Vine: Serverless Library Protocol

[dthain]

(Note this is relative to PR #3436 that hasn't been merged yet.)

The protocol between the worker and the serverless library has several infelicities that require a coordinated fix between the library code and vine_process_invoke_function

• [x] The output of each function invocation is written as a single line with a maximum of VINE_LINE_MAX. It should instead be written as an integer length followed by the actual data. (Similar to that of the function input.)
• [x] The function invocation has a default timeout of 5 minutes. The should be configurable using an existing property of a standard task, like the maximum run time.
• [x] vine_process_invoke_function assumes that the input is text and gets the length with strlen. Measure the size of the file instead so that we can send arbitrary data.
• [ ] The library startup message is very minimal JSON. Let's add in a few things there for error checking, such as a protocol number, taskvine ID, whether it is capable of forking, etc.
• [x] The manager should send the library a configuration message indicating the expected resources (cores, memory, disk) that the library has been assigned.
• [ ] The expectations of the protocol should be documented as an appendix to the TaskVine manual, so that others can implement libraries regardless of the language.

[dthain]

3436 has been merged so this now describes the master branch.

[dthain]

@tphung3 please take a crack at this one. In the current state, I think serverless is working for functions that don't violate any of the constraints above, but you should do some testing of your own. These mixups should make the system much more robust.

[dthain]

@JinZhou5042 fixed bullet 5 in #3764