[Varnish] #1759: VMOD abi check is always lenient

[Varnish]

#1759: VMOD abi check is always lenient
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 Reporter:  lkarsten  |      Owner:
     Type:  defect    |     Status:  new
 Priority:  normal    |  Milestone:
Component:  varnishd  |    Version:  trunk
 Severity:  normal    |   Keywords:
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 In 4.0, the VMOD version check is currently always a check against the
 VRT_(MAJOR|MINOR)_VERSION fields.

 If I write a VMOD that uses internal structures (like
 ctx->bo->synth_body), and Martin goes off and add something further up in
 to struct busyobj (or struct http_conn, to pick something "completely at
 random"), the VMOD compiled for the old version goes sideways when loaded
 into the new version.

 There is no way for the VMOD writer to signal it, and there is no error
 indication for the sysadmin.

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1759>
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