[release-1.6] Backport "Try to close race condition in FileWatching tests"

stdlib/FileWatching/src/FileWatching.jl

@@ -254,6 +254,10 @@ mutable struct _FDWatcher
     end
 end
 
+function iswaiting(fwd::_FDWatcher, t::Task)
+    return fwd.notify.waitq === t.queue
+end
+
 mutable struct FDWatcher
     watcher::_FDWatcher
     readable::Bool
@@ -653,10 +657,10 @@ function poll_fd(s::Union{RawFD, Sys.iswindows() ? WindowsRawSocket : Union{}},
     try
         if timeout_s >= 0
             result::FDEvent = FDEvent()
-            timer = Timer(timeout_s) do t
-                notify(wt)
-            end
-            @async begin
+            t = @async begin
+                timer = Timer(timeout_s) do t
+                    notify(wt)
+                end
                 try
                     result = wait(fdw, readable=readable, writable=writable)
                 catch e
@@ -666,6 +670,12 @@ function poll_fd(s::Union{RawFD, Sys.iswindows() ? WindowsRawSocket : Union{}},
                 notify(wt)
             end
             wait(wt)
+            # It's possible that both the timer and the poll fired on the same
+            # libuv loop. In that case, which event we see here first depends
+            # on task schedule order. If we can see that the task isn't waiting
+            # on the file watcher anymore, just let it finish so we can see
+            # the modification to `result`
+            iswaiting(fdw, t) || wait(t)
             return result
         else
             return wait(fdw, readable=readable, writable=writable)

stdlib/FileWatching/test/runtests.jl

@@ -12,7 +12,7 @@ using Base: uv_error, Experimental
 # Writable ends are always tested for write-ability before a write
 
 n = 20
-intvls = [2, .2, .1, .005]
+intvls = [2, .2, .1, .005, .00001]
 
 pipe_fds = fill((Base.INVALID_OS_HANDLE, Base.INVALID_OS_HANDLE), n)
 for i in 1:n