illumos gate

Here is my proposed fix

--- a/usr/src/uts/common/fs/zfs/spa.c
+++ b/usr/src/uts/common/fs/zfs/spa.c
@@ -27,7 +27,7 @@
  * Copyright 2013 Saso Kiselkov. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  * Copyright 2016 Toomas Soome <tsoome@me.com>
- * Copyright 2017 Joyent, Inc.
+ * Copyright 2018 Joyent, Inc.
  * Copyright (c) 2017 Datto Inc.
  * Copyright 2018 OmniOS Community Edition (OmniOSce) Association.
  */
@@ -6535,6 +6535,7 @@ spa_vdev_resilver_done_hunt(vdev_t *vd)

        /*
         * Check for a completed resilver with the 'unspare' flag set.
+        * Also potentially update faulted state.
         */
        if (vd->vdev_ops == &vdev_spare_ops) {
                vdev_t *first = vd->vdev_child[0];
@@ -6557,6 +6558,26 @@ spa_vdev_resilver_done_hunt(vdev_t *vd)
                        return (oldvd);

                /*
+                * We know the vdev is a spare (e.g. "spare-1") which just
+                * finished resilvering. If it's faulted, and one of the
+                * children is healthy, then set the spare's state to degraded
+                * so that it will handle read operations.
+                */
+               if (vd->vdev_state == VDEV_STATE_FAULTED &&
+                   vd->vdev_children >= 2) {
+                       int i;
+
+                       for (i = 0; i < vd->vdev_children; i++) {
+                               if (vd->vdev_child[i]->vdev_state ==
+                                   VDEV_STATE_HEALTHY) {
+                                       vdev_set_state(vd, B_FALSE,
+                                           VDEV_STATE_DEGRADED, VDEV_AUX_NONE);
+                                       break;
+                               }
+                       }
+               }
+
+               /*
                 * If there are more than two spares attached to a disk,
                 * and those spares are not required, then we want to
                 * attempt to free them up now so that they can be used

For testing, I created a small zpool in a VM with 3 virtual disks (2 in the mirror, 1 spare). I used fminject to inject 11 IO errors onto one side of the mirror (zinject would also work). After the spare was brought in and resilvered, here is the resulting state

> ::spa -v
ADDR                 STATE NAME
fffffe090b6a4000    ACTIVE jjpool

    ADDR             STATE     AUX          DESCRIPTION
    fffffe090c7b4800 DEGRADED  -              mirror
    fffffe090c7b6800 HEALTHY   -                /dev/dsk/c1t2d0s0
    fffffe0d267ec800 DEGRADED  -                spare
    fffffe090c7b8800 FAULTED   ERR_EXCEEDED       /dev/dsk/c1t3d0s0
    fffffe0d26665800 HEALTHY   -                  /dev/dsk/c1t4d0s0

Using DTrace, I confirmed that read IO is now occurring on both c1t2d0 and c1t4d0.