Unset TMPDIR for tempname

base/file.jl

@@ -581,10 +581,16 @@ else # !windows
 # Obtain a temporary filename.
 function tempname(parent::AbstractString=tempdir(); cleanup::Bool=true)
     isdir(parent) || throw(ArgumentError("$(repr(parent)) is not a directory"))
-    p = ccall(:tempnam, Cstring, (Cstring, Cstring), parent, temp_prefix)
-    systemerror(:tempnam, p == C_NULL)
-    s = unsafe_string(p)
-    Libc.free(p)
+    # If TMPDIR is set libc's `tempnam` will ignore `parent` as an argument (#38873)
+    # So we unset the environment variable for the call to libc, after checking
+    # in `tempdir` for the default argument.
+    s = withenv("TMPDIR" => nothing) do
+        p = ccall(:tempnam, Cstring, (Cstring, Cstring), parent, temp_prefix)
+        systemerror(:tempnam, p == C_NULL)
+        s = unsafe_string(p)
+        Libc.free(p)
+        return s
+    end
     cleanup && temp_cleanup_later(s)
     return s
 end

test/file.jl

@@ -71,6 +71,14 @@ end
         t = tempname(d)
         @test dirname(t) == d
     end
+    # 38873: check that `TMPDIR` being set does not
+    # override the parent argument to `tempname`.
+    mktempdir() do d
+        whithenv("TMPDIR"=>tmpdir()) do
+            t = tempname(d)
+            @test dirname(t) == d
+        end
+    end
     @test_throws ArgumentError tempname(randstring())
 end