prevent concurrent modifications of sources by limiting concurrency

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -23,7 +23,6 @@ object ScalafixPlugin extends AutoPlugin {
   override def requires: Plugins = JvmPlugin
 
   object autoImport {
-    @deprecated("not used internally, use your own tag", "0.9.17")
     val Scalafix = Tags.Tag("scalafix")
 
     val ScalafixConfig = config("scalafix")
@@ -207,7 +206,8 @@ object ScalafixPlugin extends AutoPlugin {
     scalafixInterfaceCache := new BlockingCache[
       ToolClasspath,
       ScalafixInterface
-    ]
+    ],
+    concurrentRestrictions += Tags.exclusiveGroup(Scalafix)
   )
 
   override def buildSettings: Seq[Def.Setting[_]] =
@@ -324,8 +324,8 @@ object ScalafixPlugin extends AutoPlugin {
   private def scalafixTask(
       shellArgs: ShellArgs,
       config: ConfigKey
-  ): Def.Initialize[Task[Unit]] =
-    Def.taskDyn {
+  ): Def.Initialize[Task[Unit]] = {
+    val task = Def.taskDyn {
       val errorLogger =
         new PrintStream(
           LoggingOutputStream(
@@ -375,6 +375,9 @@ object ScalafixPlugin extends AutoPlugin {
         }
       }
     }
+    task.tag(Scalafix)
+  }
+
   private def scalafixHelp: Def.Initialize[Task[Unit]] =
     Def.task {
       scalafixInterfaceProvider

src/sbt-test/sbt-scalafix/concurrency/build.sbt

@@ -0,0 +1,33 @@
+val V = _root_.scalafix.sbt.BuildInfo
+
+scalaVersion := V.scala212
+libraryDependencies += "ch.epfl.scala" %% "scalafix-core" % V.scalafixVersion % ScalafixConfig
+
+// make it possible to run scalafix in parallel with other tasks via the `all` command
+// (work around "Cannot mix input tasks with plain tasks/settings")
+val scalafixAddFooter = taskKey[Unit]("task wrapping scalafix")
+
+val addHeader = taskKey[Unit]("")
+
+val check = taskKey[Unit]("")
+
+inConfig(Compile)(
+  Seq(
+    scalafixAddFooter := scalafix.toTask(" --rules=class:fix.AddFooter").value,
+    addHeader := {
+      unmanagedSources.value.foreach { file =>
+        val content = IO.read(file)
+        // artificial delay between read/parsing and write/patching to exercise concurrency
+        Thread.sleep(10000)
+        IO.write(file, s"// HEADER\n$content")
+      }
+    },
+    check := {
+      unmanagedSources.value.foreach { file =>
+        val content = IO.read(file)
+        assert(content.contains("FOOTER"), s"missing footer in\n<<<$content>>>")
+        assert(content.contains("HEADER"), s"missing header in\n<<<$content>>>")
+      }
+    }
+  )
+)

src/sbt-test/sbt-scalafix/concurrency/project/plugins.sbt

@@ -0,0 +1,2 @@
+resolvers += Resolver.sonatypeRepo("public")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % sys.props("plugin.version"))

src/sbt-test/sbt-scalafix/concurrency/src/main/scala/example/Example.scala

@@ -0,0 +1 @@
+object Example

src/sbt-test/sbt-scalafix/concurrency/src/scalafix/scala/fix/AddFooter.scala

@@ -0,0 +1,11 @@
+package fix
+
+import scalafix.v1._
+
+class AddFooter extends SyntacticRule("AddFooter") {
+  override def fix(implicit doc: SyntacticDocument): Patch = {
+    // artificial delay between read/parsing and write/patching to exercise concurrency
+    Thread.sleep(1000)
+    Patch.addRight(doc.tree, "\n// FOOTER\n")
+  }
+}

src/sbt-test/sbt-scalafix/concurrency/test

@@ -0,0 +1,5 @@
+# warm up so that tasks themselves (not their dependencies) run in parallel
+> scalafix:compile
+
+> all addHeader scalafixAddFooter
+> check