Avoid calling memcpy() with a NULL source pointer and count == 0.

As in commit 0a52d378b03b7d5a, avoid doing something that has undefined
results according to the C standard, even though in practice there does
not seem to be any problem with it.

This fixes two places in numeric.c that demonstrably could call memcpy()
with such arguments.  I looked through that file and didn't see any other
places with similar hazards; this is not to claim that there are not such
places in other files.

Per report from Piotr Stefaniak.  Back-patch to 9.5 which is where the
previous commit was added.  We're more or less setting a precedent that
we will not worry about this type of issue in pre-9.5 branches unless
someone demonstrates a problem in the field.

diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c
index 7ce41b788880c1ec47485818b1a108102a16da6f..1bfa29e1b280ba8aa1025e1e71dc8490094cda4a 100644 (file)
--- a/src/backend/utils/adt/numeric.c
+++ b/src/backend/utils/adt/numeric.c
@@ -4769,7 +4769,9 @@ set_var_from_var(NumericVar *value, NumericVar *dest)
 
        newbuf = digitbuf_alloc(value->ndigits + 1);
        newbuf[0] = 0;                          /* spare digit for rounding */
-       memcpy(newbuf + 1, value->digits, value->ndigits * sizeof(NumericDigit));
+       if (value->ndigits > 0)         /* else value->digits might be null */
+               memcpy(newbuf + 1, value->digits,
+                          value->ndigits * sizeof(NumericDigit));
 
        digitbuf_free(dest->buf);
 
@@ -5090,8 +5092,9 @@ make_result(NumericVar *var)
                result->choice.n_long.n_weight = weight;
        }
 
-       memcpy(NUMERIC_DIGITS(result), digits, n * sizeof(NumericDigit));
        Assert(NUMERIC_NDIGITS(result) == n);
+       if (n > 0)
+               memcpy(NUMERIC_DIGITS(result), digits, n * sizeof(NumericDigit));
 
        /* Check for overflow of int16 fields */
        if (NUMERIC_WEIGHT(result) != weight ||