lib/fetchers.nix: factor out impure proxy vars (#18702)

Apparently everyone just copied those variables, instead of creating a
library constant for them. Some even removed the comment. -.-

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/fetchers.nix b/lib/fetchers.nix
new file mode 100644
index 00000000000..19d89d6c407
--- /dev/null
+++ b/lib/fetchers.nix
@@ -0,0 +1,12 @@
+# snippets that can be shared by mutliple fetchers (pkgs/build-support)
+{
+
+  proxyImpureEnvVars = [
+    # We borrow these environment variables from the caller to allow
+    # easy proxy configuration.  This is impure, but a fixed-output
+    # derivation like fetchurl is allowed to do so since its result is
+    # by definition pure.
+    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+  ];
+
+}