diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 5107a93de2..525e5fc7bb 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1103,16 +1103,14 @@ struct HashAndWriteSink : Sink
     HashAndWriteSink(Sink & writeSink) : writeSink(writeSink), hashSink(htSHA256)
     {
     }
-    virtual void operator ()
-        (const unsigned char * data, unsigned int len)
+    virtual void operator () (const unsigned char * data, size_t len)
     {
         writeSink(data, len);
         hashSink(data, len);
     }
     Hash currentHash()
     {
-        HashSink hashSinkClone(hashSink);
-        return hashSinkClone.finish().first;
+        return hashSink.currentHash().first;
     }
 };
 
@@ -1201,8 +1199,7 @@ struct HashAndReadSource : Source
     {
         hashing = true;
     }
-    virtual void operator ()
-        (unsigned char * data, unsigned int len)
+    virtual void operator () (unsigned char * data, size_t len)
     {
         readSource(data, len);
         if (hashing) hashSink(data, len);
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index ade9c9aa20..c1f9e3ba77 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -57,11 +57,11 @@ struct RefScanSink : Sink
 
     RefScanSink() : hashSink(htSHA256) { }
     
-    void operator () (const unsigned char * data, unsigned int len);
+    void operator () (const unsigned char * data, size_t len);
 };
 
 
-void RefScanSink::operator () (const unsigned char * data, unsigned int len)
+void RefScanSink::operator () (const unsigned char * data, size_t len)
 {
     hashSink(data, len);
 
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 8269b6a831..7bf0ad7bd4 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -374,7 +374,7 @@ Path RemoteStore::importPath(bool requireSignature, Source & source)
     openConnection();
     writeInt(wopImportPath, to);
     /* We ignore requireSignature, since the worker forces it to true
-       anyway. */    
+       anyway. */
     processStderr(0, &source);
     return readStorePath(from);
 }
diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 5334234410..bbfe7847fd 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -306,21 +306,13 @@ HashSink::HashSink(HashType ht) : ht(ht)
     start(ht, *ctx);
 }
     
-HashSink::HashSink(const HashSink & h)
-{
-    ht = h.ht;
-    bytes = h.bytes;
-    ctx = new Ctx;
-    *ctx = *h.ctx;
-}
-    
 HashSink::~HashSink()
 {
+    bufPos = 0;
     delete ctx;
 }
 
-void HashSink::operator ()
-    (const unsigned char * data, unsigned int len)
+void HashSink::write(const unsigned char * data, size_t len)
 {
     bytes += len;
     update(ht, *ctx, data, len);
@@ -328,11 +320,21 @@ void HashSink::operator ()
 
 HashResult HashSink::finish()
 {
+    flush();
     Hash hash(ht);
     nix::finish(ht, *ctx, hash.hash);
     return HashResult(hash, bytes);
 }
 
+HashResult HashSink::currentHash()
+{
+    flush();
+    Ctx ctx2 = *ctx;
+    Hash hash(ht);
+    nix::finish(ht, ctx2, hash.hash);
+    return HashResult(hash, bytes);
+}
+
 
 HashResult hashPath(
     HashType ht, const Path & path, PathFilter & filter)
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index cbdcf4c8d4..e0b6478cc4 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -91,7 +91,7 @@ string printHashType(HashType ht);
 
 union Ctx;
 
-class HashSink : public Sink
+class HashSink : public BufferedSink
 {
 private:
     HashType ht;
@@ -102,8 +102,9 @@ public:
     HashSink(HashType ht);
     HashSink(const HashSink & h);
     ~HashSink();
-    virtual void operator () (const unsigned char * data, unsigned int len);
+    void write(const unsigned char * data, size_t len);
     HashResult finish();
+    HashResult currentHash();
 };
 
 
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index e3a53c0d01..a82262704e 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -8,7 +8,16 @@
 namespace nix {
 
 
-void FdSink::operator () (const unsigned char * data, unsigned int len)
+BufferedSink::~BufferedSink()
+{
+    /* We can't call flush() here, because C++ for some insane reason
+       doesn't allow you to call virtual methods from a destructor. */
+    assert(!bufPos);
+    if (buffer) delete[] buffer;
+}
+
+    
+void BufferedSink::operator () (const unsigned char * data, size_t len)
 {
     if (!buffer) buffer = new unsigned char[bufSize];
     
@@ -16,7 +25,7 @@ void FdSink::operator () (const unsigned char * data, unsigned int len)
         /* Optimisation: bypass the buffer if the data exceeds the
            buffer size and there is no unflushed data. */
         if (bufPos == 0 && len >= bufSize) {
-            writeFull(fd, data, len);
+            write(data, len);
             break;
         }
         /* Otherwise, copy the bytes to the buffer.  Flush the buffer
@@ -29,31 +38,32 @@ void FdSink::operator () (const unsigned char * data, unsigned int len)
 }
 
