Look at non by-hash paths in copy and file methods

Ideally copy and file mirrors would support by-hash as well, but its
harder to setup and maintain especially if you want to cache an online
mirror who has by-hash enabled.

We can avoid unneeded roundtrips (in the best case) and entire cache
misses (in the usual worst case) by "just" telling methods that the URI
we passed it has the requested file perhaps also in other paths.

This is done in pseudo-relative paths as we would otherwise need to
teach redirection code to rewrite those URIs as well. A method like http
can easily ignore this value and await explicit instructions to look at
that file, but inspecting the path in local sources via file or copy is
(comparatively) free, so we just do it immediately. If that ends up
being the wrong version of the file as by-hash would have protected us
from we are in this feature branch now falling back to other mirrors
which are like the ones online and in support of by-hash.

1 files changed, 6 insertions, 5 deletions

diff --git a/apt-pkg/acquire-item.cc b/apt-pkg/acquire-item.cc
index 2c3f45076..3950ae030 100644
--- a/apt-pkg/acquire-item.cc
+++ b/apt-pkg/acquire-item.cc
@@ -473,15 +473,16 @@ bool pkgAcqTransactionItem::QueueURI(pkgAcquire::ItemDesc &Item)
       // now add the actual by-hash uris
       auto const Expected = GetExpectedHashes();
       auto const TargetHash = Expected.find(nullptr);
-      auto const PushByHashURI = [&](std::string U) {
+      auto const PushByHashURI = [&](std::string const &U) {
 	 if (unlikely(TargetHash == nullptr))
 	    return false;
-	 auto const trailing_slash = U.find_last_of("/");
+	 ::URI uri{U};
+	 auto const trailing_slash = uri.Path.find_last_of("/");
 	 if (unlikely(trailing_slash == std::string::npos))
 	    return false;
-	 auto byhashSuffix = "/by-hash/" + TargetHash->HashType() + "/" + TargetHash->HashValue();
-	 U.replace(trailing_slash, U.length() - trailing_slash, std::move(byhashSuffix));
-	 PushAlternativeURI(std::move(U), {}, false);
+	 auto altPath = uri.Path.substr(0, trailing_slash) + "/by-hash/" + TargetHash->HashType() + "/" + TargetHash->HashValue();
+	 std::swap(uri.Path, altPath);
+	 PushAlternativeURI(uri, {{"Alternate-Paths", "../../" + flNotDir(altPath)}}, false);
 	 return true;
       };
       PushByHashURI(Item.URI);