diff --git a/mozilla/netwerk/base/public/nsIFileStreams.idl b/mozilla/netwerk/base/public/nsIFileStreams.idl
index 4a186d51bd4..c8b7c3d4503 100644
--- a/mozilla/netwerk/base/public/nsIFileStreams.idl
+++ b/mozilla/netwerk/base/public/nsIFileStreams.idl
@@ -47,6 +47,9 @@ interface nsISeekableStream : nsISupports
     
     void seek(in long whence, in long offset);
     unsigned long tell();
+
+    // truncate stream to the current offset
+    void setEOF();
 };
 
 [scriptable, uuid(616f5b48-da09-11d3-8cda-0060b0fc14a3)]
diff --git a/mozilla/netwerk/base/src/nsBufferedStreams.cpp b/mozilla/netwerk/base/src/nsBufferedStreams.cpp
index 3d8d98e33c0..55f2548da82 100644
--- a/mozilla/netwerk/base/src/nsBufferedStreams.cpp
+++ b/mozilla/netwerk/base/src/nsBufferedStreams.cpp
@@ -138,6 +138,19 @@ nsBufferedStream::Tell(PRUint32 *result)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsBufferedStream::SetEOF()
+{
+    if (mStream == nsnull)
+        return NS_BASE_STREAM_CLOSED;
+    
+    nsresult rv;
+    nsCOMPtr<nsISeekableStream> ras = do_QueryInterface(mStream, &rv);
+    if (NS_FAILED(rv)) return rv;
+
+    return ras->SetEOF();
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // nsBufferedInputStream
 
diff --git a/mozilla/netwerk/base/src/nsFileStreams.cpp b/mozilla/netwerk/base/src/nsFileStreams.cpp
index 85f17d76e7c..5af2be19e76 100644
--- a/mozilla/netwerk/base/src/nsFileStreams.cpp
+++ b/mozilla/netwerk/base/src/nsFileStreams.cpp
@@ -20,6 +20,24 @@
  * Contributor(s): 
  */
 
+// file truncation support
+#if defined(__linux)
+#define _BSD_SOURCE 1
+#include <unistd.h>
+#elif defined(XP_MAC)
+#include <Files.h>
+#elif defined(XP_WIN)
+#include <windows.h>
+#else
+// XXX add necessary include file for ftruncate (or equivalent)
+#endif
+
+#if defined(XP_MAC)
+#include "pprio.h"
+#else
+#include "private/pprio.h"
+#endif
+
 #include "nsFileStreams.h"
 #include "nsILocalFile.h"
 #include "nsXPIDLString.h"
@@ -345,6 +363,46 @@ nsFileStream::Tell(PRUint32 *result)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsFileStream::SetEOF()
+{
+    if (mFD == nsnull)
+        return NS_BASE_STREAM_CLOSED;
+
+#if defined(__linux) || defined(XP_MAC) || defined(XP_OS2)
+    // Some system calls require an EOF offset.
+    PRUint32 offset;
+    nsresult rv = Tell(&offset);
+    if (NS_FAILED(rv)) return rv;
+#endif
+
+#if defined(__linux)
+    if (ftruncate(PR_FileDesc2NativeHandle(mFD), offset) != 0) {
+        NS_ERROR("ftruncate failed");
+        return NS_ERROR_FAILURE;
+    }
+#elif defined(XP_MAC)
+    if (::SetEOF(PR_FileDesc2NativeHandle(mFD), offset) != 0) {
+        NS_ERROR("SetEOF failed");
+        return NS_ERROR_FAILURE;
+    }
+#elif defined(XP_WIN)
+    if (!SetEndOfFile((HANDLE) PR_FileDesc2NativeHandle(mFD))) {
+        NS_ERROR("SetEndOfFile failed");
+        return NS_ERROR_FAILURE;
+    }
+#elif defined(XP_OS2)
+    if (DosSetFileSize((HFILE) PR_FileDesc2NativeHandle(mFD), offset) != NO_ERROR) {
+        NS_ERROR("DosSetFileSize failed");
+        return NS_ERROR_FAILURE;
+    }
+#else
+    // XXX not implemented
+#endif
+
+    return NS_OK;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // nsFileInputStream
 
diff --git a/mozilla/netwerk/base/src/nsFileTransport.cpp b/mozilla/netwerk/base/src/nsFileTransport.cpp
index ea5da21125c..6f8b42c61c7 100644
--- a/mozilla/netwerk/base/src/nsFileTransport.cpp
+++ b/mozilla/netwerk/base/src/nsFileTransport.cpp
@@ -474,12 +474,19 @@ nsFileTransport::OpenOutputStream(PRUint32 aTransferOffset,
     NS_ASSERTION(aTransferCount == PRUint32(-1), "need to wrap output stream in one that truncates");
     
