in the attempt to fix 627393, changed IFX_FileRead's readBlock to return the length it reads

fpdfsdk/fpdf_dataavail.cpp

Index: fpdfsdk/fpdf_dataavail.cpp
diff --git a/fpdfsdk/fpdf_dataavail.cpp b/fpdfsdk/fpdf_dataavail.cpp
index f208a776c4389620b480cbab90f55f462a49dadc..24ab21610a3211c6d859ed81015f2a56156b6e03 100644
--- a/fpdfsdk/fpdf_dataavail.cpp
+++ b/fpdfsdk/fpdf_dataavail.cpp
@@ -11,8 +11,10 @@
 
 #include "core/fpdfapi/fpdf_parser/include/cpdf_data_avail.h"
 #include "core/fpdfapi/fpdf_parser/include/cpdf_document.h"
+#include "core/fxcrt/include/fx_safe_types.h"
 #include "fpdfsdk/include/fsdk_define.h"
 #include "public/fpdf_formfill.h"
+#include "third_party/base/numerics/safe_conversions_impl.h"
 
 // These checks are here because core/ and public/ cannot depend on each other.
 static_assert(CPDF_DataAvail::DataError == PDF_DATA_ERROR,
@@ -58,23 +60,34 @@ class CFPDF_FileAvailWrap : public CPDF_DataAvail::FileAvail {
 
 class CFPDF_FileAccessWrap : public IFX_FileRead {
  public:
-  CFPDF_FileAccessWrap() { m_pFileAccess = nullptr; }
+  CFPDF_FileAccessWrap() {
+    m_pFileAccess = nullptr;
+    m_nCurPos = 0;
+  }
   ~CFPDF_FileAccessWrap() override {}
 
   void Set(FPDF_FILEACCESS* pFile) { m_pFileAccess = pFile; }
 
+  FX_BOOL IsEOF() override {
+    return m_nCurPos >=
+           pdfium::base::checked_cast<FX_FILESIZE>(m_pFileAccess->m_FileLen);
+  }
+
   // IFX_FileRead
   FX_FILESIZE GetSize() override { return m_pFileAccess->m_FileLen; }
 
-  FX_BOOL ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) override {
-    return m_pFileAccess->m_GetBlock(m_pFileAccess->m_Param, offset,
-                                     (uint8_t*)buffer, size);
+  size_t ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) override {
+    int blockSize = m_pFileAccess->m_GetBlock(m_pFileAccess->m_Param, offset,
+                                              (uint8_t*)buffer, size);
+    m_nCurPos = offset + pdfium::base::checked_cast<FX_FILESIZE>(blockSize);
+    return pdfium::base::checked_cast<size_t>(blockSize);
   }
 
   void Release() override {}
 
  private:
   FPDF_FILEACCESS* m_pFileAccess;
+  FX_FILESIZE m_nCurPos;
 };
 
 class CFPDF_DownloadHintsWrap : public CPDF_DataAvail::DownloadHints {