Fixed #8237: Database access error when nbackup is starting

(cherry picked from commit 1e6bd16)

Author: AlexPeshkoff

src/jrd/CryptoManager.cpp

@@ -228,7 +228,29 @@ namespace Jrd {
 				if (bak_state != Ods::hdr_nbak_normal)
 					diff_page = bm->getPageIndex(tdbb, bdb.bdb_page.getPageNum());
 
+				bool readPageAsNormal = false;
 				if (bak_state == Ods::hdr_nbak_normal || !diff_page)
+					readPageAsNormal = true;
+				else
+				{
+					if (!bm->readDifference(tdbb, diff_page, page))
+					{
+						if (page->pag_type == 0 && page->pag_generation == 0 && page->pag_scn == 0)
+						{
+							// We encountered a page which was allocated, but never written to the
+							// difference file. In this case we try to read the page from database. With
+							// this approach if the page was old we get it from DISK, and if the page
+							// was new IO error (EOF) or BUGCHECK (checksum error) will be the result.
+							// Engine is not supposed to read a page which was never written unless
+							// this is a merge process.
+							readPageAsNormal = true;
+						}
+						else
+							ERR_punt();
+					}
+				}
+
+				if (readPageAsNormal)
 				{
 					// Read page from disk as normal
 					int retryCount = 0;
@@ -250,11 +272,6 @@ namespace Jrd {
 						}
  					}
 				}
-				else
-				{
-					if (!bm->readDifference(tdbb, diff_page, page))
-						ERR_punt();
-				}
 
 				setHeader(h);
 			}