[Gradle] Implement IdeaKotlinClasspath interner

This very simple interner mechanism ensures that when import runs
within the IDE process, we de-duplicate existing file instances.

This might have an effect, because during GradleProjectResolution
we could have plenty source sets that refer to the same
files.

^KT-55492 Verification Pending

libraries/tools/kotlin-gradle-plugin-idea/src/main/kotlin/org/jetbrains/kotlin/gradle/idea/tcs/IdeaKotlinClasspath.kt

@@ -7,6 +7,8 @@ package org.jetbrains.kotlin.gradle.idea.tcs
 
 import java.io.File
 import java.io.Serializable
+import java.lang.ref.WeakReference
+import java.util.*
 
 fun IdeaKotlinClasspath(files: Iterable<File>): IdeaKotlinClasspath = IdeaKotlinClasspath.from(files)
 
@@ -76,7 +78,8 @@ class IdeaKotlinClasspath private constructor(private val files: MutableSet<File
         private const val serialVersionUID = 0L
 
         fun normalise(file: File): File {
-            return file.absoluteFile.normalize()
+            val normalized = file.absoluteFile.normalize()
+            return intern(normalized)
         }
 
         fun from(files: Iterable<File>): IdeaKotlinClasspath {
@@ -88,5 +91,14 @@ class IdeaKotlinClasspath private constructor(private val files: MutableSet<File
         }
 
         fun empty() = IdeaKotlinClasspath(mutableSetOf())
+
+        private val interner = WeakHashMap<File, WeakReference<File>>()
+
+        private fun intern(file: File): File {
+            return interner.getOrPut(file) { WeakReference(file) }.get() ?: run {
+                interner[file] = WeakReference(file)
+                file
+            }
+        }
     }
 }

libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/idea/test/tcs/IdeaKotlinClasspathTest.kt

@@ -7,10 +7,7 @@ package org.jetbrains.kotlin.gradle.idea.test.tcs
 
 import org.jetbrains.kotlin.gradle.idea.tcs.IdeaKotlinClasspath
 import java.io.File
-import kotlin.test.Test
-import kotlin.test.assertEquals
-import kotlin.test.assertNotEquals
-import kotlin.test.assertTrue
+import kotlin.test.*
 
 class IdeaKotlinClasspathTest {
 
@@ -62,4 +59,19 @@ class IdeaKotlinClasspathTest {
         classpath.remove(File("test").absoluteFile)
         assertTrue(classpath.isEmpty())
     }
+
+    @Test
+    fun `test - classpath interner`() {
+        val classpath1 = IdeaKotlinClasspath()
+        val classpath2 = IdeaKotlinClasspath()
+
+        val fileAInstance1 = File("a")
+        val fileAInstance2 = File("a")
+
+        classpath1.add(fileAInstance1)
+        classpath2.add(fileAInstance2)
+
+        /* Check that fileAInstance2 got interned and will re-use instance 1 */
+        assertSame(classpath1.single(), classpath2.single())
+    }
 }