From 1a902f5fa7d4f268d0fec3e44a48ecc2445b3b6b Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 1 Nov 2023 17:09:28 +0100
Subject: [PATCH] Merge FSAccessor into SourceAccessor

---
 src/libstore/binary-cache-store.cc | 24 +++++------
 src/libstore/binary-cache-store.hh |  2 +-
 src/libstore/derivations.cc        |  1 -
 src/libstore/dummy-store.cc        |  2 +-
 src/libstore/fs-accessor.hh        | 58 ---------------------------
 src/libstore/legacy-ssh-store.cc   |  2 +-
 src/libstore/local-fs-store.cc     | 34 +++++++++-------
 src/libstore/local-fs-store.hh     |  2 +-
 src/libstore/nar-accessor.cc       | 64 ++++++++++++------------------
 src/libstore/nar-accessor.hh       | 11 ++---
 src/libstore/remote-fs-accessor.cc | 28 ++++++-------
 src/libstore/remote-fs-accessor.hh | 21 +++++-----
 src/libstore/remote-store.cc       |  2 +-
 src/libstore/remote-store.hh       |  2 +-
 src/libstore/store-api.cc          |  8 ++--
 src/libstore/store-api.hh          |  4 +-
 src/libstore/uds-remote-store.hh   |  4 +-
 src/libutil/source-accessor.cc     |  5 +++
 src/libutil/source-accessor.hh     | 22 ++++++++--
 src/nix/bundle.cc                  |  1 -
 src/nix/cat.cc                     | 13 +++---
 src/nix/ls.cc                      | 54 +++++++++++--------------
 src/nix/run.cc                     |  8 ++--
 src/nix/why-depends.cc             | 22 +++++-----
 tests/functional/nar-access.sh     |  8 +++-
 25 files changed, 178 insertions(+), 224 deletions(-)
 delete mode 100644 src/libstore/fs-accessor.hh

diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index b61868413..dd9e2f3af 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -2,7 +2,7 @@
 #include "binary-cache-store.hh"
 #include "compression.hh"
 #include "derivations.hh"
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "globals.hh"
 #include "nar-info.hh"
 #include "sync.hh"
@@ -143,7 +143,7 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
        write the compressed NAR to disk), into a HashSink (to get the
        NAR hash), and into a NarAccessor (to get the NAR listing). */
     HashSink fileHashSink { htSHA256 };
-    std::shared_ptr<FSAccessor> narAccessor;
+    std::shared_ptr<SourceAccessor> narAccessor;
     HashSink narHashSink { htSHA256 };
     {
     FdSink fileSink(fdTemp.get());
@@ -195,7 +195,7 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
     if (writeNARListing) {
         nlohmann::json j = {
             {"version", 1},
-            {"root", listNar(ref<FSAccessor>(narAccessor), "", true)},
+            {"root", listNar(ref<SourceAccessor>(narAccessor), CanonPath::root, true)},
         };
 
         upsertFile(std::string(info.path.hashPart()) + ".ls", j.dump(), "application/json");
@@ -206,9 +206,9 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
        specify the NAR file and member containing the debug info. */
     if (writeDebugInfo) {
 
-        std::string buildIdDir = "/lib/debug/.build-id";
+        CanonPath buildIdDir("lib/debug/.build-id");
 
-        if (auto st = narAccessor->stat(buildIdDir); st && st->type == SourceAccessor::tDirectory) {
+        if (auto st = narAccessor->maybeLstat(buildIdDir); st && st->type == SourceAccessor::tDirectory) {
 
             ThreadPool threadPool(25);
 
@@ -232,16 +232,16 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
             std::regex regex2("^[0-9a-f]{38}\\.debug$");
 
             for (auto & [s1, _type] : narAccessor->readDirectory(buildIdDir)) {
-                auto dir = buildIdDir + "/" + s1;
+                auto dir = buildIdDir + s1;
 
-                if (auto st = narAccessor->stat(dir); !st || st->type != SourceAccessor::tDirectory
+                if (narAccessor->lstat(dir).type != SourceAccessor::tDirectory
                     || !std::regex_match(s1, regex1))
                     continue;
 
                 for (auto & [s2, _type] : narAccessor->readDirectory(dir)) {
-                    auto debugPath = dir + "/" + s2;
+                    auto debugPath = dir + s2;
 
-                    if (auto st = narAccessor->stat(debugPath); !st || st->type != SourceAccessor::tRegular
+                    if ( narAccessor->lstat(debugPath).type != SourceAccessor::tRegular
                         || !std::regex_match(s2, regex2))
                         continue;
 
@@ -250,7 +250,7 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
                     std::string key = "debuginfo/" + buildId;
                     std::string target = "../" + narInfo->url;
 
-                    threadPool.enqueue(std::bind(doFile, std::string(debugPath, 1), key, target));
+                    threadPool.enqueue(std::bind(doFile, std::string(debugPath.rel()), key, target));
                 }
             }
 
@@ -503,9 +503,9 @@ void BinaryCacheStore::registerDrvOutput(const Realisation& info) {
     upsertFile(filePath, info.toJSON().dump(), "application/json");
 }
 
-ref<FSAccessor> BinaryCacheStore::getFSAccessor()
+ref<SourceAccessor> BinaryCacheStore::getFSAccessor(bool requireValidPath)
 {
-    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()), localNarCache);
+    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()), requireValidPath, localNarCache);
 }
 
 void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSet & sigs)
diff --git a/src/libstore/binary-cache-store.hh b/src/libstore/binary-cache-store.hh
index 218a888e3..cea2a571f 100644
--- a/src/libstore/binary-cache-store.hh
+++ b/src/libstore/binary-cache-store.hh
@@ -148,7 +148,7 @@ public:
 
     void narFromPath(const StorePath & path, Sink & sink) override;
 
-    ref<FSAccessor> getFSAccessor() override;
+    ref<SourceAccessor> getFSAccessor(bool requireValidPath) override;
 
     void addSignatures(const StorePath & storePath, const StringSet & sigs) override;
 
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index a5ceb29dc..efdad18e1 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -6,7 +6,6 @@
 #include "split.hh"
 #include "common-protocol.hh"
 #include "common-protocol-impl.hh"
-#include "fs-accessor.hh"
 #include <boost/container/small_vector.hpp>
 #include <nlohmann/json.hpp>
 
diff --git a/src/libstore/dummy-store.cc b/src/libstore/dummy-store.cc
index 74d6ed3b5..821cda399 100644
--- a/src/libstore/dummy-store.cc
+++ b/src/libstore/dummy-store.cc
@@ -72,7 +72,7 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
         Callback<std::shared_ptr<const Realisation>> callback) noexcept override
     { callback(nullptr); }
 
-    virtual ref<FSAccessor> getFSAccessor() override
+    virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
     { unsupported("getFSAccessor"); }
 };
 
diff --git a/src/libstore/fs-accessor.hh b/src/libstore/fs-accessor.hh
deleted file mode 100644
index f6c002a2d..000000000
--- a/src/libstore/fs-accessor.hh
+++ /dev/null
@@ -1,58 +0,0 @@
-#pragma once
-///@file
-
-#include "types.hh"
-#include "source-accessor.hh"
-
-#include <optional>
-
-namespace nix {
-
-/**
- * An abstract class for accessing a filesystem-like structure, such
- * as a (possibly remote) Nix store or the contents of a NAR file.
- */
-class FSAccessor
-{
-public:
-    using Type = SourceAccessor::Type;
-
-    struct Stat
-    {
-        Type type;
-        /**
-         * For regular files only: the size of the file.
-         */
-        std::optional<uint64_t> fileSize;
-        /**
-         * For regular files only: whether this is an executable.
-         */
-        bool isExecutable = false;
-        /**
-         * For regular files only: the position of the contents of this
-         * file in the NAR.
-         */
-        std::optional<uint64_t> narOffset;
-    };
-
-    virtual ~FSAccessor() { }
-
-    virtual std::optional<Stat> stat(const Path & path) = 0;
-
-    using DirEntries = SourceAccessor::DirEntries;
-
-    virtual DirEntries readDirectory(const Path & path) = 0;
-
-    /**
-     * Read a file inside the store.
-     *
-     * If `requireValidPath` is set to `true` (the default), the path must be
-     * inside a valid store path, otherwise it just needs to be physically
-     * present (but not necessarily properly registered)
-     */
-    virtual std::string readFile(const Path & path, bool requireValidPath = true) = 0;
-
-    virtual std::string readLink(const Path & path) = 0;
-};
-
-}
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 38fdf118f..731457354 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -363,7 +363,7 @@ public:
     void ensurePath(const StorePath & path) override
     { unsupported("ensurePath"); }
 
-    virtual ref<FSAccessor> getFSAccessor() override
+    virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
     { unsupported("getFSAccessor"); }
 
     /**
diff --git a/src/libstore/local-fs-store.cc b/src/libstore/local-fs-store.cc
index 65cbb9e35..63497acbd 100644
--- a/src/libstore/local-fs-store.cc
+++ b/src/libstore/local-fs-store.cc
@@ -1,5 +1,5 @@
 #include "archive.hh"
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "store-api.hh"
 #include "local-fs-store.hh"
 #include "globals.hh"
@@ -13,26 +13,31 @@ LocalFSStore::LocalFSStore(const Params & params)
 {
 }
 
-struct LocalStoreAccessor : public FSAccessor
+struct LocalStoreAccessor : public SourceAccessor
 {
     ref<LocalFSStore> store;
+    bool requireValidPath;
 
-    LocalStoreAccessor(ref<LocalFSStore> store) : store(store) { }
+    LocalStoreAccessor(ref<LocalFSStore> store, bool requireValidPath)
+        : store(store)
+        , requireValidPath(requireValidPath)
+    { }
 
-    Path toRealPath(const Path & path, bool requireValidPath = true)
+    Path toRealPath(const CanonPath & path)
     {
-        auto storePath = store->toStorePath(path).first;
+        auto storePath = store->toStorePath(path.abs()).first;
         if (requireValidPath && !store->isValidPath(storePath))
             throw InvalidPath("path '%1%' is not a valid store path", store->printStorePath(storePath));
-        return store->getRealStoreDir() + std::string(path, store->storeDir.size());
+        return store->getRealStoreDir() + path.abs().substr(store->storeDir.size());
     }
 
