V8 12.0 snapshot version bump breaks embedder upgrades without rebuild

Answer

There is no public API to query the snapshot version that a V8 build expects before you create an Isolate. If a non‑null snapshot_blob is supplied, the API requires it to be valid; it will not silently ignore a mismatched blob.

Confirmed facts

• V8 increments an internal snapshot version with each major release (e.g., 0x1108 in V8 11.8 → 0x1200 in V8 12.0). Snapshot blobs are tied to that version and are not forward‑ or backward‑compatible.
• The version check occurs inside Isolate::CreateParams::snapshot_blob at isolate creation; failure aborts with a "Snapshot version mismatch" error.
• Setting snapshot_blob to nullptr creates a snapshot‑less isolate, which is the only way to fall back when the blob is invalid.
• V8 documentation states snapshots must be regenerated per major release; the project has not introduced a compatibility shim or version‑negotiation layer for snapshots across releases.

Steps to handle the mismatch

1. At embedder startup, obtain the running V8 version string via v8::V8::GetVersionString().
2. Compare that string to the version used when the snapshot was built (available from mksnapshot --version when the blob was generated).
3. If the versions match, proceed with the existing snapshot_blob.
4. If they differ, either rebuild the snapshot against the current V8 library or create the isolate with snapshot_blob = nullptr to fall back to a snapshot‑less isolate.