Claim a stable well known busname

Prior to this patch readd instances claim a busname with a format of:

xyz.openbmc_project.Hwmon.Hwmon<N> where N is the hwmon sysfs class
instance.

This is problematic for client applications that cache sensor object
busname mappings for objects provided by readd.  When readd instances
restart (due to udev events) the hwmon sysfs class index may have changed,
resulting in clients connecting to the wrong service instance.

Address this by ensuring readd instances claim the same name every
time they are started:

xyz.openbmc_project.Hwmon-<ID>.Hwmon1

Where ID is a std::hash of the /sys/devices path backing the hwmon
instance.

Additionally, add a trailing API version as recommended best practice
by the D-Bus specification.

Change-Id: Idd0057ce883a49c1e828fb54fede27ea14022d6a
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>

2 files changed, 20 insertions, 2 deletions

diff --git a/README.md b/README.md
index 8ac88a2..b6fce54 100644
--- a/README.md
+++ b/README.md
@@ -10,3 +10,18 @@ To build this package, do the following steps:
 
 To full clean the repository again run `./bootstrap.sh clean`.
 ```
+
+## D-Bus bus names
+
+```
+To enable the use of Linux features like cgroups prioritization and
+udev/systemd control, one instance of phosphor-hwmon is intended to
+be run per hwmon sysfs class instance.
+
+This requires an algorithm for selecting a stable, well-known D-Bus busname.
+
+The algorithm is <PREFIX>-<ID>.Hwmon<N> where PREFIX is an autoconf
+configurable prefix (BUSNAME_PREFIX, xyz.openbmc_project by default),
+ID is a std::hash of the /sys/devices path backing the hwmon class
+instance, and N is the implemented phosphor-hwmon D-Bus API version.
+```
diff --git a/mainloop.cpp b/mainloop.cpp
index bf2cacc..61de673 100644
--- a/mainloop.cpp
+++ b/mainloop.cpp
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#include <functional>
 #include <iostream>
 #include <memory>
 #include <cstdlib>
@@ -373,8 +374,10 @@ void MainLoop::run()
 
     {
         std::string busname{_prefix};
-        busname.append(1, '.');
-        busname.append(_instance);
+        busname.append(1, '-');
+        busname.append(
+                std::to_string(std::hash<decltype(_devPath)>{}(_devPath)));
+        busname.append(".Hwmon1");
         _bus.request_name(busname.c_str());
     }