-    std::optional<FSAccessor::Stat> stat(const Path & path) override
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
     {
         auto realPath = toRealPath(path);
 
+        // FIXME: use PosixSourceAccessor.
         struct stat st;
-        if (lstat(realPath.c_str(), &st)) {
+        if (::lstat(realPath.c_str(), &st)) {
             if (errno == ENOENT || errno == ENOTDIR) return std::nullopt;
             throw SysError("getting status of '%1%'", path);
         }
@@ -48,7 +53,7 @@ struct LocalStoreAccessor : public FSAccessor
             S_ISREG(st.st_mode) && st.st_mode & S_IXUSR}};
     }
 
-    DirEntries readDirectory(const Path & path) override
+    DirEntries readDirectory(const CanonPath & path) override
     {
         auto realPath = toRealPath(path);
 
@@ -61,21 +66,22 @@ struct LocalStoreAccessor : public FSAccessor
         return res;
     }
 
-    std::string readFile(const Path & path, bool requireValidPath = true) override
+    std::string readFile(const CanonPath & path) override
     {
-        return nix::readFile(toRealPath(path, requireValidPath));
+        return nix::readFile(toRealPath(path));
     }
 
-    std::string readLink(const Path & path) override
+    std::string readLink(const CanonPath & path) override
     {
         return nix::readLink(toRealPath(path));
     }
 };
 
-ref<FSAccessor> LocalFSStore::getFSAccessor()
+ref<SourceAccessor> LocalFSStore::getFSAccessor(bool requireValidPath)
 {
     return make_ref<LocalStoreAccessor>(ref<LocalFSStore>(
-            std::dynamic_pointer_cast<LocalFSStore>(shared_from_this())));
+            std::dynamic_pointer_cast<LocalFSStore>(shared_from_this())),
+        requireValidPath);
 }
 
 void LocalFSStore::narFromPath(const StorePath & path, Sink & sink)
diff --git a/src/libstore/local-fs-store.hh b/src/libstore/local-fs-store.hh
index d6bda05d1..bf855b67e 100644
--- a/src/libstore/local-fs-store.hh
+++ b/src/libstore/local-fs-store.hh
@@ -43,7 +43,7 @@ public:
     LocalFSStore(const Params & params);
 
     void narFromPath(const StorePath & path, Sink & sink) override;
-    ref<FSAccessor> getFSAccessor() override;
+    ref<SourceAccessor> getFSAccessor(bool requireValidPath) override;
 
     /**
      * Creates symlink from the `gcRoot` to the `storePath` and
diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index f1be5606e..02993680f 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -11,7 +11,7 @@ namespace nix {
 
 struct NarMember
 {
-    FSAccessor::Stat stat;
+    SourceAccessor::Stat stat;
 
     std::string target;
 
@@ -19,7 +19,7 @@ struct NarMember
     std::map<std::string, NarMember> children;
 };
 
-struct NarAccessor : public FSAccessor
+struct NarAccessor : public SourceAccessor
 {
     std::optional<const std::string> nar;
 
@@ -149,48 +149,36 @@ struct NarAccessor : public FSAccessor
         recurse(root, v);
     }
 
-    NarMember * find(const Path & path)
+    NarMember * find(const CanonPath & path)
     {
-        Path canon = path == "" ? "" : canonPath(path);
         NarMember * current = &root;
-        auto end = path.end();
-        for (auto it = path.begin(); it != end; ) {
-            // because it != end, the remaining component is non-empty so we need
-            // a directory
+
+        for (auto & i : path) {
             if (current->stat.type != Type::tDirectory) return nullptr;
-
-            // skip slash (canonPath above ensures that this is always a slash)
-            assert(*it == '/');
-            it += 1;
-
-            // lookup current component
-            auto next = std::find(it, end, '/');
-            auto child = current->children.find(std::string(it, next));
+            auto child = current->children.find(std::string(i));
             if (child == current->children.end()) return nullptr;
             current = &child->second;
-
-            it = next;
         }
 
         return current;
     }
 
-    NarMember & get(const Path & path) {
+    NarMember & get(const CanonPath & path) {
         auto result = find(path);
-        if (result == nullptr)
+        if (!result)
             throw Error("NAR file does not contain path '%1%'", path);
         return *result;
     }
 
-    std::optional<Stat> stat(const Path & path) override
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
     {
         auto i = find(path);
-        if (i == nullptr)
+        if (!i)
             return std::nullopt;
         return i->stat;
     }
 
-    DirEntries readDirectory(const Path & path) override
+    DirEntries readDirectory(const CanonPath & path) override
     {
         auto i = get(path);
 
@@ -204,7 +192,7 @@ struct NarAccessor : public FSAccessor
         return res;
     }
 
-    std::string readFile(const Path & path, bool requireValidPath = true) override
+    std::string readFile(const CanonPath & path) override
     {
         auto i = get(path);
         if (i.stat.type != Type::tRegular)
@@ -216,7 +204,7 @@ struct NarAccessor : public FSAccessor
         return std::string(*nar, *i.stat.narOffset, *i.stat.fileSize);
     }
 
-    std::string readLink(const Path & path) override
+    std::string readLink(const CanonPath & path) override
     {
         auto i = get(path);
         if (i.stat.type != Type::tSymlink)
@@ -225,40 +213,38 @@ struct NarAccessor : public FSAccessor
     }
 };
 
