal sequential playback behavior.

Validation completed:

• Checked for leftover merge conflict markers
• Ran git diff --check
• Ran the targeted DFS/VFS test suite:

go test ./pkg/mount/dfs/vfs -count=1 -v

Result: targeted DFS/VFS tests passed.

Note: I’m not claiming a full go test ./... pass here because the latest beta baseline has unrelated test failures outside this PR’s DFS/VFS changes.

[Trifocals3537]

Updated the PR based on the review feedback.

Changes made:

• Made the release stop grace period atomic-backed for safer test overrides.
• Added a test helper to restore the previous grace period value after each test.
• Increased the grace-period test timeouts to reduce flakiness.
• Renamed adaptiveStateForNewDownloader to adaptiveStateForNewDownloaderLocked.
• Preserved priority downloader behavior so priority reads do not inherit or update shared adaptive state.

Validated locally:

• git diff --check
• conflict-marker grep across changed files
• go test ./pkg/mount/dfs/vfs/... -race -count=1 -v

All passed locally.

[Trifocals3537]

Added one more lock-order fix so StopDownloaders() runs after releasing handleMu.

Validated locally with:

• git diff --check
• conflict-marker grep across changed files
• go test ./pkg/mount/dfs/vfs/... -race -count=1 -v

All passed locally.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.