diff --git a/system/jlib/jfile.cpp b/system/jlib/jfile.cpp
index df468594f7d..ceac0fb3391 100644
--- a/system/jlib/jfile.cpp
+++ b/system/jlib/jfile.cpp
@@ -7591,45 +7591,43 @@ IAPICopyClient * createApiCopyClient(IStorageApiInfo * source, IStorageApiInfo *
 }
 
 
+// NB: This implementation is not thread-safe.
+// Therefore it should only be used by use cases that are single threaded
 class CBlockedFileIO : public CSimpleInterfaceOf<IFileIO>
 {
     Owned<IFileIO> io;
-    offset_t fileSize = 0;
     size32_t blockSize = 0;
+    size32_t readLen = 0;
     void *buffer = nullptr;
     offset_t lastReadPos = (offset_t)-1;
     MemoryBuffer mb;
 public:
     CBlockedFileIO(IFileIO *_io, size32_t _blockSize) : io(_io), blockSize(_blockSize)
     {
-        fileSize = io->size();
         buffer = mb.reserveTruncate(blockSize);
     }
     virtual size32_t read(offset_t pos, size32_t len, void *data) override
     {
-        if (len > blockSize || pos+len > fileSize || len==0)
+        if (len > blockSize)
             return io->read(pos, len, data);
         size32_t totalCopied = 0;
         byte *dest = (byte *) data;
         while (len)
         {
-            offset_t readPos = (pos / blockSize) * blockSize;
-            size32_t readLen;
-            if (readPos + blockSize <= fileSize)
-                readLen = blockSize;
-            else
-                readLen = fileSize - readPos;
-            if (readPos != lastReadPos)  // NB: if readLen < blockSize then last block, won't ever read more for that block.
+            offset_t readPos = (pos / blockSize) * blockSize; // NB: could be beyond end of file
+            if (readPos != lastReadPos)
             {
-                size32_t read = io->read(readPos, readLen, buffer);
-                assertex(read == readLen);
+                readLen = io->read(readPos, blockSize, buffer); // NB: can be less than blockSize (and 0 if beyodn end of file)
                 lastReadPos = readPos;
             }
+            size32_t endPos = readPos+readLen;
             size32_t copyNow;
-            if (pos+len <= readPos+readLen)
+            if (pos+len <= endPos) // common case hopefully
                 copyNow = len;
-            else
-                copyNow = readPos+readLen-pos;
+            else if (pos < endPos)
+                copyNow = endPos-pos;
+            else // nothing to copy
+                break;
             memcpy(dest, ((byte *)buffer) + pos-readPos, copyNow);
             len -= copyNow;
             pos += copyNow;
@@ -7638,7 +7636,7 @@ class CBlockedFileIO : public CSimpleInterfaceOf<IFileIO>
         }
         return totalCopied;
     }
-    virtual offset_t size() override { return fileSize; }
+    virtual offset_t size() override { return io->size(); }
     virtual size32_t write(offset_t pos, size32_t len, const void * data) override { throwUnexpected(); }
     virtual offset_t appendFile(IFile *file, offset_t pos=0, offset_t len=(offset_t)-1) override { throwUnexpected(); }
     virtual void setSize(offset_t size) override { throwUnexpected(); }