-ref<FSAccessor> makeNarAccessor(std::string && nar)
+ref<SourceAccessor> makeNarAccessor(std::string && nar)
 {
     return make_ref<NarAccessor>(std::move(nar));
 }
 
-ref<FSAccessor> makeNarAccessor(Source & source)
+ref<SourceAccessor> makeNarAccessor(Source & source)
 {
     return make_ref<NarAccessor>(source);
 }
 
-ref<FSAccessor> makeLazyNarAccessor(const std::string & listing,
+ref<SourceAccessor> makeLazyNarAccessor(const std::string & listing,
     GetNarBytes getNarBytes)
 {
     return make_ref<NarAccessor>(listing, getNarBytes);
 }
 
 using nlohmann::json;
-json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse)
+json listNar(ref<SourceAccessor> accessor, const CanonPath & path, bool recurse)
 {
-    auto st = accessor->stat(path);
-    if (!st)
-        throw Error("path '%s' does not exist in NAR", path);
+    auto st = accessor->lstat(path);
 
     json obj = json::object();
 
-    switch (st->type) {
+    switch (st.type) {
     case SourceAccessor::Type::tRegular:
         obj["type"] = "regular";
-        if (st->fileSize)
-            obj["size"] = *st->fileSize;
-        if (st->isExecutable)
+        if (st.fileSize)
+            obj["size"] = *st.fileSize;
+        if (st.isExecutable)
             obj["executable"] = true;
-        if (st->narOffset && *st->narOffset)
-            obj["narOffset"] = *st->narOffset;
+        if (st.narOffset && *st.narOffset)
+            obj["narOffset"] = *st.narOffset;
         break;
     case SourceAccessor::Type::tDirectory:
         obj["type"] = "directory";
@@ -267,7 +253,7 @@ json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse)
             json &res2 = obj["entries"];
             for (auto & [name, type] : accessor->readDirectory(path)) {
                 if (recurse) {
-                    res2[name] = listNar(accessor, path + "/" + name, true);
+                    res2[name] = listNar(accessor, path + name, true);
                 } else
                     res2[name] = json::object();
             }
diff --git a/src/libstore/nar-accessor.hh b/src/libstore/nar-accessor.hh
index 5e19bd3c7..433774524 100644
--- a/src/libstore/nar-accessor.hh
+++ b/src/libstore/nar-accessor.hh
@@ -1,10 +1,11 @@
 #pragma once
 ///@file
 
+#include "source-accessor.hh"
+
 #include <functional>
 
 #include <nlohmann/json_fwd.hpp>
-#include "fs-accessor.hh"
 
 namespace nix {
 
@@ -14,9 +15,9 @@ struct Source;
  * Return an object that provides access to the contents of a NAR
  * file.
  */
-ref<FSAccessor> makeNarAccessor(std::string && nar);
+ref<SourceAccessor> makeNarAccessor(std::string && nar);
 
-ref<FSAccessor> makeNarAccessor(Source & source);
+ref<SourceAccessor> makeNarAccessor(Source & source);
 
 /**
  * Create a NAR accessor from a NAR listing (in the format produced by
@@ -26,7 +27,7 @@ ref<FSAccessor> makeNarAccessor(Source & source);
  */
 typedef std::function<std::string(uint64_t, uint64_t)> GetNarBytes;
 
-ref<FSAccessor> makeLazyNarAccessor(
+ref<SourceAccessor> makeLazyNarAccessor(
     const std::string & listing,
     GetNarBytes getNarBytes);
 
@@ -34,6 +35,6 @@ ref<FSAccessor> makeLazyNarAccessor(
  * Write a JSON representation of the contents of a NAR (except file
  * contents).
  */
-nlohmann::json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse);
+nlohmann::json listNar(ref<SourceAccessor> accessor, const CanonPath & path, bool recurse);
 
 }
diff --git a/src/libstore/remote-fs-accessor.cc b/src/libstore/remote-fs-accessor.cc
index 21419700c..03e57a565 100644
--- a/src/libstore/remote-fs-accessor.cc
+++ b/src/libstore/remote-fs-accessor.cc
@@ -8,8 +8,9 @@
 
 namespace nix {
 
-RemoteFSAccessor::RemoteFSAccessor(ref<Store> store, const Path & cacheDir)
+RemoteFSAccessor::RemoteFSAccessor(ref<Store> store, bool requireValidPath, const Path & cacheDir)
     : store(store)
+    , requireValidPath(requireValidPath)
     , cacheDir(cacheDir)
 {
     if (cacheDir != "")
@@ -22,7 +23,7 @@ Path RemoteFSAccessor::makeCacheFile(std::string_view hashPart, const std::strin
     return fmt("%s/%s.%s", cacheDir, hashPart, ext);
 }
 
-ref<FSAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std::string && nar)
+ref<SourceAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std::string && nar)
 {
     if (cacheDir != "") {
         try {
@@ -38,7 +39,7 @@ ref<FSAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std::str
 
     if (cacheDir != "") {
         try {
-            nlohmann::json j = listNar(narAccessor, "", true);
+            nlohmann::json j = listNar(narAccessor, CanonPath::root, true);
             writeFile(makeCacheFile(hashPart, "ls"), j.dump());
         } catch (...) {
             ignoreException();
@@ -48,11 +49,10 @@ ref<FSAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std::str
     return narAccessor;
 }
 
