Print a warning when loading a large path into memory

I.e. if you have a derivation with

  src = ./huge-directory;

you'll get a warning that this is not a good idea.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index e5a9df1d050a..3d296cf6d8d6 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -72,6 +72,8 @@ struct BufferedSource : Source
 struct FdSink : BufferedSink
 {
     int fd;
+    bool warn;
+    size_t written;
 
     FdSink() : fd(-1) { }
     FdSink(int fd) : fd(fd) { }
@@ -95,10 +97,7 @@ struct FdSource : BufferedSource
 struct StringSink : Sink
 {
     string s;
-    void operator () (const unsigned char * data, size_t len)
-    {
-        s.append((const char *) data, len);
-    }
+    void operator () (const unsigned char * data, size_t len);
 };