make PythonCall.GC more like Base.GC (#413)

Co-authored-by: Christopher Doris <github.com/cjdoris>

docs/src/faq.md

@@ -6,8 +6,8 @@ No.
 
 Some rules if you are writing multithreaded code:
 - Only call Python functions from the first thread.
-- You probably also need to call `PythonCall.GC.disable()` on the main thread before any
-  threaded block of code. Remember to call `PythonCall.GC.enable()` again afterwards.
+- You probably also need to call `on=PythonCall.GC.enable(false)` on the main thread before any
+  threaded block of code. Remember to call `PythonCall.GC.enable(on)` again afterwards.
   (This is because Julia finalizers can be called from any thread.)
 - Julia intentionally causes segmentation faults as part of the GC safepoint mechanism.
   If unhandled, these segfaults will result in termination of the process. To enable signal handling,

docs/src/releasenotes.md

@@ -1,6 +1,7 @@
 # Release Notes
 
 ## Unreleased (v1)
+* `PythonCall.GC` is now more like `Base.GC`: `enable(true)` replaces `enable()`, `enable(false)` replaces `disable()`, and `gc()` is added.
 
 ## Unreleased
 * `Py` is now treated as a scalar when broadcasting.

src/GC/GC.jl

@@ -3,7 +3,7 @@
 
 Garbage collection of Python objects.
 
-See `disable` and `enable`.
+See [`enable`](@ref) and [`gc`](@ref).
 """
 module GC
 
@@ -13,32 +13,39 @@ const ENABLED = Ref(true)
 const QUEUE = C.PyPtr[]
 
 """
-    PythonCall.GC.disable()
+    PythonCall.GC.enable(on::Bool)
 
-Disable the PythonCall garbage collector.
+Control whether garbage collection of Python objects is turned on or off.
 
-This means that whenever a Python object owned by Julia is finalized, it is not immediately
-freed but is instead added to a queue of objects to free later when `enable()` is called.
+Return the previous GC state.
+
+Disabling the GC means that whenever a Python object owned by Julia is finalized, it is not
+immediately freed but is instead added to a queue of objects to free later when GC is
+re-enabled.
 
 Like most PythonCall functions, you must only call this from the main thread.
 """
-function disable()
-    ENABLED[] = false
-    return
+function enable(on::Bool)
+    was_on = ENABLED[]
+    if on
+        ENABLED[] = true
+        if !was_on
+            gc()
+        end
+    else
+        ENABLED[] = false
+    end
+    return ans
 end
 
 """
-    PythonCall.GC.enable()
-
-Re-enable the PythonCall garbage collector.
+    PythonCall.GC.gc()
 
-This frees any Python objects which were finalized while the GC was disabled, and allows
-objects finalized in the future to be freed immediately.
+Perform garbage collection of Python objects.
 
 Like most PythonCall functions, you must only call this from the main thread.
 """
-function enable()
-    ENABLED[] = true
+function gc()
     if !isempty(QUEUE)
         C.with_gil(false) do
             for ptr in QUEUE
@@ -47,9 +54,8 @@ function enable()
                 end
             end
         end
+        empty!(QUEUE)            
     end
-    empty!(QUEUE)
-    return
 end
 
 function enqueue(ptr::C.PyPtr)