[PATCH] Use open_memstream(3) for more efficient asprintf

[Alex Brachet-Mialot <alexbrachetmialot@gmail.com>]

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Hi I wasn't able to search the lists from the online archive, so I'm not
sure if it has been talked about yet, but the current vasprintf
implementation could be made better if it didn't call vsnprintf twice. Let
me know what you think!

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diff --git a/src/stdio/vasprintf.c b/src/stdio/vasprintf.c
index 08251bc2..d55fe32f 100644
--- a/src/stdio/vasprintf.c
+++ b/src/stdio/vasprintf.c
@@ -5,11 +5,16 @@
 
 int vasprintf(char **s, const char *fmt, va_list ap)
 {
-	va_list ap2;
-	va_copy(ap2, ap);
-	int l = vsnprintf(0, 0, fmt, ap2);
-	va_end(ap2);
+	size_t l;
+	*s = 0;
+	FILE *f = open_memstream(s, &l);
+	if (!f)
+		return -1;
 
-	if (l<0 || !(*s=malloc(l+1U))) return -1;
-	return vsnprintf(*s, l+1U, fmt, ap);
+	if ((l = vfprintf(f, fmt, ap)) == -1) {
+		free(*s);
+		*s = 0;
+	}
+	fclose(f);
+	return l;
 }