 
-void FdSink::flush()
+void BufferedSink::flush()
 {
-    if (fd == -1 || bufPos == 0) return;
-    writeFull(fd, buffer, bufPos);
+    if (bufPos == 0) return;
+    write(buffer, bufPos);
     bufPos = 0;
 }
 
 
-void FdSource::operator () (unsigned char * data, unsigned int len)
+void FdSink::write(const unsigned char * data, size_t len)
+{
+    writeFull(fd, data, len);
+}
+
+
+BufferedSource::~BufferedSource()
+{
+    if (buffer) delete[] buffer;
+}
+
+
+void BufferedSource::operator () (unsigned char * data, size_t 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;
-        }
+        if (!bufPosIn) bufPosIn = read(buffer, bufSize);
             
         /* Copy out the data in the buffer. */
         size_t n = len > bufPosIn - bufPosOut ? bufPosIn - bufPosOut : len;
@@ -64,7 +74,20 @@ void FdSource::operator () (unsigned char * data, unsigned int len)
 }
 
 
-void writePadding(unsigned int len, Sink & sink)
+size_t FdSource::read(unsigned char * data, size_t len)
+{
+    ssize_t n;
+    do {
+        checkInterrupt();
+        n = ::read(fd, (char *) data, bufSize);
+    } while (n == -1 && errno == EINTR);
+    if (n == -1) throw SysError("reading from file");
+    if (n == 0) throw EndOfFile("unexpected end-of-file");
+    return n;
+}
+
+
+void writePadding(size_t len, Sink & sink)
 {
     if (len % 8) {
         unsigned char zero[8];
@@ -103,7 +126,7 @@ void writeLongLong(unsigned long long n, Sink & sink)
 
 void writeString(const string & s, Sink & sink)
 {
-    unsigned int len = s.length();
+    size_t len = s.length();
     writeInt(len, sink);
     sink((const unsigned char *) s.c_str(), len);
     writePadding(len, sink);
@@ -118,11 +141,11 @@ void writeStringSet(const StringSet & ss, Sink & sink)
 }
 
 
-void readPadding(unsigned int len, Source & source)
+void readPadding(size_t len, Source & source)
 {
     if (len % 8) {
         unsigned char zero[8];
-        unsigned int n = 8 - (len % 8);
+        size_t n = 8 - (len % 8);
         source(zero, n);
         for (unsigned int i = 0; i < n; i++)
             if (zero[i]) throw SerialisationError("non-zero padding");
@@ -162,7 +185,7 @@ unsigned long long readLongLong(Source & source)
 
 string readString(Source & source)
 {
-    unsigned int len = readInt(source);
+    size_t len = readInt(source);
     unsigned char * buf = new unsigned char[len];
     AutoDeleteArray<unsigned char> d(buf);
     source(buf, len);
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index b8d4d7a849..a0588668f0 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -11,7 +11,25 @@ namespace nix {
 struct Sink 
 {
     virtual ~Sink() { }
-    virtual void operator () (const unsigned char * data, unsigned int len) = 0;
+    virtual void operator () (const unsigned char * data, size_t len) = 0;
+};
+
+
+/* A buffered abstract sink. */
+struct BufferedSink : Sink
+{
+    size_t bufSize, bufPos;
+    unsigned char * buffer;
+
+    BufferedSink(size_t bufSize = 32 * 1024)
+        : bufSize(bufSize), bufPos(0), buffer(0) { }
+    ~BufferedSink();
+    
+    void operator () (const unsigned char * data, size_t len);
+    
+    void flush();
+    
+    virtual void write(const unsigned char * data, size_t len) = 0;
 };
 
 
@@ -20,56 +38,52 @@ struct Source
 {
     virtual ~Source() { }
     
-    /* The callee should store exactly *len bytes in the buffer
-       pointed to by data.  It should block if that much data is not
-       yet available, or throw an error if it is not going to be
-       available. */
-    virtual void operator () (unsigned char * data, unsigned int len) = 0;
+    /* Store exactly ‘len’ bytes in the buffer pointed to by ‘data’.
+       It blocks if that much data is not yet available, or throws an
+       error if it is not going to be available. */
+    virtual void operator () (unsigned char * data, size_t len) = 0;
 };
 
 
-/* A sink that writes data to a file descriptor (using a buffer). */
-struct FdSink : Sink
+/* A buffered abstract source. */
+struct BufferedSource : Source
 {
-    int fd;
-    unsigned int bufSize, bufPos;
+    size_t bufSize, bufPosIn, bufPosOut;
     unsigned char * buffer;
 
