popen: accept a negative timeout and interpret it as zero

[Totktonada]

Sometimes it is convenient to perform a substraction like deadline - now() and pass the resulting value as a timeout. I would expect the following semantic: if timeout <= 0 then perform an action if it is ready-to-go (would not trigger EWOULDBLOCK and fiber yield). Otherwise, return a timeout error.

popen now interprets any negative timeout value as an error.

tarantool/src/lua/popen.c

Lines 285 to 297 in 927e351

	/**
	* Extract a timeout value from the Lua stack.
	*
	* Return -1.0 when error occurs.
	*/
	static ev_tstamp
	luaT_check_timeout(struct lua_State *L, int idx)
	{
	if (lua_type(L, idx) == LUA_TNUMBER)
	return lua_tonumber(L, idx);
	/* FIXME: Support cdata<int64_t> and cdata<uint64_t>. */
	return -1.0;
	}

tarantool/src/lua/popen.c

Lines 1743 to 1745 in 927e351

	if (!lua_isnil(L, -1) &&
	(timeout = luaT_check_timeout(L, -1)) < 0.0)
	goto usage;

The list of affected methods:

ph:wait({timeout = -1})
ph:read({timeout = -1})
ph:write({timeout = -1})

[sergos]

It is definitely not a bug, rather a feature request (semantic change even).
We also didn't come to a resolution how it should behave in this case - it's not clear should a popen call fails with a timeout if it takes longer than proposed timeout to execute. Say, popen.new('...', {timeout=0})

[olegrok]

See also #7970

[Totktonada]

@olegrok Thanks for pointing it out!

@sergos OK, let's call it a feature. I don't care much in this case.

popen.new() has no timeout and it possibly shouldn't. I want to clarify that we're talking about calls that wait for some externally produced event: for example, arriving data from another process.

Let's take :read() as an example.

If the call has zero timeout (or a negative timeout), it should return immediately, where it means don't wait for an external event. It is OK to do some (relatively) fast local work like a buffer copying.

Let's assume the data is already there and the call can return immediately with the new data or report a timeout error. In this case, it should return the data.

In other works, zero/negative timeout means "don't wait for data arriving" in this case.

Similarly, it means "don't wait for a free space in the pipe buffer" for :write().

And, finally, it means "don't wait for the process termination" for :wait() (buf it is OK to send a zero signal and look on its result).