Github #273 Handle failure to initialise BackendContext in Runtime

Define CreateBackendContext to throw BackendUnavailableException
if necessary runtime components are missing.

Handle this in the constructor of Runtime by not adding those
backends to the internal DeviceSpec owned by the Runtime.

Signed-off-by: Matthew Bentham <Matthew.Bentham@arm.com>
Change-Id: Iadffb5240e32e1f105683c4d361276b92d1e720c

5 files changed, 28 insertions, 9 deletions

diff --git a/include/armnn/Exceptions.hpp b/include/armnn/Exceptions.hpp
index e21e974fc7..066f59f792 100644
--- a/include/armnn/Exceptions.hpp
+++ b/include/armnn/Exceptions.hpp
@@ -64,12 +64,19 @@ private:
     std::string m_Message;
 };
 
-class ClRuntimeUnavailableException : public Exception
+/// Class for non-fatal exceptions raised while initialising a backend
+class BackendUnavailableException : public Exception
 {
 public:
     using Exception::Exception;
 };
 
+class ClRuntimeUnavailableException : public BackendUnavailableException
+{
+public:
+    using BackendUnavailableException::BackendUnavailableException;
+};
+
 class InvalidArgumentException : public Exception
 {
 public:
diff --git a/include/armnn/backends/IBackendInternal.hpp b/include/armnn/backends/IBackendInternal.hpp
index 29097b4ae7..6c919ee5d4 100644
--- a/include/armnn/backends/IBackendInternal.hpp
+++ b/include/armnn/backends/IBackendInternal.hpp
@@ -115,9 +115,16 @@ public:
     virtual IWorkloadFactoryPtr CreateWorkloadFactory(
         class TensorHandleFactoryRegistry& tensorHandleFactoryRegistry) const;
 
+    /// Create the runtime context of the backend
+    ///
+    /// Implementations may return a default-constructed IBackendContextPtr if
+    /// no context is needed at runtime.
+    /// Implementations must throw BackendUnavailableException if the backend
+    /// cannot be used (for example, necessary accelerator hardware is not present).
+    /// The default implementation always returns a default-constructed pointer.
     virtual IBackendContextPtr CreateBackendContext(const IRuntime::CreationOptions&) const;
 
-    // Context specifically used for profiling interaction from backends.
+    /// Create context specifically used for profiling interaction from backends.
     virtual IBackendProfilingContextPtr CreateBackendProfilingContext(const IRuntime::CreationOptions& creationOptions,
         armnn::profiling::IBackendProfiling& backendProfiling) const;
 
diff --git a/src/armnn/DeviceSpec.hpp b/src/armnn/DeviceSpec.hpp
index 703a4b123f..a1457cf80e 100644
--- a/src/armnn/DeviceSpec.hpp
+++ b/src/armnn/DeviceSpec.hpp
@@ -14,6 +14,9 @@ namespace armnn
 class DeviceSpec : public IDeviceSpec
 {
 public:
+    DeviceSpec()
+    {}
+
     DeviceSpec(const BackendIdSet& supportedBackends)
         : m_SupportedBackends{supportedBackends} {}
 
@@ -48,7 +51,6 @@ public:
     }
 
 private:
-    DeviceSpec() = delete;
     BackendIdSet m_SupportedBackends;
     BackendIdSet m_DynamicBackends;
 };
diff --git a/src/armnn/Runtime.cpp b/src/armnn/Runtime.cpp
index 2d7269a09c..47c998a6fd 100644
--- a/src/armnn/Runtime.cpp
+++ b/src/armnn/Runtime.cpp
@@ -153,7 +153,6 @@ const std::shared_ptr<IProfiler> Runtime::GetProfiler(NetworkId networkId) const
 
 Runtime::Runtime(const CreationOptions& options)
     : m_NetworkIdCounter(0)
-    , m_DeviceSpec{BackendRegistryInstance().GetBackendIds()}
 {
     ARMNN_LOG(info) << "ArmNN v" << ARMNN_VERSION << "\n";
 
@@ -164,12 +163,11 @@ Runtime::Runtime(const CreationOptions& options)
     // goes through the backend registry
     LoadDynamicBackends(options.m_DynamicBackendsPath);
 
+    BackendIdSet supportedBackends;
     for (const auto& id : BackendRegistryInstance().GetBackendIds())
     {
         // Store backend contexts for the supported ones
-        const BackendIdSet& supportedBackends = m_DeviceSpec.GetSupportedBackends();
-        if (supportedBackends.find(id) != supportedBackends.end())
-        {
+        try {
             auto factoryFun = BackendRegistryInstance().GetFactory(id);
             auto backend = factoryFun();
             BOOST_ASSERT(backend.get() != nullptr);
@@ -182,8 +180,15 @@ Runtime::Runtime(const CreationOptions& options)
             {
                 m_BackendContexts.emplace(std::make_pair(id, std::move(context)));
             }
+            supportedBackends.emplace(id);
+        }
+        catch (const BackendUnavailableException&)
+        {
+            // Ignore backends which are unavailable
         }
+
     }
+    m_DeviceSpec.AddSupportedBackends(supportedBackends);
 }
 
 Runtime::~Runtime()
diff --git a/src/armnn/Runtime.hpp b/src/armnn/Runtime.hpp
index e5debbf9ac..2ad3c9633c 100644
--- a/src/armnn/Runtime.hpp
+++ b/src/armnn/Runtime.hpp
@@ -73,8 +73,6 @@ public:
     virtual void RegisterDebugCallback(NetworkId networkId, const DebugCallbackFunction& func) override;
 
     /// Creates a runtime for workload execution.
-    /// May throw a ClRuntimeUnavailableException if @a defaultComputeDevice requires a CL runtime but
-    /// it cannot be setup for some reason.
     Runtime(const CreationOptions& options);
 
     ~Runtime();