SI-10007 sys.process thread sync

A previous change to replace `SyncVar.set` with `SyncVar.put`
breaks things.

This commit tweaks the thread synchronizing in `sys.process`
to actually use `SyncVar` to sync and pass a var.

Joining the thread about to exit is superfluous.

A result is put exactly once, and consumers use
non-destructive `get`.

Note that as usual, avoid kicking off threads in a static
context, since class loading cycles are somewhat dicier
with 2.12 lambdas. In particular, REPL is a static context
by default.

SI-10007 Clarify deprecation message

The message on `set` was self-fulfilling, as it didn't
hint that `put` has different semantics.

So explain why `put` helps avoid errors instead of
creating them.

SI-10007 Always set exit value

Always put a value to exit code, defaulting to None.

Also clean up around tuple change to unfortunately
named Future.apply. Very hard to follow those types.

Date command pollutes output, so tweak test.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/library/scala/concurrent/SyncVar.scala b/src/library/scala/concurrent/SyncVar.scala
index 5fabf553bd..0e534a9b22 100644
--- a/src/library/scala/concurrent/SyncVar.scala
+++ b/src/library/scala/concurrent/SyncVar.scala
@@ -91,7 +91,7 @@ class SyncVar[A] {
   // [Heather] the reason why: it doesn't take into consideration
   // whether or not the SyncVar is already defined. So, set has been
   // deprecated in order to eventually be able to make "setting" private
-  @deprecated("use `put` instead, as `set` is potentially error-prone", "2.10.0")
+  @deprecated("use `put` to ensure a value cannot be overwritten without a corresponding `take`", "2.10.0")
   // NOTE: Used by SBT 0.13.0-M2 and below
   def set(x: A): Unit = setVal(x)
 
@@ -111,7 +111,7 @@ class SyncVar[A] {
   // [Heather] the reason why: it doesn't take into consideration
   // whether or not the SyncVar is already defined. So, unset has been
   // deprecated in order to eventually be able to make "unsetting" private
-  @deprecated("use `take` instead, as `unset` is potentially error-prone", "2.10.0")
+  @deprecated("use `take` to ensure a value is never discarded", "2.10.0")
   // NOTE: Used by SBT 0.13.0-M2 and below
   def unset(): Unit = synchronized {
     isDefined = false