diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 66a64a6be..e3a53c0d0 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -2,6 +2,7 @@
 #include "util.hh"
 
 #include <cstring>
+#include <cerrno>
 
 
 namespace nix {
@@ -38,7 +39,28 @@ void FdSink::flush()
 
 void FdSource::operator () (unsigned char * data, unsigned int len)
 {
-    readFull(fd, data, len);
+    if (!buffer) buffer = new unsigned char[bufSize];
+
+    while (len) {
+        if (!bufPosIn) {
+            /* Read as much data as is available (up to the buffer
+               size). */
+            checkInterrupt();
+            ssize_t n = read(fd, (char *) buffer, bufSize);
+            if (n == -1) {
+                if (errno == EINTR) continue;
+                throw SysError("reading from file");
+            }
+            if (n == 0) throw EndOfFile("unexpected end-of-file");
+            bufPosIn = n;
+        }
+            
+        /* Copy out the data in the buffer. */
+        size_t n = len > bufPosIn - bufPosOut ? bufPosIn - bufPosOut : len;
+        memcpy(data, buffer + bufPosOut, n);
+        data += n; bufPosOut += n; len -= n;
+        if (bufPosIn == bufPosOut) bufPosIn = bufPosOut = 0;
+    }
 }
 
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 711bd5e6c..b8d4d7a84 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -38,9 +38,7 @@ struct FdSink : Sink
     FdSink() : fd(-1), bufSize(32 * 1024), bufPos(0), buffer(0) { }
     
     FdSink(int fd, unsigned int bufSize = 32 * 1024)
-        : fd(fd), bufSize(bufSize), bufPos(0), buffer(0)
-    {
-    }
+        : fd(fd), bufSize(bufSize), bufPos(0), buffer(0) { }
 
     ~FdSink()
     {
@@ -58,15 +56,17 @@ struct FdSink : Sink
 struct FdSource : Source
 {
     int fd;
+    unsigned int bufSize, bufPosIn, bufPosOut;
+    unsigned char * buffer;
 
-    FdSource()
-    {
-        fd = -1;
-    }
+    FdSource() : fd(-1), bufSize(32 * 1024), bufPosIn(0), bufPosOut(0), buffer(0) { }
     
-    FdSource(int fd) 
+    FdSource(int fd, unsigned int bufSize = 32 * 1024)
+        : fd(fd), bufSize(bufSize), bufPosIn(0), bufPosOut(0), buffer(0) { }
+    
+    ~FdSource()
     {
-        this->fd = fd;
+        if (buffer) delete[] buffer;
     }
     
     void operator () (unsigned char * data, unsigned int len);