do not serve files when closing

mongoose.c

@@ -2272,10 +2272,10 @@ void mg_http_serve_file(struct mg_connection *c, struct mg_http_message *hm,
               status, mg_http_status_code_str(status), (int) mime.len, mime.buf,
               etag, (uint64_t) cl, gzip ? "Content-Encoding: gzip\r\n" : "",
               range, opts->extra_headers ? opts->extra_headers : "");
-    if (mg_strcasecmp(hm->method, mg_str("HEAD")) == 0) {
+    if (mg_strcasecmp(hm->method, mg_str("HEAD")) == 0 || c->is_closing) {
       c->is_resp = 0;
       mg_fs_close(fd);
-    } else {
+    } else { // start serving static content only if not closing, see #3354
       // Track to-be-sent content length at the end of c->data, aligned
       size_t *clp = (size_t *) &c->data[(sizeof(c->data) - sizeof(size_t)) /
                                         sizeof(size_t) * sizeof(size_t)];

src/http.c

@@ -1,7 +1,7 @@
+#include "http.h"
 #include "arch.h"
 #include "base64.h"
 #include "fmt.h"
-#include "http.h"
 #include "json.h"
 #include "log.h"
 #include "net.h"
@@ -648,10 +648,10 @@ void mg_http_serve_file(struct mg_connection *c, struct mg_http_message *hm,
               status, mg_http_status_code_str(status), (int) mime.len, mime.buf,
               etag, (uint64_t) cl, gzip ? "Content-Encoding: gzip\r\n" : "",
               range, opts->extra_headers ? opts->extra_headers : "");
-    if (mg_strcasecmp(hm->method, mg_str("HEAD")) == 0) {
+    if (mg_strcasecmp(hm->method, mg_str("HEAD")) == 0 || c->is_closing) {
       c->is_resp = 0;
       mg_fs_close(fd);
-    } else {
+    } else { // start serving static content only if not closing, see #3354
       // Track to-be-sent content length at the end of c->data, aligned
       size_t *clp = (size_t *) &c->data[(sizeof(c->data) - sizeof(size_t)) /
                                         sizeof(size_t) * sizeof(size_t)];