fixes nim-lang#5091; Ensure we don't wait on an exited process

lib/pure/osproc.nim

@@ -1383,7 +1383,8 @@ elif not defined(useNimRtl):
         s.tv_sec = b.tv_sec
         s.tv_nsec = b.tv_nsec
 
-      if p.exitFlag:
+      # Ensure we don't wait on an exited process
+      if not(p.running()) and p.exitFlag:
         return exitStatusLikeShell(p.exitStatus)
 
       if timeout == -1:

tests/osproc/twaitforexit.nim

@@ -0,0 +1,18 @@
+import std/[osproc, os, times]
+
+block: # bug #5091
+    when defined(linux):
+        const filename = "false"
+        var p = startProcess(filename, options = {poStdErrToStdOut, poUsePath})
+        os.sleep(1000) # make sure process has exited already
+
+        let atStart = getTime()
+        const msWait = 2000
+
+        try:
+            discard waitForExit(p, msWait)
+        except OSError:
+            echo getCurrentExceptionMsg()
+
+        # check that we don't have to wait msWait milliseconds
+        doAssert(getTime() <  atStart + milliseconds(msWait))