diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 07c722016..d6ded0a24 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -385,7 +385,7 @@ StorePath BinaryCacheStore::addToStore(
     if (method == FileIngestionMethod::Recursive) {
         dumpPath(srcPath, sink, filter);
     } else {
-        readFile(srcPath, sink);
+        readFileSource(srcPath)->drainInto(sink);
     }
     auto h = sink.finish().first;
 
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 7066f5c93..bae821266 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -2486,7 +2486,7 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             HashModuloSink caSink { outputHash.hashType, oldHashPart };
             std::visit(overloaded {
                 [&](const TextIngestionMethod &) {
-                    readFile(actualPath, caSink);
+                    readFileSource(actualPath)->drainInto(caSink);
                 },
                 [&](const FileIngestionMethod & m2) {
                     switch (m2) {
@@ -2494,7 +2494,7 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                         dumpPath(actualPath, caSink);
                         break;
                     case FileIngestionMethod::Flat:
-                        readFile(actualPath, caSink);
+                        readFileSource(actualPath)->drainInto(caSink);
                         break;
                     }
                 },
diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
index 5684dcd80..5f6730476 100644
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@@ -71,7 +71,7 @@ protected:
     void getFile(const std::string & path, Sink & sink) override
     {
         try {
-            readFile(binaryCacheDir + "/" + path, sink);
+            readFileSource(binaryCacheDir + "/" + path)->drainInto(sink);
         } catch (SysError & e) {
             if (e.errNo == ENOENT)
                 throw NoSuchBinaryCacheFile("file '%s' does not exist in binary cache", path);
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 5e79630c6..6f441a0a1 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1890,7 +1890,7 @@ ContentAddress LocalStore::hashCAPath(
     HashModuloSink caSink ( hashType, std::string(pathHash) );
     std::visit(overloaded {
         [&](const TextIngestionMethod &) {
-            readFile(path, caSink);
+            readFileSource(path)->drainInto(caSink);
         },
         [&](const FileIngestionMethod & m2) {
             switch (m2) {
@@ -1898,7 +1898,7 @@ ContentAddress LocalStore::hashCAPath(
                 dumpPath(path, caSink);
                 break;
             case FileIngestionMethod::Flat:
-                readFile(path, caSink);
+                readFileSource(path)->drainInto(caSink);
                 break;
             }
         },
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 55083e834..e954acff3 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -279,7 +279,7 @@ StorePath Store::addToStore(
         if (method == FileIngestionMethod::Recursive)
             dumpPath(srcPath, sink, filter);
         else
-            readFile(srcPath, sink);
+            readFileSource(srcPath)->drainInto(sink);
     });
     return addToStoreFromDump(*source, name, method, hashAlgo, repair, references);
 }
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index d573b22b4..f51f3c092 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -289,12 +289,17 @@ std::string readFile(const Path & path)
 }
 
 
-void readFile(const Path & path, Sink & sink)
+box_ptr<Source> readFileSource(const Path & path)
 {
     AutoCloseFD fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
     if (!fd)
         throw SysError("opening file '%s'", path);
-    drainFD(fd.get(), sink);
+
+    struct FileSource : FdSource {
+        AutoCloseFD fd;
+        explicit FileSource(AutoCloseFD fd) : FdSource(fd.get()), fd(std::move(fd)) {}
+    };
+    return make_box_ptr<FileSource>(std::move(fd));
 }
 
 
diff --git a/src/libutil/file-system.hh b/src/libutil/file-system.hh
index 6c1923d55..64d884227 100644
--- a/src/libutil/file-system.hh
+++ b/src/libutil/file-system.hh
@@ -5,6 +5,7 @@
  * Utiltities for working with the file sytem and file paths.
  */
 
+#include "box_ptr.hh"
 #include "types.hh"
 #include "file-descriptor.hh"
 
@@ -142,7 +143,7 @@ unsigned char getFileType(const Path & path);
  * Read the contents of a file into a string.
  */
 std::string readFile(const Path & path);
-void readFile(const Path & path, Sink & sink);
+box_ptr<Source> readFileSource(const Path & path);
 
 /**
  * Write a string to a file.
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 006b5000c..822fa150e 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -324,7 +324,7 @@ Hash hashString(HashType ht, std::string_view s)
 Hash hashFile(HashType ht, const Path & path)
 {
     HashSink sink(ht);
-    readFile(path, sink);
+    readFileSource(path)->drainInto(sink);
     return sink.finish().first;
 }
 
diff --git a/src/nix/add-to-store.cc b/src/nix/add-to-store.cc
index 39e5cc99d..7dbbbcc56 100644
--- a/src/nix/add-to-store.cc
+++ b/src/nix/add-to-store.cc
@@ -37,7 +37,7 @@ struct CmdAddToStore : MixDryRun, StoreCommand
         Hash hash = narHash;
         if (ingestionMethod == FileIngestionMethod::Flat) {
             HashSink hsink(htSHA256);
-            readFile(path, hsink);
+            readFileSource(path)->drainInto(hsink);
             hash = hsink.finish().first;
         }
 
diff --git a/src/nix/hash.cc b/src/nix/hash.cc
index 9feca9345..66c5516e7 100644
--- a/src/nix/hash.cc
+++ b/src/nix/hash.cc
@@ -85,7 +85,7 @@ struct CmdHashBase : Command
 
             switch (mode) {
             case FileIngestionMethod::Flat:
-                readFile(path, *hashSink);
+                readFileSource(path)->drainInto(*hashSink);
                 break;
             case FileIngestionMethod::Recursive:
                 dumpPath(path, *hashSink);