Skip memcpy(x, x) in qunique().

It has undefined behavior.  Follow the precedent of commit
9a9473f3cce1a21c25d6cc7569710e832d2b180b.  No back-patch, since the
master branch alone has this function.

Discussion: https://postgr.es/m/20191229070221.GA13873@gust.leadboat.com

diff --git a/src/include/lib/qunique.h b/src/include/lib/qunique.h
index 73aa10f165941f5e761aad4117ecf893eb0d1772..95d9fef637a37ad2c4d479779a006c96733c4148 100644 (file)
--- a/src/include/lib/qunique.h
+++ b/src/include/lib/qunique.h
@@ -30,8 +30,9 @@ qunique(void *array, size_t elements, size_t width,
 
    for (i = 1, j = 0; i < elements; ++i)
    {
-       if (compare(bytes + i * width, bytes + j * width) != 0)
-           memcpy(bytes + ++j * width, bytes + i * width, width);
+       if (compare(bytes + i * width, bytes + j * width) != 0 &&
+           ++j != i)
+           memcpy(bytes + j * width, bytes + i * width, width);
    }
 
    return j + 1;
@@ -55,8 +56,9 @@ qunique_arg(void *array, size_t elements, size_t width,
 
    for (i = 1, j = 0; i < elements; ++i)
    {
-       if (compare(bytes + i * width, bytes + j * width, arg) != 0)
-           memcpy(bytes + ++j * width, bytes + i * width, width);
+       if (compare(bytes + i * width, bytes + j * width, arg) != 0 &&
+           ++j != i)
+           memcpy(bytes + j * width, bytes + i * width, width);
    }
 
    return j + 1;