Comment 1 by Thomas Keller, Oct 18, 2010

Ok, I debugged into this and the easiest way to reproduce the 
problem in 3.7.3 is to do

$ mtn db init -d test.mtn
$ mtn db set foo foo "" -d test.mtn
$ mtn ls vars

The empty value column is returned as "" (zero-length 
string) in sqlite 3.7.2 and 3.6.23 (and possibly other versions 
which used to work), but as 0 in 3.7.3.

According to sqlite's docs (
http://www.sqlite.org/c3ref/column_blob.html) this seems to be 
correct

  The return value from sqlite3_column_blob() for a 
  zero-length BLOB is a NULL pointer."

but apparently was broken in earlier versions (or the paragraph has 
just been added for 3.7.3).

I'm now trying to find out whether there is a valid error case for 
which sqlite3_column_blob() also returns 0 and if not, our error in 
database.cc should probably laxed and the null pointer handled 
properly.

Comment 2 by Timothy Brownawell, Oct 21, 2010

As best I can tell, it only returns zero for empty blobs and NULLs. 
So the following should work (committed as 
9def7716bcd068fc1929ed8336432bbed775d9ee).


#
# old_revision [97939c9677047b36beef031cce4c1896849a987c]
#
# patch "database.cc"
#  from [0afa3ff4bd9c9ee3bc62b10bcf6295a9f5388d64]
#    to [8bfff559a0894259fe3668294bd3906ae837129b]
#
============================================================
--- database.cc	0afa3ff4bd9c9ee3bc62b10bcf6295a9f5388d64
+++ database.cc	8bfff559a0894259fe3668294bd3906ae837129b
@@ -1531,12 +1531,19 @@ database_impl::fetch(results & res,
       vector<string> row;
       for (int col = 0; col < ncol; col++)
         {
+          // We never store NULLs, so we should never see one.
+          int const datatype = 
sqlite3_column_type(i->second.stmt(), col);
+          E(datatype != SQLITE_NULL, origin::database,
+            F("null result in query: %s") % 
query.sql_cmd);
           const char * value = (const 
char*)sqlite3_column_blob(i->second.stmt(), col);
           int bytes = sqlite3_column_bytes(i->second.stmt(), 
col);
-          E(value, origin::database,
-            F("null result in query: %s") % 
query.sql_cmd);
-          row.push_back(string(value, value + bytes));
-          //L(FL("row %d col %d value='%s'") % nrow % col 
% value);
+          if (value) {
+            row.push_back(string(value, value + bytes));
+          } else {
+            // sqlite3_column_blob() returns null for zero-length
+            I(bytes == 0);
+            row.push_back(string());
+          }
         }
       res.push_back(row);
     }