[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
 
+using System;
+
 namespace Java.Interop {
 
 	partial class JniPeerMembers {
@@ -12,7 +14,9 @@ public bool GetBooleanValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetBooleanField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetBooleanField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, bool value)
@@ -21,6 +25,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, bool value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetBooleanField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public sbyte GetSByteValue (
@@ -30,7 +35,9 @@ public sbyte GetSByteValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetByteField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetByteField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, sbyte value)
@@ -39,6 +46,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, sbyte value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetByteField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public char GetCharValue (
@@ -48,7 +56,9 @@ public char GetCharValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetCharField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetCharField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, char value)
@@ -57,6 +67,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, char value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetCharField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public short GetInt16Value (
@@ -66,7 +77,9 @@ public short GetInt16Value (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetShortField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetShortField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, short value)
@@ -75,6 +88,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, short value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetShortField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public int GetInt32Value (
@@ -84,7 +98,9 @@ public int GetInt32Value (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetIntField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetIntField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, int value)
@@ -93,6 +109,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, int value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetIntField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public long GetInt64Value (
@@ -102,7 +119,9 @@ public long GetInt64Value (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetLongField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetLongField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, long value)
@@ -111,6 +130,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, long value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetLongField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public float GetSingleValue (
@@ -120,7 +140,9 @@ public float GetSingleValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetFloatField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetFloatField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, float value)
@@ -129,6 +151,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, float value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetFloatField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public double GetDoubleValue (
@@ -138,7 +161,9 @@ public double GetDoubleValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetDoubleField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetDoubleField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, double value)
@@ -147,6 +172,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, double value)
 
 			var f  = GetFieldInfo (encodedMember);
 			JniEnvironment.InstanceFields.SetDoubleField (self.PeerReference, f, value);
+			GC.KeepAlive (self);
 		}
 
 		public JniObjectReference GetObjectValue (
@@ -156,7 +182,9 @@ public JniObjectReference GetObjectValue (
 			JniPeerMembers.AssertSelf (self);
 
 			var f   = GetFieldInfo (encodedMember);
-			return JniEnvironment.InstanceFields.GetObjectField (self.PeerReference, f);
+			var r   = JniEnvironment.InstanceFields.GetObjectField (self.PeerReference, f);
+			GC.KeepAlive (self);
+			return r;
 		}
 
 		public void SetValue (string encodedMember, IJavaPeerable self, JniObjectReference value)
@@ -165,6 +193,7 @@ public void SetValue (string encodedMember, IJavaPeerable self, JniObjectReferen
 
 			var f  = 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);
 		}
 <#
 	}