fetchClosure: Allow a path to be rewritten to CA on the fly

The advantage is that the resulting closure doesn't need to be signed,
so you don't need to configure any binary cache keys on the client.

src/libstore/make-content-addressed.cc

@@ -0,0 +1,79 @@
+#include "make-content-addressed.hh"
+#include "references.hh"
+
+namespace nix {
+
+std::map<StorePath, StorePath> makeContentAddressed(
+    Store & srcStore,
+    Store & dstStore,
+    const StorePathSet & storePaths)
+{
+    // FIXME: use closure of storePaths.
+
+    auto paths = srcStore.topoSortPaths(storePaths);
+
+    std::reverse(paths.begin(), paths.end());
+
+    std::map<StorePath, StorePath> remappings;
+
+    for (auto & path : paths) {
+        auto pathS = srcStore.printStorePath(path);
+        auto oldInfo = srcStore.queryPathInfo(path);
+        std::string oldHashPart(path.hashPart());
+
+        StringSink sink;
+        srcStore.narFromPath(path, sink);
+
+        StringMap rewrites;
+
+        StorePathSet references;
+        bool hasSelfReference = false;
+        for (auto & ref : oldInfo->references) {
+            if (ref == path)
+                hasSelfReference = true;
+            else {
+                auto i = remappings.find(ref);
+                auto replacement = i != remappings.end() ? i->second : ref;
+                // FIXME: warn about unremapped paths?
+                if (replacement != ref)
+                    rewrites.insert_or_assign(srcStore.printStorePath(ref), srcStore.printStorePath(replacement));
+                references.insert(std::move(replacement));
+            }
+        }
+
+        sink.s = rewriteStrings(sink.s, rewrites);
+
+        HashModuloSink hashModuloSink(htSHA256, oldHashPart);
+        hashModuloSink(sink.s);
+
+        auto narHash = hashModuloSink.finish().first;
+
+        ValidPathInfo info {
+            dstStore.makeFixedOutputPath(FileIngestionMethod::Recursive, narHash, path.name(), references, hasSelfReference),
+            narHash,
+        };
+        info.references = std::move(references);
+        if (hasSelfReference) info.references.insert(info.path);
+        info.narSize = sink.s.size();
+        info.ca = FixedOutputHash {
+            .method = FileIngestionMethod::Recursive,
+            .hash = info.narHash,
+        };
+
+        printInfo("rewrote '%s' to '%s'", pathS, srcStore.printStorePath(info.path));
+
+        auto source = sinkToSource([&](Sink & nextSink) {
+            RewritingSink rsink2(oldHashPart, std::string(info.path.hashPart()), nextSink);
+            rsink2(sink.s);
+            rsink2.flush();
+        });
+
+        dstStore.addToStore(info, *source);
+
+        remappings.insert_or_assign(std::move(path), std::move(info.path));
+    }
+
+    return remappings;
+}
+
+}

src/libstore/make-content-addressed.hh

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "store-api.hh"
+
+namespace nix {
+
+std::map<StorePath, StorePath> makeContentAddressed(
+    Store & srcStore,
+    Store & dstStore,
+    const StorePathSet & storePaths);
+
+}