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Proposal: Safe pointers #1043

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Merged
filmor merged 9 commits into from
Feb 26, 2020
Merged

Proposal: Safe pointers #1043

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4a84a1e
NewReference type and an example usage
lostmsu Feb 13, 2020
88944a2
BorrowedReference + example, that exposes dangerous pattern
lostmsu Feb 13, 2020
e1ba92c
actually, borrowed refrences can be copied
lostmsu Feb 13, 2020
2d11f82
make BorrowedReference readonly ref struct
lostmsu Feb 13, 2020
dbb3dc5
updated old project file
lostmsu Feb 13, 2020
7304b25
BorrowedReference.Pointer is a private readonly field
lostmsu Feb 13, 2020
2bc218c
drop C# 8.0 requirement
lostmsu Feb 13, 2020
2cf9fc3
renamed NewReference.ToPyObject to MoveToPyObject
lostmsu Feb 23, 2020
6ae63cd
added xmldoc comments to *Reference members
lostmsu Feb 23, 2020
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added xmldoc comments to *Reference members
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@lostmsu
lostmsu committed Feb 23, 2020
commit 6ae63cd9678cd42545cd30b9be336b8ec5240045
14 changes: 5 additions & 9 deletions src/runtime/BorrowedReference.cs
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Original file line number Diff line number Diff line change
@@ -1,20 +1,16 @@
namespace Python.Runtime
{
using System;
/// <summary>
/// Represents a reference to a Python object, that is being lent, and
/// can only be safely used until execution returns to the caller.
/// </summary>
readonly ref struct BorrowedReference
{
readonly IntPtr pointer;
public bool IsNull => this.pointer == IntPtr.Zero;

public PyObject ToPyObject()
{
if (this.IsNull) throw new NullReferenceException();

Runtime.XIncref(this.pointer);
return new PyObject(this.pointer);
}

[Obsolete("Use overloads, that take BorrowedReference or NewReference")]
/// <summary>Gets a raw pointer to the Python object</summary>
public IntPtr DangerousGetAddress()
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@amos402 amos402 Feb 23, 2020

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Use a property named Address can be more fit?
Why I have to be warned if I trying use the raw pointer, seems it doesn't make any sense. Or just don't expose the interface and use explicit operator IntPtr instead.

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@lostmsu lostmsu Feb 23, 2020

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This is modeled after SafeHandle class and its DangerousGetHandle method.
The whole point of introducing these references is to get rid of IntPtrs entirely. It should warn you against converting into IntPtr. Maybe Obsolete part is unnecessary as Dangerous sounds like a warning enough. I will remove Obsolete.

=> this.IsNull ? throw new NullReferenceException() : this.pointer;

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11 changes: 10 additions & 1 deletion src/runtime/NewReference.cs
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@@ -1,6 +1,9 @@
namespace Python.Runtime
{
using System;
/// <summary>
/// Represents a reference to a Python object, that is tracked by Python's reference counting.
/// </summary>
[NonCopyable]
ref struct NewReference
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@amos402 amos402 Feb 23, 2020

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Is it necessary to limit it as a stack-allocated value? If make it to a normal struct and rename it to PyReference, it can be assigned to a collection and implement the IDisposable for using () usage.
For better usage, a BorrowedReference may be able to promote to a PyReference for using the implict operate overloading.

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@lostmsu lostmsu Feb 23, 2020

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With [NoCopyable] you won't be able to put it into a collection anyway. And without [NoCopyable] it does not provide any safety guarantees.

Having it as ref struct saves you from adding ref everywhere you take it.

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@amos402 amos402 Feb 24, 2020

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But....sometimes I just want to put them into collections...😂

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@lostmsu lostmsu Feb 24, 2020

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Then you can still do DangerousGetHandle or create a PyObject from them.

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@amos402 amos402 Feb 24, 2020

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But this type also has a purpose for reminding others that it's a reference don't forget to decref it, isn't it?

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@lostmsu lostmsu Feb 24, 2020

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Because it must be clear from code, that care is necessary around this scenario.

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@lostmsu lostmsu Feb 24, 2020

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Also, as mentioned above, [NoCopyable] would already prevent you from putting an instance into a collection, even if ref were removed. And [NoCopyable] is basically the sole purpose of this change, as if you do

NewReference refA = GetNewReferenceSomehow();
NewReference refB = refA;

Then you already miappropriated refcounts.

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@amos402 amos402 Feb 24, 2020

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Maybe more introduce a type named PyHandle for that?

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@amos402 amos402 Feb 24, 2020

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emmm, but that make BorrowedReference embarrassed.

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@lostmsu lostmsu Feb 24, 2020

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@amos402 The idea with PyHandle can be reviewed separately. This PR is only for tracking new vs borrowed references as used in Python documentation for C API.

{
Expand All @@ -11,6 +14,10 @@ ref struct NewReference
public IntPtr DangerousGetAddress()
=> this.IsNull ? throw new NullReferenceException() : this.pointer;

/// <summary>
/// Returns <see cref="PyObject"/> wrapper around this reference, which now owns
/// the pointer. Sets the original reference to <c>null</c>, as it no longer owns it.
/// </summary>
public PyObject MoveToPyObject()
{
if (this.IsNull) throw new NullReferenceException();
Expand All @@ -19,7 +26,9 @@ public PyObject MoveToPyObject()
this.pointer = IntPtr.Zero;
return result;
}

/// <summary>
/// Removes this reference to a Python object, and sets it to <c>null</c>.
/// </summary>
public void Dispose()
{
if (!this.IsNull)
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