[svn-r11054] Purpose: Bug fix.

Description: There was a bug in the calculating code of the Fletcher32
checksum in the library before v1.6.3.  The checksum value wasn't consistent
between big-endian and little-endian systems.  This bug was fixed in
Release 1.6.3.  However, after fixing the bug, the checksum value is no
longer the same as before on little-endian system.

Solution: Made the library compare both the correct checksum and incorrect
checksum generated from v1.6.2 or before.  This makes the library be
backward compatible but not forward compatible.

Platforms tested: h5committest and fuss.

Misc. update: Documented the forward incompatibility problem in the
Known Problem section in RELEASE.txt

2 files changed, 33 insertions, 8 deletions

diff --git a/release_docs/RELEASE.txt b/release_docs/RELEASE.txt
index 90f4ba3..0c1e298 100644
--- a/release_docs/RELEASE.txt
+++ b/release_docs/RELEASE.txt
@@ -732,13 +732,15 @@ Linux 2.4 IA64 Intel  y       n       y    n    y        y           y
 
 Known Problems
 ==============
-* The dataset created with the v1.6.2 library can't be read with the v1.6.3
-  library or after when Fletcher32 EDC(filter) is enabled.  There was a bug 
-  in the calculating code of the Fletcher32 checksum in the library before 
-  v1.6.3.  The checksum value wasn't consistent between big-endian and
-  little-endian systems.  This bug was fixed in Release 1.6.3.  However, after
-  fixing the bug, the checksum value is no longer the same as before on
-  little-endian system.  SLU - 2005/6/30
+* The dataset created or rewritten with the v1.6.3 library or after can't 
+  be read with the v1.6.2 library or before when Fletcher32 EDC(filter) is 
+  enabled.  There was a bug in the calculating code of the Fletcher32 
+  checksum in the library before v1.6.3.  The checksum value wasn't consistent 
+  between big-endian and little-endian systems.  This bug was fixed in 
+  Release 1.6.3.  However, after fixing the bug, the checksum value is no 
+  longer the same as before on little-endian system.  The library release 
+  after 1.6.4 can still read the dataset created or rewritten with the library
+  of v1.6.2 or before.  SLU - 2005/6/30
 * For the version 6(6.02 and 6.04) of Portland Group compiler on AMD Opteron
   processor, there's a bug in the compiler for optimization(-O2).  The library
   failed in several tests but all related to multi driver.  The problem has 
diff --git a/src/H5Zfletcher32.c b/src/H5Zfletcher32.c
index 21da7a2..c75412f 100644
--- a/src/H5Zfletcher32.c
+++ b/src/H5Zfletcher32.c
@@ -137,6 +137,9 @@ H5Z_filter_fletcher32 (unsigned flags, size_t UNUSED cd_nelmts, const unsigned U
     void    *outbuf = NULL;     /* Pointer to new buffer */
     unsigned char *src = (unsigned char*)(*buf);
     uint32_t fletcher;          /* Checksum value */
+    uint32_t reversed_fletcher; /* Possible wrong checksum value */
+    uint8_t  c[4];
+    uint8_t  tmp;
     size_t   ret_value;         /* Return value */
     
     FUNC_ENTER_NOAPI(H5Z_filter_fletcher32, 0)
@@ -159,8 +162,28 @@ H5Z_filter_fletcher32 (unsigned flags, size_t UNUSED cd_nelmts, const unsigned U
             /* Compute checksum (can't fail) */
             fletcher = H5Z_filter_fletcher32_compute(src,src_nbytes);
 
+            /* The reversed checksum.  There was a bug in the calculating code of 
+             * the Fletcher32 checksum in the library before v1.6.3.  The checksum 
+             * value wasn't consistent between big-endian and little-endian systems.  
+             * This bug was fixed in Release 1.6.3.  However, after fixing the bug, 
+             * the checksum value is no longer the same as before on little-endian 
+             * system.  We'll check both the correct checksum and the wrong 
+             * checksum to be consistent with Release 1.6.2 and before. 
+             */ 
+            HDmemcpy(c, &fletcher, 4);
+
+            tmp  = c[1];
+            c[1] = c[0];
+            c[0] = tmp;
+            
+            tmp  = c[3];
+            c[3] = c[2];
+            c[2] = tmp;
+
+            HDmemcpy(&reversed_fletcher, c, 4);
+
             /* Verify computed checksum matches stored checksum */
-            if(stored_fletcher != fletcher)
+            if(stored_fletcher != fletcher && stored_fletcher != reversed_fletcher)
 	        HGOTO_ERROR(H5E_STORAGE, H5E_READERROR, 0, "data error detected by Fletcher32 checksum")
         }