Define Base.close(::Session)

src/core.jl

@@ -554,8 +554,7 @@ mutable struct Session
         this = new(ptr, graph)
         check_status(status)
         finalizer(this, self->begin
-            status = Status()
-            @tfcall(:TF_DeleteSession, Void, (Ptr{Void}, Ptr{Void}), self.ptr, status.ptr)
+            close(self)
         end)
         return this
     end
@@ -571,6 +570,21 @@ mutable struct Session
     end
 end
 
+"""
+    close(sess::Session)
+
+Closes the TensorFlow session, freeing the associated computational resources.
+"""
+function Base.close(sess::Session)
+    if sess.ptr != C_NULL
+        status = Status()
+        @tfcall(:TF_DeleteSession, Void, (Ptr{Void}, Ptr{Void}), sess.ptr, status.ptr)
+        check_status(status)
+        sess.ptr = C_NULL
+    end
+    return nothing
+end
+
 
 mutable struct Buffer
     ptr::Ptr{Void}

src/run.jl

@@ -77,9 +77,18 @@ function build_input(tensor_map::Dict)
     input_tensors, input_values
 end
 
+struct ClosedSessionError <: Exception
+end
+
+function Base.show(io::IO, err::ClosedSessionError)
+    print(io, "An operation was attempted on a closed TensorFlow session.")
+end
+
 function run(sess::Session, inputs, input_values, outputs, targets)
     #Low level run, without size checking, and type conversion etc.
-
+    if sess.ptr == C_NULL
+        throw(ClosedSessionError())
+    end
     status = Status()
     output_values = fill(C_NULL, length(outputs))
     input_tensors = [RawTensor(x) for x in input_values]
@@ -184,6 +193,9 @@ end
 
 
 """
+    run(sess::Session, output, input_dict::Dict)
+
+
 Compute the result of one of more operations in the computation graph.
 """
 function run(sess::Session, output, input_dict)

test/core.jl

@@ -26,6 +26,15 @@ end
     end
 end
 
+@testset "Session closing" begin
+    session = tf.Session(Graph())
+    x = constant(1)
+    @test run(session, x) == 1
+    close(session)
+    close(session)  # Test that we can safely call `close` twice on the same session
+    @test_throws tf.ClosedSessionError run(session, x)
+end
+
 @testset "get_operations" begin
     let
         graph = Graph()