src: avoid copying SEA snapshot data

[colinhacks]

Description

SEA snapshots live in a resource mapped from the executable. Node.js currently copies the V8 startup data out of that resource when it loads the snapshot.

This change lets V8 read the data in place. Snapshots loaded from a file or through the embedder API still use a copy because those buffers may not live long enough.

Benchmark

Median of 240 launches in randomized order, after 30 warmups. Linux x64 on an Intel Xeon Platinum 8481C. Both executables were built from 6f41e415639b5ec3dd816e44945cc73b4d7651e3.

SEA snapshot	Before	After	Change
Empty	28.335 ms	26.610 ms	-1.725 ms (-6.1%)
Computation	28.131 ms	26.491 ms	-1.640 ms (-5.8%)
100,000 objects	56.285 ms	51.541 ms	-4.745 ms (-8.4%)

Validation

• make lint-cpp
• python3 tools/test.py --mode=release sea
• make -j22 test-only

---

AI disclosure: I used a coding agent for the investigation, patch, benchmarks, and PR text. It checked the resource lifetime against the source, built Node.js, and ran the tests above. I approved the PR and will handle review comments myself.

[nodejs-github-bot]

Review requested:

• @nodejs/startup

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[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.99%. Comparing base (46bbfc4) to head (f385b3e).

Files with missing lines	Patch %	Lines
src/node_snapshotable.cc	90.00%	1 Missing and 1 partial ⚠️
src/node.cc	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65876   +/-   ##
=======================================
  Coverage   89.98%   89.99%           
=======================================
  Files         784      784           
  Lines      268361   268373   +12     
  Branches    51117    51123    +6     
=======================================
+ Hits       241478   241512   +34     
+ Misses      17438    17425   -13     
+ Partials     9445     9436    -9     

Files with missing lines	Coverage Δ
src/env.h	97.26% <100.00%> (+0.03%)	⬆️
src/node.cc	76.39% <50.00%> (+0.02%)	⬆️
src/node_snapshotable.cc	73.62% <90.00%> (+0.06%)	⬆️

... and 25 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77203/

[panva]

cc @nodejs/single-executable

[nodejs-github-bot]

Commit Queue failed

   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
   ⚠  Commits were pushed since the last approving review:
   ⚠  - src: avoid copying SEA snapshot data
   ✖  This PR needs to wait 66 more hours to land (or 0 minutes if there is one more approval)
   ✖  GitHub CI is still running
   ⚠  Commits were pushed after the last Full PR CI run:
   ⚠  - src: avoid copying SEA snapshot data

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Full Commit Queue output

- Loading data for nodejs/node/pull/65876
✔  Done loading data for nodejs/node/pull/65876
----------------------------------- PR info ------------------------------------
Title      src: avoid copying SEA snapshot data (#65876)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     colinhacks:src-sea-borrow-snapshot-data -> nodejs:main
Labels     c++, author ready, needs-ci, commit-queue, lacks-second-approval
Commits    1
 - src: avoid copying SEA snapshot data
Committers 1
 - Colin McDonnell <3084745+colinhacks@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/65876
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/65876
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
--------------------------------------------------------------------------------
   ⚠  Commits were pushed since the last approving review:
   ⚠  - src: avoid copying SEA snapshot data
   ℹ  This PR was created on Mon, 07 Sep 2026 11:55:08 GMT
   ✔  Approvals: 1
   ✔  - Filip Skokan (@panva) (TSC): https://github.com/nodejs/node/pull/65876#pullrequestreview-5144522069
   ✖  This PR needs to wait 66 more hours to land (or 0 minutes if there is one more approval)
   ✖  GitHub CI is still running
   ℹ  Last Full PR CI on 2026-09-08T16:48:40Z: https://ci.nodejs.org/job/node-test-pull-request/77203/
   ⚠  Commits were pushed after the last Full PR CI run:
   ⚠  - src: avoid copying SEA snapshot data
- Querying data for job/node-test-pull-request/77203/
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   ✔  Last Jenkins CI successful
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