unikernel/duniverse/dune_/test/blackbox-tests/test-cases/actions/concurrent.t

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+Specification of the concurrency action:
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.7)
+  > EOF
+
+  $ cat > dune << EOF
+  > (rule
+  >  (action
+  >   (concurrent )))
+  > EOF
+
+  $ dune build
+  File "dune", line 3, characters 2-15:
+  3 |   (concurrent )))
+        ^^^^^^^^^^^^^
+  Error: 'concurrent' is only available since version 3.8 of the dune language.
+  Please update your dune-project file to have (lang dune 3.8).
+  [1]
+
+Requires Dune 3.8.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.8)
+  > EOF
+
+(concurrent ...) runs actions concurrently. Here we mock up an example where two
+subactions rely on eachother to also be running in order to terminate.
+
+We write a shell script that will simultaneously read and write to two named
+pipes. (This has to be similtaneious otherwise the read will block the write).
+They will block on the read however, which means if called with the same two
+pipes but swapped, they will only terminate when both scripts are running at the
+same time.
+  $ cat > run.sh << EOF
+  > echo foo>\$1 & read line<\$2
+  > EOF
+
+We create an action that will create named pipes a and b and then run our script
+on both of them, but importantly inside the concurrent action. This will
+demonstrate that subactions are indeed being run concurrently. 
+  $ cat > dune << EOF
+  > (rule
+  >  (deps run.sh)
+  >  (alias my-rule)
+  >  (action
+  >   (progn
+  >    (run mkfifo a b)
+  >    (concurrent
+  >     (run sh run.sh a b)
+  >     (run sh run.sh b a)))))
+  > EOF
+
+When we run the rule, we see that the two actions are indeed run concurrently.
+  $ dune build -j2 @my-rule --force
+
+Notice the need for a -j2. If Dune was configured with -j1 then the action would
+never terminate.