-std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_, bool requireValidPath)
+std::pair<ref<SourceAccessor>, CanonPath> RemoteFSAccessor::fetch(const CanonPath & path)
 {
-    auto path = canonPath(path_);
-
-    auto [storePath, restPath] = store->toStorePath(path);
+    auto [storePath, restPath_] = store->toStorePath(path.abs());
+    auto restPath = CanonPath(restPath_);
 
     if (requireValidPath && !store->isValidPath(storePath))
         throw InvalidPath("path '%1%' is not a valid store path", store->printStorePath(storePath));
@@ -63,7 +63,7 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_, boo
     std::string listing;
     Path cacheFile;
 
-    if (cacheDir != "" && pathExists(cacheFile = makeCacheFile(storePath.hashPart(), "nar"))) {
+    if (cacheDir != "" && nix::pathExists(cacheFile = makeCacheFile(storePath.hashPart(), "nar"))) {
 
         try {
             listing = nix::readFile(makeCacheFile(storePath.hashPart(), "ls"));
@@ -101,25 +101,25 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_, boo
     return {addToCache(storePath.hashPart(), std::move(sink.s)), restPath};
 }
 
-std::optional<FSAccessor::Stat> RemoteFSAccessor::stat(const Path & path)
+std::optional<SourceAccessor::Stat> RemoteFSAccessor::maybeLstat(const CanonPath & path)
 {
     auto res = fetch(path);
-    return res.first->stat(res.second);
+    return res.first->maybeLstat(res.second);
 }
 
-SourceAccessor::DirEntries RemoteFSAccessor::readDirectory(const Path & path)
+SourceAccessor::DirEntries RemoteFSAccessor::readDirectory(const CanonPath & path)
 {
     auto res = fetch(path);
     return res.first->readDirectory(res.second);
 }
 
-std::string RemoteFSAccessor::readFile(const Path & path, bool requireValidPath)
+std::string RemoteFSAccessor::readFile(const CanonPath & path)
 {
-    auto res = fetch(path, requireValidPath);
+    auto res = fetch(path);
     return res.first->readFile(res.second);
 }
 
-std::string RemoteFSAccessor::readLink(const Path & path)
+std::string RemoteFSAccessor::readLink(const CanonPath & path)
 {
     auto res = fetch(path);
     return res.first->readLink(res.second);
diff --git a/src/libstore/remote-fs-accessor.hh b/src/libstore/remote-fs-accessor.hh
index 8de3b7bcd..d09762a53 100644
--- a/src/libstore/remote-fs-accessor.hh
+++ b/src/libstore/remote-fs-accessor.hh
@@ -1,40 +1,43 @@
 #pragma once
 ///@file
 
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "ref.hh"
 #include "store-api.hh"
 
 namespace nix {
 
-class RemoteFSAccessor : public FSAccessor
+class RemoteFSAccessor : public SourceAccessor
 {
     ref<Store> store;
 
-    std::map<std::string, ref<FSAccessor>> nars;
+    std::map<std::string, ref<SourceAccessor>> nars;
+
+    bool requireValidPath;
 
     Path cacheDir;
 
-    std::pair<ref<FSAccessor>, Path> fetch(const Path & path_, bool requireValidPath = true);
+    std::pair<ref<SourceAccessor>, CanonPath> fetch(const CanonPath & path);
 
     friend class BinaryCacheStore;
 
     Path makeCacheFile(std::string_view hashPart, const std::string & ext);
 
-    ref<FSAccessor> addToCache(std::string_view hashPart, std::string && nar);
+    ref<SourceAccessor> addToCache(std::string_view hashPart, std::string && nar);
 
 public:
 
     RemoteFSAccessor(ref<Store> store,
+        bool requireValidPath = true,
         const /* FIXME: use std::optional */ Path & cacheDir = "");
 
-    std::optional<Stat> stat(const Path & path) override;
+    std::optional<Stat> maybeLstat(const CanonPath & path) override;
 
-    DirEntries readDirectory(const Path & path) override;
+    DirEntries readDirectory(const CanonPath & path) override;
 
-    std::string readFile(const Path & path, bool requireValidPath = true) override;
+    std::string readFile(const CanonPath & path) override;
 
-    std::string readLink(const Path & path) override;
+    std::string readLink(const CanonPath & path) override;
 };
 
 }
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 7bdc25433..f16949f42 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -970,7 +970,7 @@ void RemoteStore::narFromPath(const StorePath & path, Sink & sink)
     copyNAR(conn->from, sink);
 }
 
-ref<FSAccessor> RemoteStore::getFSAccessor()
+ref<SourceAccessor> RemoteStore::getFSAccessor(bool requireValidPath)
 {
     return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()));
 }
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index f0985fdc1..1cc11af86 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -185,7 +185,7 @@ protected:
 
     friend struct ConnectionHandle;
 
-    virtual ref<FSAccessor> getFSAccessor() override;
+    virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath) override;
 
     virtual void narFromPath(const StorePath & path, Sink & sink) override;
 
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 0399120d1..665b5fed7 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -1,5 +1,5 @@
 #include "crypto.hh"
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "globals.hh"
 #include "derivations.hh"
 #include "store-api.hh"
@@ -1338,12 +1338,12 @@ Derivation Store::derivationFromPath(const StorePath & drvPath)
     return readDerivation(drvPath);
 }
 
-Derivation readDerivationCommon(Store& store, const StorePath& drvPath, bool requireValidPath)
+static Derivation readDerivationCommon(Store & store, const StorePath & drvPath, bool requireValidPath)
 {
-    auto accessor = store.getFSAccessor();
+    auto accessor = store.getFSAccessor(requireValidPath);
     try {
         return parseDerivation(store,
-            accessor->readFile(store.printStorePath(drvPath), requireValidPath),
+            accessor->readFile(CanonPath(store.printStorePath(drvPath))),
             Derivation::nameFromPath(drvPath));
     } catch (FormatError & e) {
         throw Error("error parsing derivation '%s': %s", store.printStorePath(drvPath), e.msg());
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index e123fccc5..6aa317e3d 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -70,7 +70,7 @@ MakeError(InvalidStoreURI, Error);
 
 struct BasicDerivation;
 struct Derivation;
-class FSAccessor;
+struct SourceAccessor;
 class NarInfoDiskCache;
 class Store;
 
@@ -703,7 +703,7 @@ public:
     /**
      * @return An object to access files in the Nix store.
      */
-    virtual ref<FSAccessor> getFSAccessor() = 0;
+    virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath = true) = 0;
 
     /**
      * Repair the contents of the given path by redownloading it using
diff --git a/src/libstore/uds-remote-store.hh b/src/libstore/uds-remote-store.hh
index cdb28a001..a5ac9080a 100644
--- a/src/libstore/uds-remote-store.hh
+++ b/src/libstore/uds-remote-store.hh
@@ -35,8 +35,8 @@ public:
     static std::set<std::string> uriSchemes()
     { return {"unix"}; }
 
-    ref<FSAccessor> getFSAccessor() override
-    { return LocalFSStore::getFSAccessor(); }
+    ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
+    { return LocalFSStore::getFSAccessor(requireValidPath); }
 
     void narFromPath(const StorePath & path, Sink & sink) override
     { LocalFSStore::narFromPath(path, sink); }
diff --git a/src/libutil/source-accessor.cc b/src/libutil/source-accessor.cc
index 5b0c7dd34..e2114e18f 100644
--- a/src/libutil/source-accessor.cc
+++ b/src/libutil/source-accessor.cc
@@ -10,6 +10,11 @@ SourceAccessor::SourceAccessor()
 {
 }
 
+bool SourceAccessor::pathExists(const CanonPath & path)
+{
+    return maybeLstat(path).has_value();
+}
+
 std::string SourceAccessor::readFile(const CanonPath & path)
 {
     StringSink sink;
diff --git a/src/libutil/source-accessor.hh b/src/libutil/source-accessor.hh
index 80bc02b48..1a4e80361 100644
--- a/src/libutil/source-accessor.hh
+++ b/src/libutil/source-accessor.hh
@@ -40,7 +40,7 @@ struct SourceAccessor
         Sink & sink,
         std::function<void(uint64_t)> sizeCallback = [](uint64_t size){});
 
-    virtual bool pathExists(const CanonPath & path) = 0;
+    virtual bool pathExists(const CanonPath & path);
 
     enum Type {
       tRegular, tSymlink, tDirectory,
@@ -57,8 +57,24 @@ struct SourceAccessor
     struct Stat
     {
         Type type = tMisc;
-        //uint64_t fileSize = 0; // regular files only
-        bool isExecutable = false; // regular files only
+
+        /**
+         * For regular files only: the size of the file. Not all
+         * accessors return this since it may be too expensive to
+         * compute.
+         */
+        std::optional<uint64_t> fileSize;
+
+        /**
+         * For regular files only: whether this is an executable.
+         */
+        bool isExecutable = false;
+
+        /**
+         * For regular files only: the position of the contents of this
+         * file in the NAR. Only returned by NAR accessors.
+         */
+        std::optional<uint64_t> narOffset;
     };
 
     Stat lstat(const CanonPath & path);
diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index 504e35c81..54cc6a17f 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -4,7 +4,6 @@
 #include "shared.hh"
 #include "store-api.hh"
 #include "local-fs-store.hh"
-#include "fs-accessor.hh"
 #include "eval-inline.hh"
 
 using namespace nix;
diff --git a/src/nix/cat.cc b/src/nix/cat.cc
index b5fe2506f..6e5a736f2 100644
--- a/src/nix/cat.cc
+++ b/src/nix/cat.cc
@@ -1,6 +1,5 @@
 #include "command.hh"
 #include "store-api.hh"
-#include "fs-accessor.hh"
 #include "nar-accessor.hh"
 
 using namespace nix;
@@ -9,14 +8,12 @@ struct MixCat : virtual Args
 {
     std::string path;
 
