Work around replay cache creation race

If two processes try to initialize the same replay cache at the same
time, krb5_rc_io_creat can race between unlink and open, leading to a
KRB5_RC_IO_PERM error.  When this happens, make the losing process
retry so that it can continue.

This does not solve the replay cache creation race, nor is that the
only replay cache race issue.  It simply prevents the race from
causing a spurious failure.

ticket: 3498
target_version: 1.13
tags: pullup

diff --git a/src/lib/krb5/rcache/rc_io.c b/src/lib/krb5/rcache/rc_io.c
index 7e3b7e951a402d78cd5a12f03b91b6962aa11046..b9859fe9f657d59c902488d725a65863b7a2d724 100644 (file)
--- a/src/lib/krb5/rcache/rc_io.c
+++ b/src/lib/krb5/rcache/rc_io.c
@@ -158,7 +158,7 @@ krb5_rc_io_creat(krb5_context context, krb5_rc_iostuff *d, char **fn)
 {
     krb5_int16 rc_vno = htons(KRB5_RC_VNO);
     krb5_error_code retval = 0;
-    int do_not_unlink = 0;
+    int flags, do_not_unlink = 0;
     char *dir;
     size_t dirlen;
 
@@ -166,9 +166,13 @@ krb5_rc_io_creat(krb5_context context, krb5_rc_iostuff *d, char **fn)
     if (fn && *fn) {
         if (asprintf(&d->fn, "%s%s%s", dir, PATH_SEPARATOR, *fn) < 0)
             return KRB5_RC_IO_MALLOC;
-        unlink(d->fn);
-        d->fd = THREEPARAMOPEN(d->fn, O_WRONLY | O_CREAT | O_TRUNC | O_EXCL |
-                               O_BINARY, 0600);
+        d->fd = -1;
+        do {
+            if (unlink(d->fn) == -1 && errno != ENOENT)
+                break;
+            flags = O_WRONLY | O_CREAT | O_TRUNC | O_EXCL | O_BINARY;
+            d->fd = THREEPARAMOPEN(d->fn, flags, 0600);
+        } while (d->fd == -1 && errno == EEXIST);
     } else {
         retval = krb5_rc_io_mkstemp(context, d, dir);
         if (retval)