[core] write all cq MEM_CHUNK if spill to tempfile

If chunkqueue size grows large enough in memory to use tempfiles,
write all MEM_CHUNK in chunkqueue to tempfiles to free up memory.

If earlier chunks in chunkqueue are MEM_CHUNK, then a prior attempt
to write was not able to proceed, or items are being added to
chunkqueue as they are decoded from TLS.  In either case, choose to
free up memory sooner.  This may result in some cases where lighttpd
soon reads data back from disk to send to the backend, but since some
(now all) of the data is in tempfiles, sendfile() will avoid reading
back into userspace, though sendfile() will not be used if lighttpd
needs to read data back into memory to encrypt data using TLS.

src/chunk.c

@@ -627,9 +627,33 @@ static chunk *chunkqueue_get_append_tempfile(chunkqueue * const restrict cq, log
 	return c;
 }
 
+__attribute_cold__
+__attribute_noinline__
+static int chunkqueue_to_tempfiles(chunkqueue * const restrict dest, log_error_st * const restrict errh) {
+    /* transfer chunks from dest to src, adjust dest->bytes_in, and then call
+     * chunkqueue_steal_with_tempfiles() to write chunks from src back into
+     * dest, but into tempfiles.   chunkqueue_steal_with_tempfiles() calls back
+     * into chunkqueue_append_mem_to_tempfile(), but will not re-enter this func
+     * since chunks moved to src, and dest made empty before recursive call */
+    const off_t cqlen = chunkqueue_length(dest);
+    chunkqueue src = *dest; /*(copy struct)*/
+    dest->first = dest->last = NULL;
+    dest->bytes_in -= cqlen;
+    return (0 == chunkqueue_steal_with_tempfiles(dest, &src, cqlen, errh))
+      ? 0
+      : (chunkqueue_release_chunks(&src), -1);
+}
+
 int chunkqueue_append_mem_to_tempfile(chunkqueue * const restrict dest, const char * restrict mem, size_t len, log_error_st * const restrict errh) {
-	chunk *dst_c;
-	ssize_t written;
+	chunk *dst_c = dest->first;
+
+	/* check if prior MEM_CHUNK(s) exist and write to tempfile
+	 * (check first chunk only, since if we are using tempfiles, then
+	 *  we expect further chunks to be tempfiles after starting tempfiles)*/
+	if (dst_c && dst_c->type == MEM_CHUNK
+	    && 0 != chunkqueue_to_tempfiles(dest, errh)) {
+		return -1;
+	}
 
 	do {
 		/*
@@ -675,7 +699,7 @@ int chunkqueue_append_mem_to_tempfile(chunkqueue * const restrict dest, const ch
 
 		/* (dst_c->file.fd >= 0) */
 		/* coverity[negative_returns : FALSE] */
-		written = write(dst_c->file.fd, mem, len);
+		const ssize_t written = write(dst_c->file.fd, mem, len);
 
 		if ((size_t) written == len) {
 			dst_c->file.length += len;