-    void cat(ref<FSAccessor> accessor)
+    void cat(ref<SourceAccessor> accessor)
     {
-        if (auto st = accessor->stat(path)) {
-            if (st->type != FSAccessor::Type::tRegular)
-                throw Error("path '%1%' is not a regular file", path);
-            writeFull(STDOUT_FILENO, accessor->readFile(path));
-        } else
-            throw Error("path '%1%' does not exist", path);
+        auto st = accessor->lstat(CanonPath(path));
+        if (st.type != SourceAccessor::Type::tRegular)
+            throw Error("path '%1%' is not a regular file", path);
+        writeFull(STDOUT_FILENO, accessor->readFile(CanonPath(path)));
     }
 };
 
diff --git a/src/nix/ls.cc b/src/nix/ls.cc
index da978f379..231456c9c 100644
--- a/src/nix/ls.cc
+++ b/src/nix/ls.cc
@@ -1,6 +1,5 @@
 #include "command.hh"
 #include "store-api.hh"
-#include "fs-accessor.hh"
 #include "nar-accessor.hh"
 #include "common-args.hh"
 #include <nlohmann/json.hpp>
@@ -39,63 +38,58 @@ struct MixLs : virtual Args, MixJSON
         });
     }
 
-    void listText(ref<FSAccessor> accessor)
+    void listText(ref<SourceAccessor> accessor)
     {
-        std::function<void(const FSAccessor::Stat &, const Path &, const std::string &, bool)> doPath;
+        std::function<void(const SourceAccessor::Stat &, const CanonPath &, std::string_view, bool)> doPath;
 
-        auto showFile = [&](const Path & curPath, const std::string & relPath) {
+        auto showFile = [&](const CanonPath & curPath, std::string_view relPath) {
             if (verbose) {
-                auto st = accessor->stat(curPath);
-                assert(st);
+                auto st = accessor->lstat(curPath);
                 std::string tp =
-                    st->type == FSAccessor::Type::tRegular ?
-                        (st->isExecutable ? "-r-xr-xr-x" : "-r--r--r--") :
-                    st->type == FSAccessor::Type::tSymlink ? "lrwxrwxrwx" :
+                    st.type == SourceAccessor::Type::tRegular ?
+                        (st.isExecutable ? "-r-xr-xr-x" : "-r--r--r--") :
+                    st.type == SourceAccessor::Type::tSymlink ? "lrwxrwxrwx" :
                     "dr-xr-xr-x";
-                auto line = fmt("%s %20d %s", tp, st->fileSize.value_or(0), relPath);
-                if (st->type == FSAccessor::Type::tSymlink)
+                auto line = fmt("%s %20d %s", tp, st.fileSize.value_or(0), relPath);
+                if (st.type == SourceAccessor::Type::tSymlink)
                     line += " -> " + accessor->readLink(curPath);
                 logger->cout(line);
-                if (recursive && st->type == FSAccessor::Type::tDirectory)
-                    doPath(*st, curPath, relPath, false);
+                if (recursive && st.type == SourceAccessor::Type::tDirectory)
+                    doPath(st, curPath, relPath, false);
             } else {
                 logger->cout(relPath);
                 if (recursive) {
-                    auto st = accessor->stat(curPath);
-                    assert(st);
-                    if (st->type == FSAccessor::Type::tDirectory)
-                        doPath(*st, curPath, relPath, false);
+                    auto st = accessor->lstat(curPath);
+                    if (st.type == SourceAccessor::Type::tDirectory)
+                        doPath(st, curPath, relPath, false);
                 }
             }
         };
 
-        doPath = [&](const FSAccessor::Stat & st, const Path & curPath,
-            const std::string & relPath, bool showDirectory)
+        doPath = [&](const SourceAccessor::Stat & st, const CanonPath & curPath,
+            std::string_view relPath, bool showDirectory)
         {
-            if (st.type == FSAccessor::Type::tDirectory && !showDirectory) {
+            if (st.type == SourceAccessor::Type::tDirectory && !showDirectory) {
                 auto names = accessor->readDirectory(curPath);
                 for (auto & [name, type] : names)
-                    showFile(curPath + "/" + name, relPath + "/" + name);
+                    showFile(curPath + name, relPath + "/" + name);
             } else
                 showFile(curPath, relPath);
         };
 
-        auto st = accessor->stat(path);
-        if (!st)
-            throw Error("path '%1%' does not exist", path);
-        doPath(*st, path,
-            st->type == FSAccessor::Type::tDirectory ? "." : std::string(baseNameOf(path)),
+        auto path2 = CanonPath(path);
+        auto st = accessor->lstat(path2);
+        doPath(st, path2,
+            st.type == SourceAccessor::Type::tDirectory ? "." : path2.baseName().value_or(""),
             showDirectory);
     }
 
-    void list(ref<FSAccessor> accessor)
+    void list(ref<SourceAccessor> accessor)
     {
-        if (path == "/") path = "";
-
         if (json) {
             if (showDirectory)
                 throw UsageError("'--directory' is useless with '--json'");
-            logger->cout("%s", listNar(accessor, path, recursive));
+            logger->cout("%s", listNar(accessor, CanonPath(path), recursive));
         } else
             listText(accessor);
     }
diff --git a/src/nix/run.cc b/src/nix/run.cc
index 07806283c..1465e8cde 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -6,7 +6,7 @@
 #include "derivations.hh"
 #include "local-store.hh"
 #include "finally.hh"
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "progress-bar.hh"
 #include "eval.hh"
 #include "build/personality.hh"
@@ -119,9 +119,9 @@ struct CmdShell : InstallablesCommand, MixEnvironment
             if (true)
                 unixPath.push_front(store->printStorePath(path) + "/bin");
 
