diff --git a/include/libwebsockets/lws-lwsac.h b/include/libwebsockets/lws-lwsac.h
index 1c0d69b8a..a59e45942 100644
--- a/include/libwebsockets/lws-lwsac.h
+++ b/include/libwebsockets/lws-lwsac.h
@@ -1,7 +1,7 @@
 /*
  * libwebsockets - small server side websockets and web server implementation
  *
- * Copyright (C) 2010 - 2019 Andy Green <andy@warmcat.com>
+ * Copyright (C) 2010 - 2020 Andy Green <andy@warmcat.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to
@@ -198,6 +198,38 @@ lwsac_reference(struct lwsac *head);
 LWS_VISIBLE LWS_EXTERN void
 lwsac_unreference(struct lwsac **head);
 
+/**
+ * lwsac_extend() - try to increase the size of the last block
+ *
+ * \param head: pointer to the lwsac list object
+ * \param amount: amount to try to increase usage for
+ *
+ * This will either increase the usage reservation of the last allocated block
+ * by amount and return 0, or fail and return 1.
+ *
+ * This is very cheap to call and is designed to optimize usage after a static
+ * struct for vari-sized additional content which may flow into an additional
+ * block in a new chunk if necessary, but wants to make the most of the space
+ * in front of it first to try to avoid gaps and the new chunk if it can.
+ *
+ * The additional area if the call succeeds will have been memset to 0.
+ *
+ * To use it, the following must be true:
+ *
+ * - only the last lwsac use can be extended
+ *
+ * - if another use happens inbetween the use and extend, it will break
+ *
+ * - the use cannot have been using backfill
+ *
+ * - a user object must be tracking the current allocated size of the last use
+ *   (lwsac doesn't know it) and increment by amount if the extend call succeeds
+ *
+ * Despite these restrictions this can be an important optimization for some
+ * cases
+ */
+LWS_VISIBLE LWS_EXTERN int
+lwsac_extend(struct lwsac *head, int amount);
 
 /* helpers to keep a file cached in memory */
 
diff --git a/lib/misc/lwsac/lwsac.c b/lib/misc/lwsac/lwsac.c
index 426fdb381..3ca48a707 100644
--- a/lib/misc/lwsac/lwsac.c
+++ b/lib/misc/lwsac/lwsac.c
@@ -1,7 +1,7 @@
 /*
  * libwebsockets - small server side websockets and web server implementation
  *
- * Copyright (C) 2010 - 2019 Andy Green <andy@warmcat.com>
+ * Copyright (C) 2010 - 2020 Andy Green <andy@warmcat.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to
@@ -69,6 +69,29 @@ lwsac_get_next(struct lwsac *lac)
 	return lac->next;
 }
 
+int
+lwsac_extend(struct lwsac *head, int amount)
+{
+	struct lwsac_head *lachead;
+	struct lwsac *bf;
+
+	assert(head);
+	lachead = (struct lwsac_head *)&head[1];
+
+	bf = lachead->curr;
+	assert(bf);
+
+	if (bf->alloc_size - bf->ofs < lwsac_align(amount))
+		return 1;
+
+	/* memset so constant folding never sees uninitialized data */
+
+	memset(((uint8_t *)bf) + bf->ofs, 0, lwsac_align(amount));
+	bf->ofs += lwsac_align(amount);
+
+	return 0;
+}
+
 static void *
 _lwsac_use(struct lwsac **head, size_t ensure, size_t chunk_size, char backfill)
 {
diff --git a/lib/misc/lwsac/private-lib-misc-lwsac.h b/lib/misc/lwsac/private-lib-misc-lwsac.h
index 8c09595fc..026e7adbd 100644
--- a/lib/misc/lwsac/private-lib-misc-lwsac.h
+++ b/lib/misc/lwsac/private-lib-misc-lwsac.h
@@ -1,7 +1,7 @@
 /*
  * libwebsockets - small server side websockets and web server implementation
  *
- * Copyright (C) 2010 - 2019 Andy Green <andy@warmcat.com>
+ * Copyright (C) 2010 - 2020 Andy Green <andy@warmcat.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to
@@ -38,18 +38,26 @@
  * count reaches zero.
  */
 
+/*
+ * One of these per chunk
+ */
+
 struct lwsac {
 	struct lwsac *next;
 	struct lwsac *head; /* pointer back to the first chunk */
-	size_t alloc_size;
+	size_t alloc_size; /* alloc size of the whole chunk */
 	size_t ofs; /* next writeable position inside chunk */
 };
 
+/*
+ * One of these per lwsac, at start of first chunk
+ */
+
 struct lwsac_head {
-	struct lwsac *curr; /* applies to head chunk only */
-	size_t total_alloc_size; /* applies to head chunk only */
-	int refcount; /* applies to head chunk only */
-	int total_blocks; /* applies to head chunk only */
+	struct lwsac *curr;
+	size_t total_alloc_size;
+	int refcount;
+	int total_blocks;
 	char detached; /* if our refcount gets to zero, free the chunk list */
 };