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You are here: English WTF CodeSOD: Isn't There a Vaccine For MUMPS?

CodeSOD: Isn't There a Vaccine For MUMPS?

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Alex F is suffering from a disease. No, it’s not disfiguring, it’s not fatal. It’s something much worse than that.

It’s MUMPS.

MUMPS is a little bit infamous. MUMPS is its own WTF.

Alex is a support tech, which in their organization means that they sometimes write up tickets, or for simple problems even fix the code themselves. For this issue, Alex wrote up a ticket, explaining that the users was submitting a background job to run a report, but instead got an error.

Alex sent it to the developer, and the developer replied with a one line code fix:

 i $$zRunAsBkgUser(desc_$H,"runReportBkg^KHUTILLOCMAP",$na(%ZeData)) d
 . w !,"Search has been started in the background."
 e  w !,"Search failed to start in the background."

Alex tested it, and… it didn’t work. So, fully aware of the risks they were taking, Alex dug into the code, starting with the global function $$zRunAsBkgUser.

Before I post any more code, I am legally required to offer a content warning: the rest of this article is going to be full of MUMPS code. This is not for the faint of heart, and TDWTF accepts no responsibility for your mental health if you continue. Don’t read the rest of this article if you have eaten any solid food in the past twenty minutes. If you experience a rash, this may be a sign of a life threatening condition, and you should seek immediate treatment. Do not consume alcohol while reading this article. Save that for after, you’ll need it.

 ;---------
  ; NAME:         zRunAsBkgUser
  ; SCOPE:        PUBLIC
  ; DESCRIPTION:  Run the specified tag as the correct OS-level background user. The process will always start in the system default time zone.
  ; PARAMETERS:
  ;  %uJobID (I,REQ)      - Free text string uniquely identifying the request
  ;                         If null, the tag will be used instead but -- as this is not guaranteed unique -- this ID should be considered required
  ;  %uBkgTag (I,REQ)     - The tag to run
  ;  %uVarList (I,OPT)    - Variables to be passed from the current process' symbol table
  ;  %uJobParams (I,OPT)  - An array of additional parameters to be passed to %ZdUJOB
  ;                         Should be passed with the names of the parameters in %ZdUJOB, e.g. arr("%ZeDIR")="MGR"
  ;                         Currently supports only: %ZeDIR, %ZeNODE, %ZeBkOv
  ;  %uError (O,OPT)      - Error message in case of failure
  ;  %uForceBkg (I,OPT)   - If true, will force the request to be submitted to %ZeUMON
  ;  %uVerifyCond (I,OPT) - If null, this tag will return immediately after submitting the request
  ;                         If non-null, should contain code that will be evaluated to determine the success or failure of the request
  ;                         Will be executed as s @("result=("_%uVerifyCond_")")
  ;  %uVerifyTmo (I,OPT)  - Length of time, in seconds, to try to verify the success of the request
  ;                         Defaults to 1 second
  ; RETURNS:      If %uVerifyCond is not set: 1 if it's acceptable to run, 0 otherwise
  ;               If %uVerifyCond is set: 1 if the condition is verified after the specified timeout, 0 otherwise
zRunAsBkgUser(%uJobID,%uBkgTag,%uVarList,%uJobParams,%uError,%uForceBkg,%uVerifyCond,%uVerifyTmo) q $$RunBkgJob^%ZeUMON($$zCurrRou(),%uJobID,%uBkgTag,%uVarList,.%uJobParams,.%uError,%uForceBkg,%uVerifyCond,%uVerifyTmo) ;;#eof#  ;;#inline#

Thank the gods for comments, I guess. Alex’s eyes locked upon the sixth parameter- %uForceBkg. That seems a bit odd, for a function which is supposed to be submitting a background job. The zRunAsBkgUser function is otherwise quite short- it’s a wrapper around RunBkgJob.

Let’s just look at the comments:

 ;---------
  ; NAME:         RunBkgJob
  ; SCOPE:        INTERNAL
  ; DESCRIPTION:  Submit request to monitor daemon to run the specified tag as a background process
  ;               Used to ensure the correct OS-level user in the child process
  ;               Will fork off from the current process if the correct OS-level user is already specified,
  ;               unless the %uForceBkg flag is set. It will always start in the system default time zone.
  ; KEYWORDS:     run,background,job,submit,%ZeUMON,correct,user
  ; CALLED BY:    ($$)zRunAsBkgUser
  ; PARAMETERS:
  ;  %uOrigRou (I,REQ)    - The routine submitting the request
  ;  %uJobID (I,REQ)      - Free text string uniquely identifying the request
  ;                         If null, the tag will be used instead but -- as this is not guaranteed unique -- this ID should be considered required
  ;  %uBkgTag (I,REQ)     - The tag to run
  ;  %uVarList (I,OPT)    - Variables to be passed from the current process' symbol table
  ;                         If "", pass nothing; if 1, pass everything
  ;  %uJobParams (I,OPT)  - An array of additional parameters to be passed to %ZdUJOB
  ;                         Should be passed with the names of the parameters in %ZdUJOB, e.g. arr("%ZeDIR")="MGR"
  ;                         Currently supports only: %ZeDIR, %ZeNODE, %ZeBkOv
  ;  %uError (O,OPT)      - Error message in case of failure
  ;  %uForceBkg (I,OPT)   - If true, will force the request to be submitted to %ZeUMON
  ;  %uVerifyCond (I,OPT) - If null, this tag will return immediately after submitting the request
  ;                         If non-null, should contain code that will be evaluated to determine the success or failure of the request
  ;                         Will be executed as s @("result=("_%uVerifyCond_")")
  ;  %uVerifyTmo (I,OPT)  - Length of time, in seconds, to try to verify the success of the request
  ;                         Defaults to 1 second
  ; RETURNS:      If %uVerifyCond is not set: 1 if it's acceptable to run, 0 otherwise
  ;               If %uVerifyCond is set: 1 if the condition is verified after the specified timeout, 0 otherwise

