[GHC] #8309: traceEvent truncates to 512 bytes

[GHC]

#8309: traceEvent truncates to 512 bytes
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       Reporter:  duncan            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Runtime System    |           Version:  7.6.3
       Keywords:  eventlog tracing  |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 The `Debug.Trace.traceEvent`  (& `traceEventIO`) use a code path that
 unnecessarily uses printf format strings and uses a fixed size 512-byte
 buffer, hence truncating all user trace messages at that size.

 Here is the call path:

  * `Debug.Trace.traceEvent`  (& `traceEventIO`) call `traceEvent#` primop
  * cmm impl of the primop `stg_traceEventzh` calls C RTS function
 `traceUserMsg`
  * `traceUserMsg` calls `traceFormatUserMsg(cap, "%s", msg);`, using the
 printf format string
  * `traceFormatUserMsg` uses `postUserMsg` which eventually calls
 `postLogMsg`
  * `postLogMsg` does the printf stuff

 Here's what `postLogMsg` does:

 {{{
 #define BUF 512

 void postLogMsg(EventsBuf *eb, EventTypeNum type, char *msg, va_list ap)
 {
     char buf[BUF];
     nat size;

     size = vsnprintf(buf,BUF,msg,ap);
     if (size > BUF) {
         buf[BUF-1] = '\0';
         size = BUF;
     }

    ....
 }}}

 This is obviously designed for RTS-internal users, not the user path.

 So the problem starts with this bit:

 {{{
 void traceUserMsg(Capability *cap, char *msg)
 {
     traceFormatUserMsg(cap, "%s", msg);
 }
 }}}

 It just should not use that code path. It should call something that
 directly posts the message, without any silly printf strings. In fact, the
 code path from the primop down should really be changed to use an explicit
 given length, rather than using a null-terminated string and having to
 call strlen.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8309>
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