[Repo Assist] Fix #1419: Add cycle guard for XSD group references to prevent StackOverflow (#1626)

* Update test/build dependencies: FAKE 6.1.4, NUnit 3.13.3, FsUnit 4.2.0, FsCheck 2.16.6

- FAKE packages: 6.1.3 → 6.1.4 (patch)
- NUnit: 3.13.1 → 3.13.3 (patch)
- FsUnit: 4.0.4 → 4.2.0 (minor)
- FsCheck: 2.15.1 → 2.16.6 (minor)

Build: passes (0 errors)
Tests: all offline tests pass; network tests skip due to sandbox

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix #1419: Add cycle guard for XSD group references in XsdInference

Add a VisitingGroups HashSet to ParsingContext and guard the GroupRef
case in parseParticle. When a group is already being visited, return
Empty (an empty particle) instead of recursing infinitely.

This prevents a StackOverflowException when parsing XSD schemas that
contain groups forming cycles (e.g. group A contains group ref B,
and group B's resolved content eventually leads back to group A via
the compiled XmlSchema object graph).

Add a test verifying that a recursive content model schema with a
group referencing itself through element content completes without error.

Closes #1419

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Repo Assist <repo-assist@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: 4 people

src/FSharp.Data.Xml.Core/XsdInference.fs

@@ -104,9 +104,12 @@ module XsdParsing =
         let elements =
             System.Collections.Generic.Dictionary<XmlSchemaElement, XsdModel.XsdElement>()
 
+        let visitingGroups = System.Collections.Generic.HashSet<XmlQualifiedName>()
+
         member x.GetElement name = getElm name
         member x.GetSubstitutions elm = getSubst elm
         member x.Elements = elements
+        member x.VisitingGroups = visitingGroups
 
 
     open XsdModel
@@ -183,7 +186,17 @@ module XsdParsing =
         match par with
         | :? XmlSchemaAny -> Any occurs
         | :? XmlSchemaGroupBase as grp -> parseParticles grp
-        | :? XmlSchemaGroupRef as grpRef -> parseParticle ctx grpRef.Particle
+        | :? XmlSchemaGroupRef as grpRef ->
+            // Guard against circular group references (e.g. group A → group B → group A).
+            // XSD allows groups to reference each other transitively; without a cycle guard
+            // this causes unbounded recursion and a StackOverflowException.
+            if ctx.VisitingGroups.Contains grpRef.RefName then
+                Empty // break the cycle
+            else
+                ctx.VisitingGroups.Add grpRef.RefName |> ignore
+                let result = parseParticle ctx grpRef.Particle
+                ctx.VisitingGroups.Remove grpRef.RefName |> ignore
+                result
         | :? XmlSchemaElement as elm ->
             let e =
                 if elm.RefName.IsEmpty then

tests/FSharp.Data.DesignTime.Tests/InferenceTests.fs

@@ -934,3 +934,35 @@ let ``abstract elements can be recursive``() =
     getInferedTypes formulaXsd [| sample1; sample2 |]
     |> ignore
 
+[<Test>]
+let ``circular group references do not cause a stack overflow``() =
+
+    // Schema with a group that (indirectly) creates a recursive content model:
+    // RecursiveGroup contains element "children" whose anonymous type references RecursiveGroup.
+    // Without a GroupRef cycle guard, this can cause unbounded recursion in parseParticle
+    // on .NET runtimes where the compiled XmlSchema object graph contains shared group particle references.
+    let xsd =
+        """
+        <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
+                   elementFormDefault="qualified">
+            <xs:group name="RecursiveGroup">
+                <xs:sequence>
+                    <xs:element name="value" type="xs:string" minOccurs="0"/>
+                    <xs:element name="children" minOccurs="0">
+                        <xs:complexType>
+                            <xs:group ref="RecursiveGroup" minOccurs="0" maxOccurs="unbounded"/>
+                        </xs:complexType>
+                    </xs:element>
+                </xs:sequence>
+            </xs:group>
+            <xs:element name="Root">
+                <xs:complexType>
+                    <xs:group ref="RecursiveGroup"/>
+                </xs:complexType>
+            </xs:element>
+        </xs:schema>
+        """
+
+    // Must complete without StackOverflowException
+    getInferedTypeFromSchema xsd |> ignore
+