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Proposal: Safe pointers #1043

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Merged
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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22 changes: 22 additions & 0 deletions src/runtime/BorrowedReference.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
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;

/// <summary>Gets a raw pointer to the Python object</summary>
public IntPtr DangerousGetAddress()
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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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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;

BorrowedReference(IntPtr pointer)
{
this.pointer = pointer;
}
}
}
39 changes: 39 additions & 0 deletions src/runtime/NewReference.cs
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@@ -0,0 +1,39 @@
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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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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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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But....sometimes I just want to put them into collections...😂

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

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

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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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Maybe more introduce a type named PyHandle for that?

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emmm, but that make BorrowedReference embarrassed.

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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.

{
IntPtr pointer;
public bool IsNull => this.pointer == IntPtr.Zero;

/// <summary>Gets a raw pointer to the Python object</summary>
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();

var result = new PyObject(this.pointer);
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)
Runtime.XDecref(this.pointer);
this.pointer = IntPtr.Zero;
}
}
}
6 changes: 6 additions & 0 deletions src/runtime/NonCopyableAttribute.cs
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@@ -0,0 +1,6 @@
namespace Python.Runtime
{
using System;
[AttributeUsage(AttributeTargets.Struct)]
class NonCopyableAttribute : Attribute { }
}
7 changes: 7 additions & 0 deletions src/runtime/Python.Runtime.15.csproj
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Expand Up @@ -129,6 +129,13 @@
<PackageReference Include="Microsoft.TargetingPack.NETFramework.v4.5" Version="1.0.1" ExcludeAssets="All" PrivateAssets="All" />
</ItemGroup>

<ItemGroup>
<PackageReference Include="NonCopyableAnalyzer" Version="0.5.1">
<PrivateAssets>all</PrivateAssets>
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>
</ItemGroup>

<Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />

<PropertyGroup>
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3 changes: 3 additions & 0 deletions src/runtime/Python.Runtime.csproj
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Expand Up @@ -83,6 +83,7 @@
</Compile>
<Compile Include="arrayobject.cs" />
<Compile Include="assemblymanager.cs" />
<Compile Include="BorrowedReference.cs" />
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Is it good for separate these three items to different files? They're small, related, and it make the project files be scattered.

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The might get more code later on. Generally, I prefer a type per file approach for anything except delegate types.

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Or create a folder for them? Just just too far if they sorted by name I thought🤣

<Compile Include="classderived.cs" />
<Compile Include="classbase.cs" />
<Compile Include="classmanager.cs" />
Expand Down Expand Up @@ -119,6 +120,8 @@
<Compile Include="moduleobject.cs" />
<Compile Include="modulepropertyobject.cs" />
<Compile Include="nativecall.cs" />
<Compile Include="NewReference.cs" />
<Compile Include="NonCopyableAttribute.cs" />
<Compile Include="overload.cs" />
<Compile Include="propertyobject.cs" />
<Compile Include="pyansistring.cs" />
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4 changes: 2 additions & 2 deletions src/runtime/assemblymanager.cs
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Expand Up @@ -145,7 +145,7 @@ internal static void UpdatePath()
probed.Clear();
for (var i = 0; i < count; i++)
{
IntPtr item = Runtime.PyList_GetItem(list, i);
BorrowedReference item = Runtime.PyList_GetItem(list, i);
string path = Runtime.GetManagedString(item);
if (path != null)
{
Expand Down Expand Up @@ -492,4 +492,4 @@ internal static Type[] GetTypes(Assembly a)
}
}
}
}
}
2 changes: 1 addition & 1 deletion src/runtime/methodbinder.cs
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Expand Up @@ -292,7 +292,7 @@ internal Binding Bind(IntPtr inst, IntPtr args, IntPtr kw, MethodBase info, Meth
for (int i = 0; i < pynkwargs; ++i)
{
var keyStr = Runtime.GetManagedString(Runtime.PyList_GetItem(keylist, i));
kwargDict[keyStr] = Runtime.PyList_GetItem(valueList, i);
kwargDict[keyStr] = Runtime.PyList_GetItem(valueList, i).DangerousGetAddress();
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This generates a compiler warning, that uncovers a potentially dangerous pattern, where kwargsDict may contain borrowed references.

}
Runtime.XDecref(keylist);
Runtime.XDecref(valueList);
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16 changes: 12 additions & 4 deletions src/runtime/pydict.cs
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Expand Up @@ -139,12 +139,20 @@ public PyObject Values()
/// </remarks>
public PyObject Items()
{
IntPtr items = Runtime.PyDict_Items(obj);
if (items == IntPtr.Zero)
var items = Runtime.PyDict_Items(this.obj);
try
{
throw new PythonException();
if (items.IsNull)
{
throw new PythonException();
}

return items.MoveToPyObject();
}
finally
{
items.Dispose();
}
return new PyObject(items);
}


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8 changes: 5 additions & 3 deletions src/runtime/runtime.cs
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Expand Up @@ -1509,6 +1509,8 @@ internal static IntPtr PyUnicode_FromString(string s)
return PyUnicode_FromUnicode(s, s.Length);
}

internal static string GetManagedString(in BorrowedReference borrowedReference)
=> GetManagedString(borrowedReference.DangerousGetAddress());
/// <summary>
/// Function to access the internal PyUnicode/PyString object and
/// convert it to a managed string with the correct encoding.
Expand Down Expand Up @@ -1591,7 +1593,7 @@ internal static bool PyDict_Check(IntPtr ob)
internal static extern IntPtr PyDict_Values(IntPtr pointer);

[DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
internal static extern IntPtr PyDict_Items(IntPtr pointer);
internal static extern NewReference PyDict_Items(IntPtr pointer);

[DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
internal static extern IntPtr PyDict_Copy(IntPtr pointer);
Expand Down Expand Up @@ -1631,13 +1633,13 @@ internal static IntPtr PyList_New(long size)
[DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
internal static extern IntPtr PyList_AsTuple(IntPtr pointer);

internal static IntPtr PyList_GetItem(IntPtr pointer, long index)
internal static BorrowedReference PyList_GetItem(IntPtr pointer, long index)
{
return PyList_GetItem(pointer, new IntPtr(index));
}

[DllImport(_PythonDll, CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr PyList_GetItem(IntPtr pointer, IntPtr index);
private static extern BorrowedReference PyList_GetItem(IntPtr pointer, IntPtr index);

internal static int PyList_SetItem(IntPtr pointer, long index, IntPtr value)
{
Expand Down