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From Selva Govindarajan <selva.govindara...@esgyn.com>
Subject RE: Set_SqlParser_Flags
Date Thu, 19 May 2016 17:17:07 GMT
 A better technique would be to construct a class

Class SqlParserFlags {
   ULng32 flagbits_;
   SqlParserFlags(ULng32 flagbits) {
      flagbits_  = flagbits;
      SqlParser_Flags |= flagbits;
  }
  ~SqlParserFlags() {
    SqlParser_Flags &= ~flagbits_
  }
}

In the function where you want to set the flags

SqlParserFlags sqlParserFlags(flagbits);

When this function exits, the parser flag would get reset.

Selva


-----Original Message-----
From: Dave Birdsall [mailto:dave.birdsall@esgyn.com]
Sent: Wednesday, May 18, 2016 3:55 PM
To: dev@trafodion.incubator.apache.org
Subject: RE: Set_SqlParser_Flags

Hi,

I'm concerned that these bugs propagate to some extent by code cloning. For
that, documentation doesn't help, and a review of all calls is still
necessary to make sure there are no bugs to clone. The example I debugged
today had a copy with exactly the same variable name and comment so I assume
it was cloned.

By the way, the earlier example I debugged some months ago is here:
https://issues.apache.org/jira/browse/TRAFODION-1678.

Perhaps I can make a slightly different proposal?

Perhaps we provide replacements for Set_SqlParser_Flags and
Reset_SqlParser_Flags that have cleaner semantics. But leave the old ones
there. And we can encourage folks to change their usage over time.

What I have in mind is something like:

inline static ULng32 TurnOn_SqlParser_Flags(ULng32 flagbits) {
   ULng32 newlyOn = flagbits & ~SqlParser_Flags; // remember the bits that
weren't already turned on
   SqlParser_Flags |= flagbits;
   return newlyOn;
}

inline static void TurnOff_SqlParser_Flags(ULng32 flagbits) {
   SqlParser_Flags &= ~flagbits;
}

The usage model is like this:

Class1::method1()
{
    ULng32 turnOffOnExit = TurnOn_SqlParser_Flags(<whatever bits I want to
turn on>);

    ... do some stuff ...

    TurnOff_SqlParser_Flags(turnOffOnExit);
}

This is in comparison to current examples:

Class1::method1()
{
  ULng32 savedParserFlags = Get_SqlParser_Flags(0xFFFFFFFF);  // have to
remember to use 0xFFFFFFFF here

   Set_SqlParser_Flags(<whatever bits I want to turn on but it needs to be
non-zero to work right>);

   … do a bunch of stuff …

   // Restore parser flags to prior settings.

   Assign_SqlParser_Flags(savedParserFlags);  // note Assign, not Set }

Or, the following, which is incorrect since it doesn't take into account
that some of the bits might have already been on when the method is entered:

Class1::method1()
{
    Set_SqlParser_Flags(<whatever bits I want to turn on but it needs to be
non-zero to work right>);

    ... do some stuff ...

    Reset_SqlParserFlags(<whatever bits I turned on above>); }

What do you think?

Dave

-----Original Message-----
From: Anoop Sharma [mailto:anoop.sharma@esgyn.com]
Sent: Wednesday, May 18, 2016 2:36 PM
To: dev@trafodion.incubator.apache.org
Subject: RE: Set_SqlParser_Flags

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
email.
The method AssignParserFlags method was added later to correctly save and
restore 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
broken because of incorrect usage of parserflags.

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

anoop

-----Original Message-----
From: Dave Birdsall [mailto:dave.birdsall@esgyn.com]
Sent: Wednesday, May 18, 2016 2:02 PM
To: dev@trafodion.incubator.apache.org
Subject: Set_SqlParser_Flags

Hi,



In parser/SqlParserGlobalsCmn.h are three functions:



  inline static void Set_SqlParser_Flags(ULng32 flagbits)

  {

    if (flagbits)

      SqlParser_Flags |= flagbits;

    else

      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;

    else

      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:



Class1::method1()

{

  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.

   Set_SqlParser_Flags(savedParserFlags);

}



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.



Thanks,



Dave

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