[nginx] Reset imap tag to empty after authentication attempt

[Robert Mueller]

# HG changeset patch
# User Rob Mueller <robm at fastmailteam.com>
Reset imap tag to empty after authentication attempt

We need to reset the imap tag to empty after an authentication attempt
completes, otherwise if the next line parsed is incomplete with no tag
(e.g. empty line) then we use the "tag" from the previous buffer which
is now definitely wrong and has been partially overwritten with the most
recently read data (e.g. CRLF).

An example before this patch:

S: * OK IMAP4 ready
C: foobar login a b
S: foobar NO Incorrect username or password.
C:
S:
S: obar BAD invalid command

Then with this patch:

S: * OK IMAP4 ready
C: foobar login a b
S: foobar NO Incorrect username or password.
C:
S: * BAD invalid command
---
 src/mail/ngx_mail_auth_http_module.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/mail/ngx_mail_auth_http_module.c b/src/mail/ngx_mail_auth_http_module.c
index 543d1c4eb..55280d66c 100644
--- a/src/mail/ngx_mail_auth_http_module.c
+++ b/src/mail/ngx_mail_auth_http_module.c
@@ -981,6 +981,7 @@ ngx_mail_auth_sleep_handler(ngx_event_t *rev)
 
         s->mail_state = 0;
         s->auth_method = NGX_MAIL_AUTH_PLAIN;
+        s->tag.len = 0;
 
         c->log->action = "in auth state";