     nsresult rv = mStreamIO->GetOutputStream(aResult);
-    if (NS_SUCCEEDED(rv) && aTransferOffset) {
-        nsCOMPtr<nsISeekableStream> seekable(do_QueryInterface(*aResult, &rv));
-        if (NS_SUCCEEDED(rv) && seekable) {
+    if (NS_FAILED(rv)) return rv;
+
+    nsCOMPtr<nsISeekableStream> seekable(do_QueryInterface(*aResult, &rv));
+    if (seekable) {
+        if (aTransferOffset)
             rv = seekable->Seek(nsISeekableStream::NS_SEEK_SET, aTransferOffset);
-        }
+        if (NS_SUCCEEDED(rv))
+            rv = seekable->SetEOF();
     }
+    // a stream need not support nsISeekableStream unless a transfer offset
+    // greater than zero was requested.
+    else if (aTransferOffset == 0)
+        rv = NS_OK;
     
     return rv;
 }
@@ -673,14 +680,12 @@ nsFileTransport::Process(void)
         if (mOffset > 0) {
             // if we need to set a starting offset, QI for the nsISeekableStream
             // and set it
-            nsCOMPtr<nsISeekableStream> ras = do_QueryInterface(source, &mStatus);
-            if (NS_FAILED(mStatus)) {
-                mXferState = END_READ;
-                return;
+            nsCOMPtr<nsISeekableStream> seekable = do_QueryInterface(source, &mStatus);
+            if (NS_SUCCEEDED(mStatus)) {
+                // for now, assume the offset is always relative to the start of the
+                // file (position 0) so use PR_SEEK_SET
+                mStatus = seekable->Seek(PR_SEEK_SET, mOffset);
             }
-            // for now, assume the offset is always relative to the start of the
-            // file (position 0) so use PR_SEEK_SET
-            mStatus = ras->Seek(PR_SEEK_SET, mOffset);
             if (NS_FAILED(mStatus)) {
                 mXferState = END_READ;
                 return;
@@ -866,23 +871,24 @@ nsFileTransport::Process(void)
             return;
         }
 
-        if (mOffset > 0) {
-            // If we need to set a starting offset, QI for the nsISeekableStream
-            // and set it.
-            nsCOMPtr<nsISeekableStream> ras = do_QueryInterface(mSink, &mStatus);
-            if (NS_FAILED(mStatus)) {
-                mXferState = END_WRITE;
-                return;
-            }
-            // For now, assume the offset is always relative to the start of the
-            // file (position 0) so use PR_SEEK_SET.
-            mStatus = ras->Seek(PR_SEEK_SET, mOffset);
-            if (NS_FAILED(mStatus)) {
-                mXferState = END_WRITE;
-                return;
-            }
-            mOffset = 0;
+        // QI to a seekable stream so we can set EOF and Seek (if required)
+        nsCOMPtr<nsISeekableStream> seekable(do_QueryInterface(mSink, &mStatus));
+        if (seekable) {
+            if (mOffset)
+                mStatus = seekable->Seek(nsISeekableStream::NS_SEEK_SET, mOffset);
+            if (NS_SUCCEEDED(mStatus))
+                mStatus = seekable->SetEOF();
         }
+        // a stream need not support nsISeekableStream unless a transfer offset
+        // greater than zero was requested.
+        else if (mOffset == 0)
+            mStatus = NS_OK;
+
+        if (NS_FAILED(mStatus)) {
+            mXferState = END_WRITE;
+            return;
+        }
+        mOffset = 0;
 
         if (!mSinkWrapper) {
             //
diff --git a/mozilla/netwerk/base/src/nsSocketTransport.cpp b/mozilla/netwerk/base/src/nsSocketTransport.cpp
index d8258be20d4..c05870c7736 100644
--- a/mozilla/netwerk/base/src/nsSocketTransport.cpp
+++ b/mozilla/netwerk/base/src/nsSocketTransport.cpp
@@ -2329,6 +2329,12 @@ nsSocketIS::Tell(PRUint32 *offset)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsSocketIS::SetEOF()
+{
+    return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 //
 //----------------------------------------------------------------------------
 // nsSocketOS
@@ -2456,6 +2462,12 @@ nsSocketOS::Tell(PRUint32 *offset)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsSocketOS::SetEOF()
+{
+    return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 //
 //----------------------------------------------------------------------------
 // nsSocketRequest