Once again, the suspicious uForceBkg parameter is getting passed it. The comments claim that this only controls the timezone, which implies either the parameter is horribly misnamed, or the comments are wrong. Or, possibly, both. Wait, no, it's talking about ZeUMON. My brain wants it to be timezones. MUMPS is getting to me. Since the zRunAsBkgUser has different comments, I suspect it’s both, but it’s MUMPS. I have no idea what could happen. Let’s look at the code.

  RunBkgJob(%uOrigRou,%uJobID,%uBkgTag,%uVarList,%uJobParams,%uError,%uForceBkg,%uVerifyCond,%uVerifyTmo) ;
  n %uSecCount,%uIsStarted,%uCondCode,%uVarCnt,%uVar,%uRet,%uTempFeat
  k %uError
  i %uBkgTag="" s %uError="Need to pass a tag" q 0
  i '$$validrou(%uBkgTag) s %uError="Tag does not exist" q 0
  ;if we're already the right user, just fork off directly
  i '%uForceBkg,$$zValidBkgOSUser() d  q %uRet
  . d inheritOff^%ZdDEBUG()
  . s %uRet=$$^%ZdUJOB(%uBkgTag,"",%uVarList,%uJobParams("%ZeDIR"),%uJobParams("%ZeNODE"),$$zTZSystem(1),"","","","",%uJobParams("%ZeOvBk"))
  . d inheritOn^%ZdDEBUG()
  ;
  s:%uJobID="" %uJobID=%uBkgTag   ;this *should* be uniquely identifying, though it might not be...
  s ^%ZeUMON("START","J",%uJobID,"TAG")=%uBkgTag
  s ^%ZeUMON("START","J",%uJobID,"CALLER")=%uOrigRou
  i $$zFeatureCanUseTempFeatGlobal() s %uTempFeat=$$zFeatureSerializeTempGlo() s:%uTempFeat'="" ^%ZeUMON("START","J",%uJobID,"FEAT")=%uTempFeat
  m:$D(%uJobParams) ^%ZeUMON("START","J",%uJobID,"PARAMS")=%uJobParams
  i %uVarList]"" d
  . s ^%ZeUMON("START","J",%uJobID,"VARS")=%uVarList
  . d inheritOff^%ZdDEBUG()
  . i %uVarList=1 d %zSavVbl($name(^%ZeUMON("START","J",%uJobID,"VARS"))) i 1   ;Save whole symbol table if %uVarList is 1
  . e  f %uVarCnt=1:1:$L(%uVarList,",") s %uVar=$p(%uVarList,",",%uVarCnt) m:%uVar]"" ^%ZeUMON("START","J",%uJobID,"VARS",%uVar)=@%uVar
  . d inheritOn^%ZdDEBUG()
  s ^%ZeUMON("START","G",%uJobID)=""   ;avoid race conditions by setting pointer only after the data is complete
  d log("BKG","Request to launch tag "_%uBkgTag_" from "_%uOrigRou)
  q:%uVerifyCond="" 1   ;don't hang around if there's no need
  d
  . s %uError="Verification tag crashed"
  . d SetTrap^%ZeERRTRAP("","","Error verifying launch of background tag "_%uBkgTag)
  . s:%uVerifyTmo<1 %uVerifyTmo=1
  . s %uIsStarted=0
  . s %uCondCode="%uIsStarted=("_%uVerifyCond_")"
  . f %uSecCount=1:1:%uVerifyTmo h 1 s @%uCondCode q:%uIsStarted
  . d ClearTrap^%ZeERRTRAP
  . k %uError
  i %uError="",'%uIsStarted s %uError="Could not verify that job started successfully"
  q %uIsStarted
  ;
  q  ;;#eor#

Well, there you have it, the bug is so simple to spot, I’ll leave it as an exercise to the readers.

I’m kidding. The smoking gun, as Alex calls it, is the block:

  i '%uForceBkg,$$zValidBkgOSUser() d  q %uRet
  . d inheritOff^%ZdDEBUG()
  . s %uRet=$$^%ZdUJOB(%uBkgTag,"",%uVarList,%uJobParams("%ZeDIR"),%uJobParams("%ZeNODE"),$$zTZSystem(1),"","","","",%uJobParams("%ZeOvBk"))
  . d inheritOn^%ZdDEBUG()
  ;

This is what passes for an “if” statement in MUMPS. Specifically, if the %uForceBkg parameter is set, and the zValidBkgOSUser function returns true, then we’ll submit the job. Otherwise, we don’t submit the job, and thus get errors when we check on whether or not it’s done.

So, the underlying bug, such as it were, is a confusing parameter with an unreasonable default. This is not all that much of a WTF, I admit, but I really really wanted you all to see this much MUMPS code in a single sitting, and I wanted to remind you: there are people who work with this every day.

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