stdio: *memstream: decouple the buffer size from the stream length

It's useful to be able to track both facts with a single variable, but
it also makes it more difficult to change how the buffer size scales.

As an example, Apple's implementation seems to scale the buffer size by
1.5x on growth, presumably in an attempt to reduce trips into realloc().
This might be questionable in the face of stdio buffering, but avoiding
serious churn in the line- or un-buffered case is a net positive if
doing so isn't incredibly invasive.

Reviewed by:	des, jhb, obiwac
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D57354

lib/libc/stdio/open_memstream.c

@@ -46,6 +46,7 @@
 struct memstream {
 	char **bufp;
 	size_t *sizep;
+	size_t size;
 	ssize_t len;
 	fpos_t offset;
 };
@@ -60,19 +61,22 @@ memstream_grow(struct memstream *ms, fpos_t newoff)
 		newsize = SSIZE_MAX - 1;
 	else
 		newsize = newoff;
-	if (newsize > ms->len) {
+	if (newsize > ms->size) {
 		buf = realloc(*ms->bufp, newsize + 1);
 		if (buf == NULL)
 			return (0);
 
 #ifdef DEBUG
 		fprintf(stderr, "MS: %p growing from %zd to %zd\n",
-		    ms, ms->len, newsize);
+		    ms, ms->size, newsize);
 #endif
-		memset(buf + ms->len + 1, 0, newsize - ms->len);
+		memset(buf + ms->size + 1, 0, newsize - ms->size);
 		*ms->bufp = buf;
-		ms->len = newsize;
+		ms->size = newsize;
 	}
+
+	if (newsize > ms->len)
+		ms->len = newsize;
 	return (1);
 }
 
@@ -192,6 +196,7 @@ open_memstream(char **bufp, size_t *sizep)
 	}
 	ms->bufp = bufp;
 	ms->sizep = sizep;
+	ms->size = 0;
 	ms->len = 0;
 	ms->offset = 0;
 	memstream_update(ms);

lib/libc/stdio/open_wmemstream.c

@@ -46,6 +46,7 @@
 struct wmemstream {
 	wchar_t **bufp;
 	size_t *sizep;
+	size_t size;
 	ssize_t len;
 	fpos_t offset;
 	mbstate_t mbstate;
@@ -61,18 +62,20 @@ wmemstream_grow(struct wmemstream *ms, fpos_t newoff)
 		newsize = SSIZE_MAX / sizeof(wchar_t) - 1;
 	else
 		newsize = newoff;
-	if (newsize > ms->len) {
+	if (newsize > ms->size) {
 		buf = reallocarray(*ms->bufp, newsize + 1, sizeof(wchar_t));
 		if (buf == NULL)
 			return (0);
 #ifdef DEBUG
 		fprintf(stderr, "WMS: %p growing from %zd to %zd\n",
-		    ms, ms->len, newsize);
+		    ms, ms->size, newsize);
 #endif
-		wmemset(buf + ms->len + 1, 0, newsize - ms->len);
+		wmemset(buf + ms->size + 1, 0, newsize - ms->size);
 		*ms->bufp = buf;
-		ms->len = newsize;
+		ms->size = newsize;
 	}
+	if (newsize > ms->len)
+		ms->len = newsize;
 	return (1);
 }
 
@@ -252,6 +255,7 @@ open_wmemstream(wchar_t **bufp, size_t *sizep)
 	}
 	ms->bufp = bufp;
 	ms->sizep = sizep;
+	ms->size = 0;
 	ms->len = 0;
 	ms->offset = 0;
 	memset(&ms->mbstate, 0, sizeof(mbstate_t));