THRIFT_PORT_OUT env var can be used to find thrift port
THRIFT_PORT_OUT env var can be used to find thrift port

Summary: If a thrift server binary is run with the env var `THRIFT_PORT_OUT=xxxxx` with value of length 5 (enough to store a `uint16_t`), during setup the env var will be updated to be the port on which the thrift server is listening.

Reviewed By: roddym

Differential Revision: D28971248

fbshipit-source-id: cae87750a256abb9d50cd1575b21b05e0327b130

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