[webfansplz]

Summary

Opening a Rolldown session in DevTools reads the build's logs.json, which gets very large for big projects. On a lobehub SPA build (~29k modules, 6.9M events, 4 GB log), reopening an already-parsed session still took almost a second and blocked the DevTools server in one ~700 ms chunk. This PR makes reopening near-instant and keeps the server responsive while large logs are read.

Changes

• Summary restore skips the reader index. Restoring a session summary used to parse every sidecar cache, including the reader index (148 MB). It now reads only session-summary.json and the module metrics the module list and chunk sizes rely on (41 MB); the index is loaded on the first full read (module or plugin details).
• No duplicate cache write. A summary write already writes every sidecar, so the following full read no longer writes the same files again.
• Binary reader index. reader-index.json is replaced by reader-index.bin (new reader-index-codec.ts). Encoding and decoding yield to the event loop regularly, offsets are stored as f64 so logs over 4 GB work, and a header with the log size and mtime rejects stale or truncated files. The cache version is bumped to 3, so existing caches are rebuilt.
• Time-sliced parsing. Reading logs.json yields to the event loop about every 8 ms instead of once per 1 MB chunk.

Impact

Measured locally on the lobehub log above (the build's log writer timed out on the last line, so the session is truncated by one event):

	Before	After
Reopen session (summary, cache present)	0.83 s, longest stall 710 ms, heap 369 MB	0.11 s, longest stall 15 ms, heap 66 MB
First full read after reopening (details)	included above	0.35 s, longest stall 58 ms
Cold open (no cache)	14.7 s, longest stall 579 ms	~15.5 s, longest stall 175 ms
Full read right after a cold open	1.1 s, longest stall 494 ms	0 s
Reader index cache	148 MB	77 MB

Cold open is ~5% slower in total because parsing now yields, in exchange for the server staying responsive. Its time is dominated by per-line JSON.parse, GC, and building metrics; skipping large content strings and buffer-based action detection were tried and were slower, so they aren't included.

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commit: 1d3a5ad

[Copilot]

🟡 Changes recommended

Restoring cached sessions without module metrics hides metrics and plugin badges and produces zero module-based sizes, while later full reads do not refresh the affected client data.

1 open issue

• Preserve module metrics when restoring cached sessions · New

What changed in this PR

This PR reduces Rolldown session reopening time and improves DevTools server responsiveness when processing large build logs.

Changes:

• Defers reader-index and module-metrics loading until a full read and avoids duplicate cache writes.
• Adds a binary reader-index cache with log validation and support for offsets beyond 4 GB.
• Adds time-sliced parsing and tests for codec integrity and summary-to-full restoration.

File	Description
packages/​rolldown/​src/​node/​rolldown/​reader-index-codec.ts	Adds binary index encoding and decoding with periodic yields.
packages/​rolldown/​src/​node/​rolldown/​log-cache.ts	Integrates binary caches and lightweight summary restoration.
packages/​rolldown/​src/​node/​rolldown/​events-reader.ts	Updates summary state handling and yields during parsing.
packages/​rolldown/​src/​node/​rolldown/​__tests__/​reader-index-cache.test.ts	Tests codec round trips, invalid data rejection, and cache restoration.

_{Review effort: Balanced}