libfetchers: log fetches by URL just before they happen

Addresses but does not close #305, as we still need an indicator for
frozen fetches.

Change-Id: Iba34ad42dc1c8772f7da249b90fe794b041bbf73

src/libfetchers/fetchers.cc

@@ -133,6 +133,10 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
     }
 
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
+        // *sighs*, we print the base URL, rather than the full URL because the Nixpkgs
+        // fileset lib tests assume that fetching shallow and non-shallow prints exactly the
+        // same stderr...
+        printInfo("fetching %s input '%s'", this->getType(), this->toURL().base);
         try {
             return scheme->fetch(store, *this);
         } catch (Error & e) {

tests/functional/nix-profile.sh

@@ -155,6 +155,7 @@ diff -u <(
     nix --offline profile install $flake2Dir 2>&1 1> /dev/null \
         | grep -vE "^warning: " \
         | grep -vE "^error \(ignored\): " \
+        | grep -vE "^fetching .+ input" \
         || true
 ) <(cat << EOF
 error: An existing package already provides the following file: