osproc: fix double close on POSIX (#5724)

Calling close() in some cases issued two close() syscalls to one FD, which is incorrect in multithreaded programs.

lib/pure/osproc.nim

@@ -1001,12 +1001,20 @@ elif not defined(useNimRtl):
   {.pop}
 
   proc close(p: Process) =
-    if p.inStream != nil: close(p.inStream)
-    if p.outStream != nil: close(p.outStream)
-    if p.errStream != nil: close(p.errStream)
     if poParentStreams notin p.options:
+      if p.inStream != nil:
+        close(p.inStream)
+      else:
         discard close(p.inHandle)
+
+      if p.outStream != nil:
+        close(p.outStream)
+      else:
         discard close(p.outHandle)
+
+      if p.errStream != nil:
+        close(p.errStream)
+      else:
         discard close(p.errHandle)
 
   proc suspend(p: Process) =

tests/osproc/tclose.nim

@@ -0,0 +1,24 @@
+discard """
+  exitcode: 0
+"""
+
+when defined(linux):
+  import osproc, os
+
+  proc countFds(): int =
+    result = 0
+    for i in walkDir("/proc/self/fd"):
+      result += 1
+
+  let initCount = countFds()
+
+  let p = osproc.startProcess("echo", options={poUsePath})
+  assert countFds() == initCount + 3
+  p.close
+  assert countFds() == initCount
+
+  let p1 = osproc.startProcess("echo", options={poUsePath})
+  discard p1.inputStream
+  assert countFds() == initCount + 3
+  p.close
+  assert countFds() == initCount