-    FdSink() : fd(-1), bufSize(32 * 1024), bufPos(0), buffer(0) { }
+    BufferedSource(size_t bufSize = 32 * 1024)
+        : bufSize(bufSize), bufPosIn(0), bufPosOut(0), buffer(0) { }
+    ~BufferedSource();
     
-    FdSink(int fd, unsigned int bufSize = 32 * 1024)
-        : fd(fd), bufSize(bufSize), bufPos(0), buffer(0) { }
-
-    ~FdSink()
-    {
-        flush();
-        if (buffer) delete[] buffer;
-    }
+    void operator () (unsigned char * data, size_t len);
     
-    void operator () (const unsigned char * data, unsigned int len);
+    /* Store up to ‘len’ in the buffer pointed to by ‘data’, and
+       return the number of bytes stored.  If should block until at
+       least one byte is available. */
+    virtual size_t read(unsigned char * data, size_t len) = 0;
+};
 
-    void flush();
+
+/* A sink that writes data to a file descriptor. */
+struct FdSink : BufferedSink
+{
+    int fd;
+
+    FdSink() : fd(-1) { }
+    FdSink(int fd) : fd(fd) { }
+    ~FdSink() { flush(); }
+    
+    void write(const unsigned char * data, size_t len);
 };
 
 
 /* A source that reads data from a file descriptor. */
-struct FdSource : Source
+struct FdSource : BufferedSource
 {
     int fd;
-    unsigned int bufSize, bufPosIn, bufPosOut;
-    unsigned char * buffer;
-
-    FdSource() : fd(-1), bufSize(32 * 1024), bufPosIn(0), bufPosOut(0), buffer(0) { }
-    
-    FdSource(int fd, unsigned int bufSize = 32 * 1024)
-        : fd(fd), bufSize(bufSize), bufPosIn(0), bufPosOut(0), buffer(0) { }
-    
-    ~FdSource()
-    {
-        if (buffer) delete[] buffer;
-    }
-    
-    void operator () (unsigned char * data, unsigned int len);
+    FdSource() : fd(-1) { }
+    FdSource(int fd) : fd(fd) { }
+    size_t read(unsigned char * data, size_t len);
 };
 
 
@@ -77,7 +91,7 @@ struct FdSource : Source
 struct StringSink : Sink
 {
     string s;
-    virtual void operator () (const unsigned char * data, unsigned int len)
+    void operator () (const unsigned char * data, size_t len)
     {
         s.append((const char *) data, len);
     }
@@ -88,9 +102,9 @@ struct StringSink : Sink
 struct StringSource : Source
 {
     const string & s;
-    unsigned int pos;
+    size_t pos;
     StringSource(const string & _s) : s(_s), pos(0) { }
-    virtual void operator () (unsigned char * data, unsigned int len)
+    virtual void operator () (unsigned char * data, size_t len)
     {
         s.copy((char *) data, len, pos);
         pos += len;
@@ -100,13 +114,13 @@ struct StringSource : Source
 };
 
 
-void writePadding(unsigned int len, Sink & sink);
+void writePadding(size_t len, Sink & sink);
 void writeInt(unsigned int n, Sink & sink);
 void writeLongLong(unsigned long long n, Sink & sink);
 void writeString(const string & s, Sink & sink);
 void writeStringSet(const StringSet & ss, Sink & sink);
 
-void readPadding(unsigned int len, Source & source);
+void readPadding(size_t len, Source & source);
 unsigned int readInt(Source & source);
 unsigned long long readLongLong(Source & source);
 string readString(Source & source);
diff --git a/src/nix-worker/nix-worker.cc b/src/nix-worker/nix-worker.cc
index 6c222420e0..a898526382 100644
--- a/src/nix-worker/nix-worker.cc
+++ b/src/nix-worker/nix-worker.cc
@@ -202,8 +202,7 @@ struct TunnelSink : Sink
 {
     Sink & to;
     TunnelSink(Sink & to) : to(to) { }
-    virtual void operator ()
-        (const unsigned char * data, unsigned int len)
+    virtual void operator () (const unsigned char * data, size_t len)
     {
         writeInt(STDERR_WRITE, to);
         writeString(string((const char *) data, len), to);
@@ -215,8 +214,7 @@ struct TunnelSource : Source
 {
     Source & from;
     TunnelSource(Source & from) : from(from) { }
-    virtual void operator ()
-        (unsigned char * data, unsigned int len)
+    virtual void operator () (unsigned char * data, size_t len)
     {
         /* Careful: we're going to receive data from the client now,
            so we have to disable the SIGPOLL handler. */
@@ -267,7 +265,7 @@ struct SavingSourceAdapter : Source
     Source & orig;
     string s;
     SavingSourceAdapter(Source & orig) : orig(orig) { }
-    void operator () (unsigned char * data, unsigned int len)
+    void operator () (unsigned char * data, size_t len)
     {
         orig(data, len);
         s.append((const char *) data, len);