Hannes Siebenhandl pushed to branch wip/fendor/post-string-len-assertion at Glasgow Haskell Compiler / GHC

Commits:

• 50a62b35
  by fendor at 2025-10-30T13:16:32+01:00

  Fix assertion in `postStringLen` to account for \0 byte
  
  We fix the assertion to handle trailing \0 bytes in `postStringLen`.
  Before this change, the assertion looked like this:
  
    ASSERT(eb->begin + eb->size > eb->pos + len + 1);
  
  Let's assume some values to see why this is actually off by one:
  
    eb->begin = 0
    eb->size  = 1
    eb->pos   = 0
  
    len       = 1
  
  then the assertion would trigger correctly:
  
    0 + 1 > 0 + 1 + 1 => 1 > 2 => false
  
  as there is not enough space for the \0 byte (which is the trailing +1).
  However, if we change `eb->size = 2`, then we do have enough space for a
  string of length 1, but the assertion still fails:
  
    0 + 2 > 0 + 1 + 1 => 2 > 2 => false
  
  Which causes the assertion to fail if there is exactly enough space for
  the string with a trailing \0 byte.
  
  Clearly, the assertion should be `>=`!
  If we switch around the operand, it should become more obvious that `<=`
  is the correct comparison:
  
    ASSERT(eb->pos + len + 1 <= eb->begin + eb->size);
  
  This is expresses more naturally that the current position plus the
  length of the string (and the null byte) must be smaller or equal to the
  overall size of the buffer.
  
  This change also is in line with the implementation in
  `hasRoomForEvent` and `hasRoomForVariableEvent`:
  
    ```
    StgBool hasRoomForEvent(EventsBuf *eb, EventTypeNum eNum)
    {
      uint32_t size = ...;
      if (eb->pos + size > eb->begin + eb->size)
      ...
    ```
  
  the check `eb->pos + size > eb->begin + eb->size` is identical to
  `eb->pos + size <= eb->begin + eb->size` plus a negation.
  

1 changed file:

• rts/eventlog/EventLog.c

Changes:

 static inline void postStringLen(EventsBuf *eb, const char *buf, StgWord len)

 {

     if (buf) {

-        ASSERT(eb->begin + eb->size > eb->pos + len + 1);

+        ASSERT(eb->pos + len + 1 <= eb->begin + eb->size);

         memcpy(eb->pos, buf, len);

         eb->pos += len;

     }