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From Anoop Sharma <>
Subject RE: Set_SqlParser_Flags
Date Wed, 18 May 2016 21:35:58 GMT
Set and Reset parserflags have been in use for a long time,
probably for multiple years.

Initially, use of parserflags was very limited and restricted to parser
component only. And the current Set and Reset functionality was correct for
that usage.

But over time, parserflags started to get used in many other components
and code.

Some of the places did use Set incorrectly as has been mentioned Dave's
The method AssignParserFlags method was added later to correctly save and
parserflags, if that is what was needed.

We need to be bit careful if all of traf code is to be fixed for Set/Reset
usage. One
would need to make sure that they understand it well in terms of its impact
on the
immediate code as well as surrounding code.
It may be good to do that though at the same time I don’t think the code is
because of incorrect usage of parserflags.

It maybe good enough to document the Set/Reset/Assign functionality so folks
are adding new code do the right thing.


-----Original Message-----
From: Dave Birdsall []
Sent: Wednesday, May 18, 2016 2:02 PM
Subject: Set_SqlParser_Flags


In parser/SqlParserGlobalsCmn.h are three functions:

  inline static void Set_SqlParser_Flags(ULng32 flagbits)


    if (flagbits)

      SqlParser_Flags |= flagbits;


      SqlParser_Flags = 0;


  inline static void Assign_SqlParser_Flags(ULng32 flagbits)


    SqlParser_Flags = flagbits;


inline static void Reset_SqlParser_Flags(ULng32 flagbits)


    if (flagbits)

      SqlParser_Flags &= ~flagbits;


      SqlParser_Flags = 0;


These functions have some error-prone semantics. First problem is that they
modify a global variable, but I’ll leave that discussion for another day.
The second problem is some odd semantics. Set_SqlParser_Flags turns on bits,
unless you pass it a zero value. Then it turns everything off.
Reset_SqlParser_Flags turns off some bits, unless you pass it a zero value.
Then it turns off EVERY bit. The third problem is that the name
“Set_SqlParser_Flags” is misleading. I have debugged more than one set of
code where the author appears to have thought that this function set ALL the
bits to his particular input value.

Here’s an example of a bug that I debugged today:



  ULng32 savedParserFlags = Get_SqlParser_Flags(0xFFFFFFFF);

   Set_SqlParser_Flags(0x100000); // ORs this bit into the flags

   … do a bunch of stuff …

   // Restore parser flags to prior settings.



Here’s what this code does. If the parser flags happen to be zero on entry,
the savedParserFlags variable gets a value of zero. The first
Set_SqlParser_Flags call turns on bit 0x100000. The last Set_SqlParser_Flags
call resets the parser flags to zero *because of the weird semantic
concerning a zero value.*

If the parser flags happen to be non-zero on entry, the savedParserFlags
variable gets a non-zero value. The first Set_SqlParser_Flags call turns on
the 0x100000 bit. The last Set_SqlParser_Flags call *does nothing!* Why?
Because we pass a non-zero value, it just turns on those bits. But those
bits were already on! Notice that the 0x100000 bit remains on at exit, in
contrast to the first case above where it is reset to zero.

It’s clear from the comment that the author intended to restore the parser
flags to their previous state. And most of the time, the parser flags are
zero, so the code works as intended. But if there is another bug somewhere
else that leaves a bit set, then this code will leave another bit set
unintentionally, and we snowball.

The bug is, the author should have used Assign_SqlParser_Flags for the last
call, instead of Set_SqlParser_Flags.

Debugging mis-set global variables is hard in general, and this is the
second instance of this kind of bug that I’ve run into.

I think the misleading function names confuse developers as to the right
call to use. And the weird zero semantics tends to hide the bugs so they
occur only in weird situations.

So I would like to propose the following refactoring:

1.       Rename the functions. Set_SqlParser_Flags à
TurnOn_SqlParser_FlagBits, and similarly for Reset. Leave
Assign_SqlParser_Flags as-is.

2.       Remove the weird zero semantics. In TurnOn_SqlParser_FlagBits, if
a zero value is passed in, it does nothing. This is more mathematically
consistent; it is the behavior of the Boolean OR operator, and the Set UNION
operator. Similarly, in TurnOff_SqlParser_FlagBits, if a zero value is
passed in, it does nothing. That’s more like a Set DIFFERENCE.

3.       Go through the Trafodion code, looking for places that call
Set_SqlParser_Flags that seem to intend to assign rather than OR, and
replace those calls with Assign_SqlParser_Flags.

If the community does not object, I will create a JIRA for this work and
assign it to myself.

In the meantime, please be careful when coding Set_SqlParser_Flags calls.



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