csn1: Avoid storing existence bit as true if content was actually NULL

If we decode Exist bit as "1" but we are at the end of the message, and
all the Next items we'd read are expected to be possibly NULL, then swap
the Exist bit in the decoded structure as "0" in order to tell the
decoder user that the related information structure is actually unset,
as if "0" was received.

Related: SYS#5552
Related: OS#4955
Related: OS#5020
Change-Id: I38602e4b680ed87297c7e440691a494c07cad446

1 files changed, 36 insertions, 6 deletions

diff --git a/src/csn1_dec.c b/src/csn1_dec.c
index 4bc74f36..fa1f0c39 100644
--- a/src/csn1_dec.c
+++ b/src/csn1_dec.c
@@ -909,6 +909,7 @@ csnStreamDecoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec *vector
       case CSN_NEXT_EXIST:
       {
         guint8 fExist;
+        guint8 isnull;
 
         pui8  = pui8DATA(data, pDescr->offset);
 
@@ -927,17 +928,30 @@ csnStreamDecoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec *vector
         /* the "regular" M_NEXT_EXIST description element */
 
         fExist = 0x00;
+        isnull = 1;
 	if (bitvec_read_field(vector, readIndex, 1))
         {
           fExist = 0x01;
+          if (remaining_bits_len == 1) {
+             /* If { 1 < end > } and all next items may be null, store it as { 0 } */
+            const CSN_DESCR* pDescrNext = pDescr + 1;
+            guint8 i;
+            for (i = 0; i < pDescr->i; i++, pDescrNext++)
+            {
+              if (!pDescrNext->may_be_null)
+                isnull = 0;
+            }
+          } else {
+            isnull = 0;
+          }
         }
 
-        *pui8     = fExist;
+        *pui8     = !isnull;
         remaining_bits_len -= 1;
-        LOGPC(DCSN1, LOGL_DEBUG, "%s = %u | ", pDescr->sz , (unsigned)*pui8);
+        LOGPC(DCSN1, LOGL_DEBUG, "%s = %u%s | ", pDescr->sz, (unsigned)fExist, fExist && isnull ? " (NULL)" : "");
         ++bit_offset;
 
-        if (fExist == 0)
+        if (isnull)
         { /* Skip 'i' entries */
           pDescr += pDescr->i;
         }
@@ -949,6 +963,7 @@ csnStreamDecoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec *vector
       case CSN_NEXT_EXIST_LH:
       {
         guint8 fExist;
+        guint8 isnull;
         pui8  = pui8DATA(data, pDescr->offset);
 
         /* this if-statement represents the M_NEXT_EXIST_OR_NULL_LH description element */
@@ -964,14 +979,29 @@ csnStreamDecoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec *vector
         }
 
         /* the "regular" M_NEXT_EXIST_LH description element */
-        fExist = get_masked_bits8(vector, readIndex, bit_offset, 1);
-        LOGPC(DCSN1, LOGL_DEBUG, "%s = %u | ", pDescr->sz , (unsigned)fExist);
+        isnull = 1;
+        if ((fExist = get_masked_bits8(vector, readIndex, bit_offset, 1)))
+        {
+          if (remaining_bits_len == 1) {
+             /* If { 1 < end > } and all next items may be null, store it as { 0 } */
+            const CSN_DESCR* pDescrNext = pDescr + 1;
+            guint8 i;
+            for (i = 0; i < pDescr->i; i++, pDescrNext++)
+            {
+              if (!pDescrNext->may_be_null)
+                isnull = 0;
+            }
+          } else {
+            isnull = 0;
+          }
+        }
+        LOGPC(DCSN1, LOGL_DEBUG, "%s = %u%s | ", pDescr->sz , (unsigned)fExist, fExist && isnull ? " (NULL)" : "");
         *pui8++   = fExist;
         remaining_bits_len -= 1;
 
         bit_offset++;
 
-        if (fExist == 0)
+        if (isnull)
         { /* Skip 'i' entries */
           pDescr += pDescr->i;
         }