Restrict `readFile` context to references that appear in the string

When calling `builtins.readFile` on a store path, the references of that
path are currently added to the resulting string's context.

This change makes those references the *possible* context of the string,
but filters them to keep only the references whose hash actually appears
in the string, similarly to what is done for determining the runtime
references of a path.

3 files changed, 53 insertions, 18 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 28b998474..ff620ca63 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -5,6 +5,7 @@
 #include "globals.hh"
 #include "json-to-value.hh"
 #include "names.hh"
+#include "references.hh"
 #include "store-api.hh"
 #include "util.hh"
 #include "json.hh"
@@ -1542,6 +1543,10 @@ static void prim_readFile(EvalState & state, const PosIdx pos, Value * * args, V
             refs = state.store->queryPathInfo(state.store->toStorePath(path).first)->references;
         } catch (Error &) { // FIXME: should be InvalidPathError
         }
+        // Re-scan references to filter down to just the ones that actually occur in the file.
+        auto refsSink = PathRefScanSink::fromPaths(refs);
+        refsSink << s;
+        refs = refsSink.getResultPaths();
     }
     auto context = state.store->printStorePathSet(refs);
     v.mkString(s, context);
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index 34dce092c..3bb297fc8 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -67,20 +67,12 @@ void RefScanSink::operator () (std::string_view data)
 }
 
 
-std::pair<StorePathSet, HashResult> scanForReferences(
-    const std::string & path,
-    const StorePathSet & refs)
-{
-    HashSink hashSink { htSHA256 };
-    auto found = scanForReferences(hashSink, path, refs);
-    auto hash = hashSink.finish();
-    return std::pair<StorePathSet, HashResult>(found, hash);
-}
+PathRefScanSink::PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap)
+    : RefScanSink(std::move(hashes))
+    , backMap(std::move(backMap))
+{ }
 
-StorePathSet scanForReferences(
-    Sink & toTee,
-    const Path & path,
-    const StorePathSet & refs)
+PathRefScanSink PathRefScanSink::fromPaths(const StorePathSet & refs)
 {
     StringSet hashes;
     std::map<std::string, StorePath> backMap;
@@ -92,14 +84,14 @@ StorePathSet scanForReferences(
         hashes.insert(hashPart);
     }
 
-    /* Look for the hashes in the NAR dump of the path. */
-    RefScanSink refsSink(std::move(hashes));
-    TeeSink sink { refsSink, toTee };
-    dumpPath(path, sink);
+    return PathRefScanSink(std::move(hashes), std::move(backMap));
+}
 
+StorePathSet PathRefScanSink::getResultPaths()
+{
     /* Map the hashes found back to their store paths. */
     StorePathSet found;
-    for (auto & i : refsSink.getResult()) {
+    for (auto & i : getResult()) {
         auto j = backMap.find(i);
         assert(j != backMap.end());
         found.insert(j->second);
@@ -109,6 +101,31 @@ StorePathSet scanForReferences(
 }
 
 
+std::pair<StorePathSet, HashResult> scanForReferences(
+    const std::string & path,
+    const StorePathSet & refs)
+{
+    HashSink hashSink { htSHA256 };
+    auto found = scanForReferences(hashSink, path, refs);
+    auto hash = hashSink.finish();
+    return std::pair<StorePathSet, HashResult>(found, hash);
+}
+
+StorePathSet scanForReferences(
+    Sink & toTee,
+    const Path & path,
+    const StorePathSet & refs)
+{
+    PathRefScanSink refsSink = PathRefScanSink::fromPaths(refs);
+    TeeSink sink { refsSink, toTee };
+
+    /* Look for the hashes in the NAR dump of the path. */
+    dumpPath(path, sink);
+
+    return refsSink.getResultPaths();
+}
+
+
 RewritingSink::RewritingSink(const std::string & from, const std::string & to, Sink & nextSink)
     : from(from), to(to), nextSink(nextSink)
 {
diff --git a/src/libstore/references.hh b/src/libstore/references.hh
index a6119c861..6f381f96c 100644
--- a/src/libstore/references.hh
+++ b/src/libstore/references.hh
@@ -27,6 +27,19 @@ public:
     void operator () (std::string_view data) override;
 };
 
+class PathRefScanSink : public RefScanSink
+{
+    std::map<std::string, StorePath> backMap;
+
+    PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap);
+
+public:
+
+    static PathRefScanSink fromPaths(const StorePathSet & refs);
+
+    StorePathSet getResultPaths();
+};
+
 struct RewritingSink : Sink
 {
     std::string from, to, prev;