Add setting to warn about copying/hashing large paths

This is useful for diagnosing whether an evaluation is copying large
paths to the store. Example:

   $ nix build .#packages.x86_64-linux.default --large-path-warning-threshold 1000000
   warning: copied large path '/home/eelco/Dev/nix-master/' to the store (6271792 bytes)
   warning: copied large path '«github:NixOS/nixpkgs/b550fe4b4776908ac2a861124307045f8e717c8e?narHash=sha256-7kkJQd4rZ%2BvFrzWu8sTRtta5D1kBG0LSRYAfhtmMlSo%3D»/' to the store (155263768 bytes)
   warning: copied large path '«github:libgit2/libgit2/45fd9ed7ae1a9b74b957ef4f337bc3c8b3df01b5?narHash=sha256-oX4Z3S9WtJlwvj0uH9HlYcWv%2Bx1hqp8mhXl7HsLu2f0%3D»/' to the store (22175416 bytes)
   warning: copied large path '/nix/store/z985088mcd6w23qwdlirsinnyzayagki-source' to the store (5885872 bytes)

src/libutil/file-content-address.cc

@@ -112,17 +112,19 @@ HashResult hashPath(
 }
 
 
-Hash hashPath(
+std::pair<Hash, std::optional<uint64_t>> hashPath(
     const SourcePath & path,
     FileIngestionMethod method, HashAlgorithm ht,
     PathFilter & filter)
 {
     switch (method) {
     case FileIngestionMethod::Flat:
-    case FileIngestionMethod::Recursive:
-        return hashPath(path, (FileSerialisationMethod) method, ht, filter).first;
+    case FileIngestionMethod::Recursive: {
+        auto res = hashPath(path, (FileSerialisationMethod) method, ht, filter);
+        return {res.first, {res.second}};
+    }
     case FileIngestionMethod::Git:
-        return git::dumpHash(ht, path, filter).hash;
+        return {git::dumpHash(ht, path, filter).hash, std::nullopt};
     }
     assert(false);
 }

src/libutil/file-content-address.hh

@@ -132,14 +132,15 @@ std::string_view renderFileIngestionMethod(FileIngestionMethod method);
 
 /**
  * Compute the hash of the given file system object according to the
- * given method.
+ * given method, and for some ingestion methods, the size of the
+ * serialisation.
  *
  * Unlike the other `hashPath`, this works on an arbitrary
  * `FileIngestionMethod` instead of `FileSerialisationMethod`, but
- * doesn't return the size as this is this is not a both simple and
+ * may not return the size as this is this is not a both simple and
  * useful defined for a merkle format.
  */
-Hash hashPath(
+std::pair<Hash, std::optional<uint64_t>> hashPath(
     const SourcePath & path,
     FileIngestionMethod method, HashAlgorithm ha,
     PathFilter & filter = defaultPathFilter);

src/libutil/serialise.hh

@@ -283,6 +283,26 @@ struct LengthSink : Sink
     }
 };
 
+/**
+ * A wrapper source that counts the number of bytes read from it.
+ */
+struct LengthSource : Source
+{
+    Source & next;
+
+    LengthSource(Source & next) : next(next)
+    { }
+
+    uint64_t total = 0;
+
+    size_t read(char * data, size_t len) override
+    {
+        auto n = next.read(data, len);
+        total += n;
+        return n;
+    }
+};
+
 /**
  * Convert a function into a sink.
  */