Persistently cache InputAccessor::fetchToStore()

This especially speeds up repeated evaluations that copy a large
source tree (e.g. 'nix.nixPath = [ "nixpkgs=${nixpkgs}" ];').

src/libfetchers/fetchers.cc

@@ -160,11 +160,14 @@ void Input::checkLocks(Input & input) const
 
 std::pair<ref<InputAccessor>, Input> Input::getAccessor(ref<Store> store) const
 {
+    // FIXME: cache the accessor
+
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
     try {
         auto [accessor, final] = scheme->getAccessor(store, *this);
+        accessor->fingerprint = scheme->getFingerprint(store, *this);
         checkLocks(final);
         return {accessor, std::move(final)};
     } catch (Error & e) {
@@ -256,10 +259,7 @@ std::optional<time_t> Input::getLastModified() const
 
 std::optional<std::string> Input::getFingerprint(ref<Store> store) const
 {
-    if (auto rev = getRev())
+    return scheme ? scheme->getFingerprint(store, *this) : std::nullopt;
-        return rev->gitRev();
-    assert(scheme);
-    return scheme->getFingerprint(store, *this);
 }
 
 ParsedURL InputScheme::toURL(const Input & input) const
@@ -293,4 +293,12 @@ void InputScheme::clone(const Input & input, const Path & destDir) const
     throw Error("do not know how to clone input '%s'", input.to_string());
 }
 
+std::optional<std::string> InputScheme::getFingerprint(ref<Store> store, const Input & input) const
+{
+    if (auto rev = input.getRev())
+        return rev->gitRev();
+    else
+        return std::nullopt;
+}
+
 }

src/libfetchers/fetchers.hh

@@ -141,8 +141,7 @@ struct InputScheme
     virtual std::optional<CanonPath> isRelative(const Input & input) const
     { return std::nullopt; }
 
-    virtual std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const
+    virtual std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const;
-    { return std::nullopt; }
 };
 
 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);

src/libfetchers/input-accessor.cc

@@ -1,6 +1,7 @@
 #include "input-accessor.hh"
 #include "util.hh"
 #include "store-api.hh"
+#include "cache.hh"
 
 #include <atomic>
 
@@ -96,6 +97,21 @@ StorePath InputAccessor::fetchToStore(
     // FIXME: add an optimisation for the case where the accessor is
     // an FSInputAccessor pointing to a store path.
 
+    std::optional<std::string> cacheKey;
+
+    if (!filter && fingerprint) {
+        cacheKey = *fingerprint + "|" + name + "|" + path.abs();
+        if (auto storePathS = fetchers::getCache()->queryFact(*cacheKey)) {
+            if (auto storePath = store->maybeParseStorePath(*storePathS)) {
+                if (store->isValidPath(*storePath)) {
+                    debug("store path cache hit for '%s'", showPath(path));
+                    return *storePath;
+                }
+            }
+        }
+    } else
+        debug("source path '%s' is uncacheable", showPath(path));
+
     auto source = sinkToSource([&](Sink & sink) {
         dumpPath(path, sink, filter ? *filter : defaultPathFilter);
     });
@@ -105,6 +121,9 @@ StorePath InputAccessor::fetchToStore(
         ? store->computeStorePathFromDump(*source, name).first
         : store->addToStoreFromDump(*source, name, FileIngestionMethod::Recursive, htSHA256, repair);
 
+    if (cacheKey)
+        fetchers::getCache()->upsertFact(*cacheKey, store->printStorePath(storePath));
+
     return storePath;
 }
 

src/libfetchers/input-accessor.hh

@@ -21,6 +21,8 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
 
     std::string displayPrefix, displaySuffix;
 
+    std::optional<std::string> fingerprint;
+
     InputAccessor();
 
     virtual ~InputAccessor()