Track double writes (prepare for literal snapshots).

selfie-lib/src/commonMain/kotlin/com/diffplug/selfie/SnapshotFile.kt

@@ -127,9 +127,6 @@ class SnapshotFile {
 
   var wasSetAtTestTime: Boolean = false
   fun setAtTestTime(key: String, snapshot: Snapshot) {
-    // TODO: track whenever a snapshot is set, so that we can:
-    //  - warn about duplicate snapshots when they are equal
-    //  - give good errors when they are not
     val newSnapshots = snapshots.plusOrReplace(key, snapshot)
     if (newSnapshots !== snapshots) {
       snapshots = newSnapshots

selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/RW.kt

@@ -21,34 +21,39 @@ package com.diffplug.selfie
  * - if environment variable or system property named `ci` or `CI` with value `true` or `TRUE`
  *     - then Selfie is read-only and errors out on a snapshot mismatch
  * - if environment variable or system property named `selfie` or `SELFIE`
- *     - its value should be either `read` or `write` (case-insensitive)
- *     - that will override the presence of `CI`
+ *     - its value should be either `read`, `write`, or `writeonce` (case-insensitive)
+ *     - `write` allows a single snapshot to be set multiple times within a test, so long as it is
+ *       the same value. `writeonce` errors as soon as a snapshot is set twice even to the same
+ *       value.
+ *     - selfie, if set, will override the presence of `CI`
  */
-internal object RW {
-  private fun lowercaseFromEnvOrSys(key: String): String? {
-    val env = System.getenv(key)?.lowercase()
-    if (!env.isNullOrEmpty()) {
-      return env
-    }
-    val system = System.getProperty(key)?.lowercase()
-    if (!system.isNullOrEmpty()) {
-      return system
+internal enum class RW {
+  read,
+  write,
+  writeonce;
+
+  companion object {
+    private fun lowercaseFromEnvOrSys(key: String): String? {
+      val env = System.getenv(key)?.lowercase()
+      if (!env.isNullOrEmpty()) {
+        return env
+      }
+      val system = System.getProperty(key)?.lowercase()
+      if (!system.isNullOrEmpty()) {
+        return system
+      }
+      return null
     }
-    return null
-  }
-  private fun calcIsWrite(): Boolean {
-    val override = lowercaseFromEnvOrSys("selfie") ?: lowercaseFromEnvOrSys("SELFIE")
-    if (override != null) {
-      return when (override) {
-        "read" -> false
-        "write" -> true
-        else ->
-            throw IllegalArgumentException(
-                "Expected 'selfie' to be 'read' or 'write', but was '$override'")
+    private fun calcRW(): RW {
+      val override = lowercaseFromEnvOrSys("selfie") ?: lowercaseFromEnvOrSys("SELFIE")
+      if (override != null) {
+        return RW.valueOf(override)
       }
+      val ci = lowercaseFromEnvOrSys("ci") ?: lowercaseFromEnvOrSys("CI")
+      return if (ci == "true") read else write
     }
-    val ci = lowercaseFromEnvOrSys("ci") ?: lowercaseFromEnvOrSys("CI")
-    return ci != "true"
+    val mode = calcRW()
+    val isWrite = mode != read
+    val isWriteOnce = mode == writeonce
   }
-  val isWrite = calcIsWrite()
 }

selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieTestExecutionListener.kt

@@ -93,6 +93,7 @@ internal class ClassProgress(val className: String) {
 
   private var file: SnapshotFile? = null
   private var methods = ArrayMap.empty<String, MethodSnapshotGC>()
+  private var diskWriteTracker: DiskWriteTracker? = DiskWriteTracker()
   // the methods below called by the TestExecutionListener on its runtime thread
   @Synchronized fun startMethod(method: String) {
     assertNotTerminated()
@@ -132,6 +133,7 @@ internal class ClassProgress(val className: String) {
     }
     // now that we are done, allow our contents to be GC'ed
     methods = TERMINATED
+    diskWriteTracker = null
     file = null
   }
   // the methods below are called from the test thread for I/O on snapshots
@@ -145,8 +147,10 @@ internal class ClassProgress(val className: String) {
   }
   @Synchronized fun write(method: String, suffix: String, snapshot: Snapshot) {
     assertNotTerminated()
+    val key = "$method$suffix"
+    diskWriteTracker!!.record(key, snapshot, recordCall())
     methods[method]!!.keepSuffix(suffix)
-    read().setAtTestTime("$method$suffix", snapshot)
+    read().setAtTestTime(key, snapshot)
   }
   @Synchronized fun read(
       method: String,

selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/WriteTracker.kt

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2023 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.selfie.junit5
+
+import com.diffplug.selfie.RW
+import com.diffplug.selfie.Snapshot
+import java.util.stream.Collectors
+
+/** Represents the line at which user code called into Selfie. */
+data class CallLocation(val subpath: String, val line: Int) : Comparable<CallLocation> {
+  override fun compareTo(other: CallLocation): Int {
+    val subpathCompare = subpath.compareTo(other.subpath)
+    return if (subpathCompare != 0) subpathCompare else line.compareTo(other.line)
+  }
+  override fun toString(): String = "$subpath:$line"
+}
+/** Represents the callstack above a given CallLocation. */
+class CallStack(val location: CallLocation, val restOfStack: List<CallLocation>)
+/** Generates a CallLocation and the CallStack behind it. */
+fun recordCall(): CallStack {
+  val calls =
+      StackWalker.getInstance().walk { frames ->
+        frames
+            .skip(1)
+            .map { CallLocation(it.className.replace('.', '/') + ".kt", it.lineNumber) }
+            .collect(Collectors.toList())
+      }
+  return CallStack(calls.removeAt(0), calls)
+}
+/** The first write at a given spot. */
+internal class FirstWrite<T>(val snapshot: T, val callStack: CallStack)
+
+/** For tracking the writes of disk snapshots literals. */
+internal open class WriteTracker<K : Comparable<K>, V> {
+  val writes = mutableMapOf<K, FirstWrite<V>>()
+  protected fun recordInternal(key: K, snapshot: V, call: CallStack) {
+    val existing = writes.putIfAbsent(key, FirstWrite(snapshot, call))
+    if (existing != null) {
+      if (existing.snapshot != snapshot) {
+        throw org.opentest4j.AssertionFailedError(
+            "Snapshot was set to multiple values:\nfirst time:${existing.callStack}\n\nthis time:${call}",
+            existing.snapshot,
+            snapshot)
+      } else if (RW.isWriteOnce) {
+        throw org.opentest4j.AssertionFailedError(
+            "Snapshot was set to the same value multiple times.", existing.callStack, call)
+      }
+    }
+  }
+}
+
+internal class DiskWriteTracker : WriteTracker<String, Snapshot>() {
+  fun record(key: String, snapshot: Snapshot, call: CallStack) {
+    recordInternal(key, snapshot, call)
+  }
+}
+
+class LiteralValue {
+  // TODO: String, Int, Long, Boolean, etc
+}
+
+internal class InlineWriteTracker : WriteTracker<CallLocation, LiteralValue>() {
+  fun record(call: CallStack, snapshot: LiteralValue) {
+    recordInternal(call.location, snapshot, call)
+  }
+}

selfie-runner-junit5/src/test/kotlin/com/diffplug/selfie/junit5/DuplicateWriteTest.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2023 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.selfie.junit5
+
+import io.kotest.matchers.shouldBe
+import kotlin.test.Test
+import org.junit.jupiter.api.MethodOrderer
+import org.junit.jupiter.api.Order
+import org.junit.jupiter.api.TestMethodOrder
+import org.junitpioneer.jupiter.DisableIfTestFails
+
+/** Simplest test for verifying read/write of a snapshot. */
+@TestMethodOrder(MethodOrderer.OrderAnnotation::class)
+@DisableIfTestFails
+class DuplicateWriteTest : Harness("undertest-junit5") {
+  @Test @Order(1)
+  fun noSelfie() {
+    ut_snapshot().deleteIfExists()
+    ut_snapshot().assertDoesNotExist()
+  }
+
+  @Test @Order(2)
+  fun cannot_write_multiple_things_to_one_snapshot() {
+    ut_mirror().linesFrom("fun shouldFail()").toFirst("}").uncomment()
+    ut_mirror().linesFrom("fun shouldPass()").toFirst("}").commentOut()
+    gradlew("underTest", "-Pselfie=write")?.printStackTrace()
+  }
+
+  @Test @Order(3)
+  fun can_write_one_thing_multiple_times_to_one_snapshot() {
+    ut_mirror().linesFrom("fun shouldFail()").toFirst("}").commentOut()
+    ut_mirror().linesFrom("fun shouldPass()").toFirst("}").uncomment()
+    gradlew("underTest", "-Pselfie=write") shouldBe null
+  }
+
+  @Test @Order(4)
+  fun can_read_one_thing_multiple_times_from_one_snapshot() {
+    ut_mirror().linesFrom("fun shouldFail()").toFirst("}").commentOut()
+    ut_mirror().linesFrom("fun shouldPass()").toFirst("}").uncomment()
+    gradlew("underTest", "-Pselfie=read") shouldBe null
+  }
+
+  @Test @Order(5)
+  fun writeonce_mode() {
+    ut_mirror().linesFrom("fun shouldFail()").toFirst("}").commentOut()
+    ut_mirror().linesFrom("fun shouldPass()").toFirst("}").uncomment()
+    gradlew("underTest", "-Pselfie=writeonce")?.printStackTrace()
+  }
+
+  @Test @Order(6)
+  fun deleteSelfie() {
+    ut_snapshot().deleteIfExists()
+  }
+}

selfie-runner-junit5/src/test/kotlin/testpkg/RecordCallTest.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2023 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package testpkg
+
+import com.diffplug.selfie.junit5.recordCall
+import io.kotest.matchers.ints.shouldBeGreaterThan
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+
+class RecordCallTest {
+  @Test
+  fun testRecordCall() {
+    val stack = recordCall()
+    stack.location.toString() shouldBe "testpkg/RecordCallTest.kt:26"
+    stack.restOfStack.size shouldBeGreaterThan 0
+  }
+}

undertest-junit5/src/test/kotlin/undertest/junit5/UT_DuplicateWriteTest.kt

@@ -0,0 +1,15 @@
+package undertest.junit5
+
+import com.diffplug.selfie.expectSelfie
+import org.junit.jupiter.api.Test
+
+class UT_DuplicateWriteTest {
+//  @Test fun shouldFail() {
+//    expectSelfie("apples").toMatchDisk()
+//    expectSelfie("oranges").toMatchDisk()
+//  }
+  @Test fun shouldPass() {
+    expectSelfie("twins").toMatchDisk()
+    expectSelfie("twins").toMatchDisk()
+  }
+}