Use TMPDIR env var if available to get the temp dir name (#11443) [bugfix]

Additionally, use normalizePathEnd to suffix the dir name with "/" or
"\" as appropriate for the current OS.

Fixes https://github.com/nim-lang/Nim/issues/11439.

lib/pure/os.nim

@@ -798,13 +798,16 @@ proc getTempDir*(): string {.rtl, extern: "nos$1",
   ## * `expandTilde proc <#expandTilde,string>`_
   ## * `getCurrentDir proc <#getCurrentDir>`_
   ## * `setCurrentDir proc <#setCurrentDir,string>`_
-  const tempDirDefault = "/tmp/"
+  const tempDirDefault = "/tmp"
+  result = tempDirDefault
   when defined(tempDir):
     const tempDir {.strdefine.}: string = tempDirDefault
-    return tempDir
-  elif defined(windows): return string(getEnv("TEMP")) & "\\"
-  elif defined(android): return getHomeDir()
-  else: return tempDirDefault
+    result = tempDir
+  elif defined(windows): result = string(getEnv("TEMP"))
+  elif defined(android): result = getHomeDir()
+  else:
+    if existsEnv("TMPDIR"): result = string(getEnv("TMPDIR"))
+  normalizePathEnd(result, trailingSep=true)
 
 proc expandTilde*(path: string): string {.
   tags: [ReadEnvEffect, ReadIOEffect].} =

tests/stdlib/tos.nim

@@ -333,3 +333,15 @@ block ospaths:
   doAssert joinPath("", "lib") == "lib"
   doAssert joinPath("", "/lib") == unixToNativePath"/lib"
   doAssert joinPath("usr/", "/lib") == unixToNativePath"usr/lib"
+
+block getTempDir:
+  block TMPDIR:
+    # TMPDIR env var is not used if either of these are defined.
+    when not (defined(tempDir) or defined(windows) or defined(android)):
+      if existsEnv("TMPDIR"):
+        let origTmpDir = getEnv("TMPDIR")
+        putEnv("TMPDIR", "/mytmp")
+        doAssert getTempDir() == "/mytmp/"
+        putEnv("TMPDIR", origTmpDir)
+      else:
+        doAssert getTempDir() == "/tmp/"