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On 2018-12-17 2:51 p.m., Andrew Gierth wrote:
"Roberto" == Roberto Ierusalimschy <> writes:
  >> The problem for me is that if the Lua code does anything that raises
  >> a PostgreSQL error, I have to limit what further calls happen to
  >> other backend functions until some subtransaction level that
  >> encloses the error is properly aborted and released. At present, I
  >> can do that easily because Lua code has no way to catch errors
  >> except for pcall(), and I replace the normal pcall() function with
  >> one that creates a subtransaction, lua_pcall's the specified
  >> function, and releases the subtransaction on either return or error
  >> (and before calling the supplied error handler in the case of
  >> xpcall). This guarantees that the Lua code can not do anything
  >> unsafe in between the generation of the error and its being cleaned
  >> up.

  Roberto> What about Sony's suggestion, of running the sandboxed code
  Roberto> inside a coroutine and using yields to stop the code? (BTW,
  Roberto> that seems a good practice for any sandbox, independently of
  Roberto> this to-be-closed issue.)

I'm not sure that helps. It certainly presents a problem of how to
unwind the C stack safely, especially in the case where Lua code has
called a C function that calls a Lua callback.

where you'd use pcall/error you use coroutine.create+pcallk/yield instead. when it yields, throw it away, and you never(?) run the toclose that way.