[Java.Interop] Prevent premature collection w/ JniInstance* (#768)

Context: dotnet/android@e88cfbc
While writing the commit message for xamarin/xamarin-android/@e88cfbcf,
it occurred to me that the same fundamental scenario of:
CallIntoJava (new JavaLangObjectSubclass ().Handle);
// GC collects instance after `.Handle`, before `CallIntoJava()`
could apply to `JniPeerMembers.JniInstanceMethods.Invoke*()`
invocations:
JniArgumentValue* __args = …;
_members.InstanceMethods.InvokeVirtualObjectMethod (__id, this, __args);
// What prevents `this` from being collected "too soon"?
Address this: update `JniPeerMembers.JniInstanceMethods.Invoke*()`
so that there is a `GC.KeepAlive(self)` after accessing
`self.PeerReference`. This will ensure that `self` isn't collected
"during" `JniEnvironment.InstanceMethods.Call*Method()` invocations.
Likewise update `JniPeerMembers.JniInstanceFields.Get*Value()` and
`JniPeerMembers.JniInstanceFields.Set*Value()` so that there is a
`GC.KeepAlive(self)` after the `JniEnvironment.InstanceFields.*`
invocation.

Author: jonpryor

src/Java.Interop/Java.Interop/JniPeerMembers.JniFields.cs

@@ -1,5 +1,7 @@
 #nullable enable
 
+usingSystem;
+
 namespaceJava.Interop{
 
 partialclassJniPeerMembers{
@@ -12,7 +14,9 @@ public bool GetBooleanValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetBooleanField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetBooleanField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,boolvalue)
@@ -21,6 +25,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, bool value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetBooleanField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicsbyteGetSByteValue(
@@ -30,7 +35,9 @@ public sbyte GetSByteValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetByteField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetByteField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,sbytevalue)
@@ -39,6 +46,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, sbyte value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetByteField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publiccharGetCharValue(
@@ -48,7 +56,9 @@ public char GetCharValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetCharField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetCharField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,charvalue)
@@ -57,6 +67,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, char value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetCharField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicshortGetInt16Value(
@@ -66,7 +77,9 @@ public short GetInt16Value (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetShortField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetShortField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,shortvalue)
@@ -75,6 +88,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, short value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetShortField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicintGetInt32Value(
@@ -84,7 +98,9 @@ public int GetInt32Value (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetIntField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetIntField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,intvalue)
@@ -93,6 +109,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, int value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetIntField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publiclongGetInt64Value(
@@ -102,7 +119,9 @@ public long GetInt64Value (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetLongField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetLongField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,longvalue)
@@ -111,6 +130,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, long value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetLongField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicfloatGetSingleValue(
@@ -120,7 +140,9 @@ public float GetSingleValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetFloatField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetFloatField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,floatvalue)
@@ -129,6 +151,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, float value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetFloatField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicdoubleGetDoubleValue(
@@ -138,7 +161,9 @@ public double GetDoubleValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetDoubleField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetDoubleField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,doublevalue)
@@ -147,6 +172,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, double value)
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetDoubleField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 
 publicJniObjectReferenceGetObjectValue(
@@ -156,7 +182,9 @@ public JniObjectReference GetObjectValue (
 JniPeerMembers.AssertSelf(self);
 
 varf=GetFieldInfo(encodedMember);
-returnJniEnvironment.InstanceFields.GetObjectField(self.PeerReference,f);
+varr=JniEnvironment.InstanceFields.GetObjectField(self.PeerReference,f);
+GC.KeepAlive(self);
+returnr;
 }
 
 publicvoidSetValue(stringencodedMember,IJavaPeerableself,JniObjectReferencevalue)
@@ -165,6 +193,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, JniObjectReferen
 
 varf=GetFieldInfo(encodedMember);
 JniEnvironment.InstanceFields.SetObjectField(self.PeerReference,f,value);
+GC.KeepAlive(self);
 }
 }
 

src/Java.Interop/Java.Interop/JniPeerMembers.JniFields.tt

@@ -19,6 +19,8 @@
 #>
 #nullable enable
 
+using System;
+
 namespace Java.Interop {
 
 	partial class JniPeerMembers {
@@ -34,7 +36,9 @@ namespace Java.Interop {
  JniPeerMembers.AssertSelf (self);
 
  var f = GetFieldInfo (encodedMember);
- return JniEnvironment.InstanceFields.Get<#= info.JniCallType #>Field (self.PeerReference, f);
+ var r = JniEnvironment.InstanceFields.Get<#= info.JniCallType #>Field (self.PeerReference, f);
+ GC.KeepAlive (self);
+ return r;
  }
 
  public void SetValue (string encodedMember, IJavaPeerable self, <#= info.ParameterType #> value)
@@ -43,6 +47,7 @@ namespace Java.Interop {
 
  var f = GetFieldInfo (encodedMember);
  JniEnvironment.InstanceFields.Set<#= info.JniCallType #>Field (self.PeerReference, f, value);
+ GC.KeepAlive (self);
  }
 <#
 	}