From a69b99ade04482fe8580e9a9f87172dbb9e0bee9 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 6 Jun 2025 19:40:57 +0200
Subject: [PATCH 1/3] Add ForwardingSourceAccessor

---
 .../nix/util/forwarding-source-accessor.hh    | 57 +++++++++++++++++++
 src/libutil/include/nix/util/meson.build      |  1 +
 2 files changed, 58 insertions(+)
 create mode 100644 src/libutil/include/nix/util/forwarding-source-accessor.hh

diff --git a/src/libutil/include/nix/util/forwarding-source-accessor.hh b/src/libutil/include/nix/util/forwarding-source-accessor.hh
new file mode 100644
index 000000000..bdba2addc
--- /dev/null
+++ b/src/libutil/include/nix/util/forwarding-source-accessor.hh
@@ -0,0 +1,57 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+/**
+ * A source accessor that just forwards every operation to another
+ * accessor. This is not useful in itself but can be used as a
+ * superclass for accessors that do change some operations.
+ */
+struct ForwardingSourceAccessor : SourceAccessor
+{
+    ref<SourceAccessor> next;
+
+    ForwardingSourceAccessor(ref<SourceAccessor> next)
+        : next(next)
+    {
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        return next->readFile(path);
+    }
+
+    void readFile(const CanonPath & path, Sink & sink, std::function<void(uint64_t)> sizeCallback) override
+    {
+        next->readFile(path, sink, sizeCallback);
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        return next->maybeLstat(path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return next->readDirectory(path);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        return next->readLink(path);
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        return next->showPath(path);
+    }
+
+    std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override
+    {
+        return next->getPhysicalPath(path);
+    }
+};
+
+}
diff --git a/src/libutil/include/nix/util/meson.build b/src/libutil/include/nix/util/meson.build
index 329d40612..3dacfafc6 100644
--- a/src/libutil/include/nix/util/meson.build
+++ b/src/libutil/include/nix/util/meson.build
@@ -34,6 +34,7 @@ headers = files(
   'file-system.hh',
   'finally.hh',
   'fmt.hh',
+  'forwarding-source-accessor.hh',
   'fs-sink.hh',
   'git.hh',
   'hash.hh',

From e18b1637dc7311724b264000556a94fd65766492 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 6 Jun 2025 19:41:12 +0200
Subject: [PATCH 2/3] Fix display of paths in substituted source trees
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

These got displayed as e.g.

  «github:NixOS/nixpkgs/adaa24fbf46737f3f1b5497bf64bae750f82942e?narHash=sha256-qhFMmDkeJX9KJwr5H32f1r7Prs7XbQWtO0h3V0a0rFY%3D»/nix/store/x9wnkly3k1gkq580m90jjn32q9f05q2v-source/pkgs/stdenv/generic/source-stdenv.sh

Now we get

  «github:NixOS/nixpkgs/adaa24fbf46737f3f1b5497bf64bae750f82942e?narHash=sha256-qhFMmDkeJX9KJwr5H32f1r7Prs7XbQWtO0h3V0a0rFY%3D»/pkgs/stdenv/generic/source-stdenv.sh
---
 src/libfetchers/fetchers.cc | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 614b3c90e..9beef69f0 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -5,6 +5,7 @@
 #include "nix/util/json-utils.hh"
 #include "nix/fetchers/store-path-accessor.hh"
 #include "nix/fetchers/fetch-settings.hh"
+#include "nix/util/forwarding-source-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -293,6 +294,21 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessor(ref<Store> store) const
     }
 }
 
+/**
+ * Helper class that ensures that paths in substituted source trees
+ * are rendered as `«input»/path` rather than
+ * `«input»/nix/store/<hash>-source/path`.
+ */
+struct SubstitutedSourceAccessor : ForwardingSourceAccessor
+{
+    using ForwardingSourceAccessor::ForwardingSourceAccessor;
+
+    std::string showPath(const CanonPath & path) override
+    {
+        return displayPrefix + path.abs() + displaySuffix;;
+    }
+};
+
 std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> store) const
 {
     // FIXME: cache the accessor
@@ -320,10 +336,12 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
             debug("using substituted/cached input '%s' in '%s'",
                 to_string(), store->printStorePath(storePath));
 
-            auto accessor = makeStorePathAccessor(store, storePath);
+            auto accessor = make_ref<SubstitutedSourceAccessor>(makeStorePathAccessor(store, storePath));
 
             accessor->fingerprint = getFingerprint(store);
 
+            // FIXME: ideally we would use the `showPath()` of the
+            // "real" accessor for this fetcher type.
             accessor->setPathDisplay("«" + to_string() + "»");
 
             return {accessor, *this};

From 74af43ee9151fa71345f0dc980527fa2ece14728 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 6 Jun 2025 20:03:33 +0200
Subject: [PATCH 3/3] Remove superfluous semicolon

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>
---
 src/libfetchers/fetchers.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 9beef69f0..5764f310d 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -305,7 +305,7 @@ struct SubstitutedSourceAccessor : ForwardingSourceAccessor
 
     std::string showPath(const CanonPath & path) override
     {
-        return displayPrefix + path.abs() + displaySuffix;;
+        return displayPrefix + path.abs() + displaySuffix;
     }
 };