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

nmisch · nmisch · commit 5b630501e9fa · 2020-01-04T11:31:42.000-08:00
It has undefined behavior. Follow the precedent of commit 9a9473f. 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
@@ -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;

Original file line number	Diff line number	Diff line change
`@@ -30,8 +30,9 @@ qunique(void *array, size_t elements, size_t width,`
`30`	`30`
`31`	`31`	`for (i = 1, j = 0; i < elements; ++i)`
`32`	`32`	`{`
`33`		`- if (compare(bytes + i * width, bytes + j * width) != 0)`
`34`		`- memcpy(bytes + ++j * width, bytes + i * width, width);`
	`33`	`+ if (compare(bytes + i * width, bytes + j * width) != 0 &&`
	`34`	`+ ++j != i)`
	`35`	`+ memcpy(bytes + j * width, bytes + i * width, width);`
`35`	`36`	`}`
`36`	`37`
`37`	`38`	`return j + 1;`
`@@ -55,8 +56,9 @@ qunique_arg(void *array, size_t elements, size_t width,`
`55`	`56`
`56`	`57`	`for (i = 1, j = 0; i < elements; ++i)`
`57`	`58`	`{`
`58`		`- if (compare(bytes + i * width, bytes + j * width, arg) != 0)`
`59`		`- memcpy(bytes + ++j * width, bytes + i * width, width);`
	`59`	`+ if (compare(bytes + i * width, bytes + j * width, arg) != 0 &&`
	`60`	`+ ++j != i)`
	`61`	`+ memcpy(bytes + j * width, bytes + i * width, width);`
`60`	`62`	`}`
`61`	`63`
`62`	`64`	`return j + 1;`