fix(spdx): Avoid running into cycles when computing linkage types

Previously, the function could run into an infinite loop. This has been
spotted from reading the code, but has not been observed when ORT
executes (yet).

Signed-off-by: Frank Viernau <frank.viernau@gmail.com>

Author: fviernau

plugins/reporters/spdx/src/main/kotlin/SpdxDocumentModelMapper.kt

@@ -218,6 +218,7 @@ private fun SpdxPackage.filterChecksums(wantSpdx23: Boolean): SpdxPackage =
 private fun OrtResult.getLinkageTypesForDependencyRelationships():
     Map<Pair<Identifier, Identifier>, Set<PackageLinkage>> {
     val result = mutableMapOf<Pair<Identifier, Identifier>, MutableSet<PackageLinkage>>()
+    val visitedNodeReferenceKeys = mutableSetOf<Any>()
 
     // Traverse all non-excluded edges and collect the linkage types used in between any pair of ids.
     getProjects(omitExcluded = true, includeSubProjects = true).forEach { project ->
@@ -235,6 +236,11 @@ private fun OrtResult.getLinkageTypesForDependencyRelationships():
             while (queue.isNotEmpty()) {
                 val parent = queue.removeFirst()
 
+                when (val key = parent.getInternalId()) {
+                    in visitedNodeReferenceKeys -> continue
+                    else -> visitedNodeReferenceKeys += key
+                }
+
                 val children = parent.visitDependencies { children ->
                     // Map to a list instead of to a sequence, so that the conversion to the stable reference is done
                     // within this code block. After leaving the block, the node reference are not valid anymore.