-            auto propPath = store->printStorePath(path) + "/nix-support/propagated-user-env-packages";
-            if (auto st = accessor->stat(propPath); st && st->type == SourceAccessor::tRegular) {
-                for (auto & p : tokenizeString<Paths>(readFile(propPath)))
+            auto propPath = CanonPath(store->printStorePath(path)) + "nix-support" + "propagated-user-env-packages";
+            if (auto st = accessor->maybeLstat(propPath); st && st->type == SourceAccessor::tRegular) {
+                for (auto & p : tokenizeString<Paths>(accessor->readFile(propPath)))
                     todo.push(store->parseStorePath(p));
             }
         }
diff --git a/src/nix/why-depends.cc b/src/nix/why-depends.cc
index 04c1a0c1c..aecf65922 100644
--- a/src/nix/why-depends.cc
+++ b/src/nix/why-depends.cc
@@ -1,7 +1,7 @@
 #include "command.hh"
 #include "store-api.hh"
 #include "progress-bar.hh"
-#include "fs-accessor.hh"
+#include "source-accessor.hh"
 #include "shared.hh"
 
 #include <queue>
@@ -175,7 +175,7 @@ struct CmdWhyDepends : SourceExprCommand, MixOperateOnOptions
         struct BailOut { };
 
         printNode = [&](Node & node, const std::string & firstPad, const std::string & tailPad) {
-            auto pathS = store->printStorePath(node.path);
+            CanonPath pathS(store->printStorePath(node.path));
 
             assert(node.dist != inf);
             if (precise) {
@@ -183,7 +183,7 @@ struct CmdWhyDepends : SourceExprCommand, MixOperateOnOptions
                     firstPad,
                     node.visited ? "\e[38;5;244m" : "",
                     firstPad != "" ? "→ " : "",
-                    pathS);
+                    pathS.abs());
             }
 
             if (node.path == dependencyPath && !all
@@ -210,25 +210,25 @@ struct CmdWhyDepends : SourceExprCommand, MixOperateOnOptions
                contain the reference. */
             std::map<std::string, Strings> hits;
 
-            std::function<void(const Path &)> visitPath;
+            std::function<void(const CanonPath &)> visitPath;
 
-            visitPath = [&](const Path & p) {
-                auto st = accessor->stat(p);
+            visitPath = [&](const CanonPath & p) {
+                auto st = accessor->maybeLstat(p);
                 assert(st);
 
-                auto p2 = p == pathS ? "/" : std::string(p, pathS.size() + 1);
+                auto p2 = p == pathS ? "/" : p.abs().substr(pathS.abs().size() + 1);
 
                 auto getColour = [&](const std::string & hash) {
                     return hash == dependencyPathHash ? ANSI_GREEN : ANSI_BLUE;
                 };
 
-                if (st->type == FSAccessor::Type::tDirectory) {
+                if (st->type == SourceAccessor::Type::tDirectory) {
                     auto names = accessor->readDirectory(p);
                     for (auto & [name, type] : names)
-                        visitPath(p + "/" + name);
+                        visitPath(p + name);
                 }
 
-                else if (st->type == FSAccessor::Type::tRegular) {
+                else if (st->type == SourceAccessor::Type::tRegular) {
                     auto contents = accessor->readFile(p);
 
                     for (auto & hash : hashes) {
@@ -246,7 +246,7 @@ struct CmdWhyDepends : SourceExprCommand, MixOperateOnOptions
                     }
                 }
 
-                else if (st->type == FSAccessor::Type::tSymlink) {
+                else if (st->type == SourceAccessor::Type::tSymlink) {
                     auto target = accessor->readLink(p);
 
                     for (auto & hash : hashes) {
diff --git a/tests/functional/nar-access.sh b/tests/functional/nar-access.sh
index d487d58d2..13d23c342 100644
--- a/tests/functional/nar-access.sh
+++ b/tests/functional/nar-access.sh
@@ -25,6 +25,12 @@ diff -u baz.cat-nar $storePath/foo/baz
 nix store cat $storePath/foo/baz > baz.cat-nar
 diff -u baz.cat-nar $storePath/foo/baz
 
+# Check that 'nix store cat' fails on invalid store paths.
+invalidPath="$(dirname $storePath)/99999999999999999999999999999999-foo"
+mv $storePath $invalidPath
+(! nix store cat $invalidPath/foo/baz)
+mv $invalidPath $storePath
+
 # Test --json.
 diff -u \
     <(nix nar ls --json $narFile / | jq -S) \
@@ -46,7 +52,7 @@ diff -u \
     <(echo '{"type":"regular","size":0}' | jq -S)
 
 # Test missing files.
-expect 1 nix store ls --json -R $storePath/xyzzy 2>&1 | grep 'does not exist in NAR'
+expect 1 nix store ls --json -R $storePath/xyzzy 2>&1 | grep 'does not exist'
 expect 1 nix store ls $storePath/xyzzy 2>&1 | grep 'does not exist'
 
 # Test failure to dump.