pathtree: Allow get() on subtrees

When running recent PathTree modifications on a BMC we can see failures
in the log such as "Failed to invoke ObjectMapper method" from the
phosphor-fru-fault-monitor. This can be triggered via the
openbmc-test-automation suite, for example with the following command

$ robot -v OPENBMC_HOST:w5.aus.stglabs.ibm.com --include \
	Create_Test_Error_Callout_And_Verify extended/test_association.robot

Capturing the DBus traffic across this test case
(`busctl capture > dbus.pcap`) and analysing it with Wireshark showed the
following method invocation:

```
0000   6c 01 00 01 53 00 00 00 16 00 00 00 c6 00 00 00  l...S...........
0010   01 01 6f 00 22 00 00 00 2f 78 79 7a 2f 6f 70 65  ..o.".../xyz/ope
0020   6e 62 6d 63 5f 70 72 6f 6a 65 63 74 2f 6f 62 6a  nbmc_project/obj
0030   65 63 74 5f 6d 61 70 70 65 72 00 00 00 00 00 00  ect_mapper......
0040   03 01 73 00 09 00 00 00 47 65 74 4f 62 6a 65 63  ..s.....GetObjec
0050   74 00 00 00 00 00 00 00 02 01 73 00 20 00 00 00  t.........s. ...
0060   78 79 7a 2e 6f 70 65 6e 62 6d 63 5f 70 72 6f 6a  xyz.openbmc_proj
0070   65 63 74 2e 4f 62 6a 65 63 74 4d 61 70 70 65 72  ect.ObjectMapper
0080   00 00 00 00 00 00 00 00 06 01 73 00 20 00 00 00  ..........s. ...
0090   78 79 7a 2e 6f 70 65 6e 62 6d 63 5f 70 72 6f 6a  xyz.openbmc_proj
00a0   65 63 74 2e 4f 62 6a 65 63 74 4d 61 70 70 65 72  ect.ObjectMapper
00b0   00 00 00 00 00 00 00 00 08 01 67 00 03 73 61 73  ..........g..sas
00c0   00 00 00 00 00 00 00 00 07 01 73 00 05 00 00 00  ..........s.....
00d0   3a 31 2e 35 37 00 00 00 20 00 00 00 2f 78 79 7a  :1.57... .../xyz
00e0   2f 6f 70 65 6e 62 6d 63 5f 70 72 6f 6a 65 63 74  /openbmc_project
00f0   2f 6c 65 64 2f 67 72 6f 75 70 73 2f 00 00 00 00  /led/groups/....
0100   27 00 00 00 22 00 00 00 6f 72 67 2e 66 72 65 65  '..."...org.free
0110   64 65 73 6b 74 6f 70 2e 44 42 75 73 2e 4f 62 6a  desktop.DBus.Obj
0120   65 63 74 4d 61 6e 61 67 65 72 00                 ectManager.
```

and the response

```
0000   6c 03 01 01 3f 00 00 00 9d 08 00 00 5e 00 00 00  l...?.......^...
0010   06 01 73 00 05 00 00 00 3a 31 2e 35 37 00 00 00  ..s.....:1.57...
0020   04 01 73 00 27 00 00 00 6f 72 67 2e 66 72 65 65  ..s.'...org.free
0030   64 65 73 6b 74 6f 70 2e 44 42 75 73 2e 45 72 72  desktop.DBus.Err
0040   6f 72 2e 46 69 6c 65 4e 6f 74 46 6f 75 6e 64 00  or.FileNotFound.
0050   05 01 75 00 16 00 00 00 08 01 67 00 01 73 00 00  ..u.......g..s..
0060   07 01 73 00 05 00 00 00 3a 31 2e 31 35 00 00 00  ..s.....:1.15...
0070   3a 00 00 00 70 61 74 68 20 6f 72 20 6f 62 6a 65  :...path or obje
0080   63 74 20 6e 6f 74 20 66 6f 75 6e 64 3a 20 2f 78  ct not found: /x
0090   79 7a 2f 6f 70 65 6e 62 6d 63 5f 70 72 6f 6a 65  yz/openbmc_proje
00a0   63 74 2f 6c 65 64 2f 67 72 6f 75 70 73 2f 00     ct/led/groups/.
```

These packets translate to the following `busctl` invocation and response:

```
root@witherspoon:~# busctl call \
	xyz.openbmc_project.ObjectMapper \
	/xyz/openbmc_project/object_mapper \
	xyz.openbmc_project.ObjectMapper \
	GetObject sas \
		/xyz/openbmc_project/led/groups/ \
		1 \
		org.freedesktop.Dbus.ObjectManager
path or object not found: /xyz/openbmc_project/led/groups/
root@witherspoon:~#
```

From `busctl tree` we can see that the objects do actually appear on the
bus:

root@witherspoon:~# busctl tree
...
Service xyz.openbmc_project.LED.GroupManager:
└─/xyz
  └─/xyz/openbmc_project
    └─/xyz/openbmc_project/led
      └─/xyz/openbmc_project/led/groups
        ├─/xyz/openbmc_project/led/groups/bmc_booted
        ├─/xyz/openbmc_project/led/groups/bmc_fault
        ├─/xyz/openbmc_project/led/groups/boxelder_fault
        ├─/xyz/openbmc_project/led/groups/core0_fault
        ├─/xyz/openbmc_project/led/groups/core10_fault
...
```

However, the /xyz/openbmc_project/led/groups/ path represents a subtree
root, not a leaf element. I didn't realise this was a supported
use-case, and over-aggressively removed support for it in the recent
patch adding the flat dict cache.

Add in a fallback to iterating the PathTree datastructure when we take a
cache miss.

Change-Id: I658c3fdd6b87be9d814c59a654e63ec5bb83e2bd
Fixes: 52aeb314c0eb ("pathtree: Cache keys in a flat dict for fast lookup")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>

1 files changed, 4 insertions, 1 deletions

diff --git a/obmc/utils/pathtree.py b/obmc/utils/pathtree.py
index dc48609..9896bfe 100644
--- a/obmc/utils/pathtree.py
+++ b/obmc/utils/pathtree.py
@@ -157,7 +157,10 @@ class PathTree:
         d[elements[-1]].update({kids: children, 'data': value})
 
     def __getitem__(self, key):
-        return self.cache[key]
+        if key in self.cache:
+            return self.cache[key]
+
+        return self._get_node(key).get('data')
 
     def setdefault(self, key, default):
         if not self.get(key):