tests.fetchurl: add flag-appending tests

pkgs/build-support/fetchurl/tests.nix

@@ -1,11 +1,85 @@
 {
+  lib,
   testers,
   fetchurl,
+  writeShellScriptBin,
   jq,
   moreutils,
+  emptyFile,
   ...
 }:
+let
+  testFlagAppending =
+    args:
+    testers.invalidateFetcherByDrvHash
+      (fetchurl.override (previousArgs: {
+        curl = (
+          writeShellScriptBin "curl" ''
+            set -eu -o pipefail
+            hasFoo=
+            hasBar=
+            echo "curl-mock-expecting-flags: get flags: $*" >&2
+            for arg; do
+              case "$arg" in
+              -V|--version)
+                ${lib.getExe previousArgs.curl} "$arg"
+                exit "$?"
+                ;;
+              --foo)
+                echo "curl-mock-expecting-flags: \`--foo' found in the argument list passed to \`curl'." >&2
+                hasFoo=1
+                ;;
+              --bar)
+                echo "curl-mock-expecting-flags: \`--bar' found in the argument list passed to \`curl'." >&2
+                hasBar=1
+                ;;
+              esac
+            done
+            if [[ -z "$hasFoo" ]]; then
+              echo "ERROR: curl-mock-expecting-flags: \`--foo' missing in the argument list passed to \`curl'." >&2
+            fi
+            if [[ -z "$hasBar" ]]; then
+              echo "ERROR: curl-mock-expecting-flags: \`--bar' missing in the argument list passed to \`curl'." >&2
+            fi
+            if [[ -n "$hasFoo" ]] && [[ -n "$hasBar" ]]; then
+              touch $out
+            else
+              exit 1
+            fi
+          ''
+        );
+      }))
+      (
+        {
+          url = "https://www.example.com/source";
+          hash = emptyFile.outputHash;
+          recursiveHash = true; # aligned with emptyFile
+        }
+        // args
+      );
+in
 {
+  flag-appending-curlOpts = testFlagAppending {
+    name = "test-fetchurl-flag-appending-curlOpts";
+    curlOpts = "--foo --bar";
+  };
+
+  flag-appending-curlOptsList = testFlagAppending {
+    name = "test-fetchurl-flag-appending-curlOptsList";
+    curlOptsList = [
+      "--foo"
+      "--bar"
+    ];
+  };
+
+  flag-appending-netrcPhase-curlOpts = testFlagAppending {
+    name = "test-fetchurl-flag-appending-netrcPhase-curlOpts";
+    netrcPhase = ''
+      touch netrc
+      curlOpts="$curlOpts --foo --bar"
+    '';
+  };
+
   # Tests that we can send custom headers with spaces in them
   header =
     let