[2.x] dependencyTree: add `--append` for file output in multi-module builds (#8878)

**Summary**
This adds an --append flag to the dependencyTree input task when used with --out. It allows running dependencyTree in multiple subprojects and appending their outputs to the same target file instead of overwriting.

**Solution**
- Add Arg.Append in the CLI parsing
- When --append is set, write using IO.write(..., append = true)

main/src/main/scala/sbt/plugins/DependencyTreeSettings.scala

@@ -36,6 +36,7 @@ private[sbt] object DependencyTreeSettings:
     case Quiet
     case Format(format: Fmt)
     case Out(out: String)
+    case Append
     case Browse
 
   enum Fmt:
@@ -67,6 +68,7 @@ private[sbt] object DependencyTreeSettings:
 
   lazy val ArgOptionParser: Parser[Arg] =
     Space ~> (("--quiet" ^^^ Arg.Quiet)
+      | ("--append" ^^^ Arg.Append)
       | ("--browse" ^^^ Arg.Browse)
       | ("--out" ~> Space ~> StringBasic)
         .map(Arg.Out(_))
@@ -97,6 +99,7 @@ SUBCOMMAND
 
 OPTIONS
   --quiet      Returns the output as task value, replacing asString
+  --append     Append to the output file when used with --out
   --out <file> Writes the output to the specified file;
                The file extension will influence the default subcommand
   --browse     Opens the browser when combined with graph or html subcommand
@@ -164,6 +167,7 @@ OPTIONS
         val args = ArgsParser.parsed.toList
         val isHelp = args.contains(Arg.Help)
         val isQuiet = args.contains(Arg.Quiet)
+        val isAppend = args.contains(Arg.Append)
         val isBrowse = args.contains(Arg.Browse)
         if isHelp then Def.task { s.log.info(usageText); "" }
         else
@@ -195,19 +199,19 @@ OPTIONS
                   case Fmt.List  => rendering.FlatList.render(_.id.idString)(graph)
                   case Fmt.Stats => rendering.Statistics.renderModuleStatsList(graph)
                   case _         => rendering.AsciiTree.asciiTree(graph, asciiGraphWidth.value)
-                handleOutput(output, outFileOpt, isQuiet, s.log)
+                handleOutput(output, outFileOpt, isQuiet, isAppend, s.log)
               }
             case Fmt.Json =>
               Def.task {
                 val graph = dependencyTreeModuleGraph0.value
                 val output = TreeView.createJson(graph)
-                handleOutput(output, outFileOpt, isQuiet, s.log)
+                handleOutput(output, outFileOpt, isQuiet, isAppend, s.log)
               }
             case Fmt.Xml =>
               Def.task {
                 val graph = dependencyTreeModuleGraph0.value
                 val output = rendering.GraphML.graphMLAsString(graph)
-                handleOutput(output, outFileOpt, isQuiet, s.log)
+                handleOutput(output, outFileOpt, isQuiet, isAppend, s.log)
               }
             case Fmt.Html =>
               Def.task {
@@ -227,7 +231,8 @@ OPTIONS
                   rendering.DOT.HTMLLabelRendering.AngleBrackets,
                   dependencyDotNodeColors.value
                 )
-                if format == Fmt.Graph then handleOutput(output, outFileOpt, isQuiet, s.log)
+                if format == Fmt.Graph then
+                  handleOutput(output, outFileOpt, isQuiet, isAppend, s.log)
                 else
                   val outputFile = DagreHTML.createFile(output, targetDir)
                   if isBrowse then openBrowser(outputFile.toURI)
@@ -281,7 +286,7 @@ OPTIONS
             val output = format match
               case Fmt.Json => rendering.LicenseInfo.renderJson(graph)
               case _        => rendering.LicenseInfo.render(graph)
-            handleOutput(output, outFileOpt, isQuiet, s.log)
+            handleOutput(output, outFileOpt, isQuiet, appendToFile = false, s.log)
           }
       }).evaluated,
     )
@@ -306,11 +311,15 @@ OPTIONS
       content: String,
       outputFileOpt: Option[File],
       isQuiet: Boolean,
+      appendToFile: Boolean,
       log: Logger,
   ): String =
     outputFileOpt match
       case Some(output) =>
-        IO.write(output, content, IO.utf8)
+        val toWrite =
+          if appendToFile && output.exists() && output.length() > 0 then "\n" + content
+          else content
+        IO.write(output, toWrite, IO.utf8, append = appendToFile)
         if !isQuiet then log.info(s"wrote dependencies to $output")
         output.toString
       case None =>

main/src/test/scala/sbt/plugins/DependencyTreeTest.scala

@@ -27,6 +27,7 @@ object DependencyTreeTest extends verify.BasicTestSuite:
     assert(parseArgs(List("help")) == List(Arg.Help))
     assert(parseArgs(List("--help")) == List(Arg.Help))
     assert(parseArgs(List("--quiet")) == List(Arg.Quiet))
+    assert(parseArgs(List("--append")) == List(Arg.Append))
     assert(parseArgs(List("tree")) == List(Arg.Format(Fmt.Tree)))
     assert(parseArgs(List("--out", "/tmp/deps.txt")) == List(Arg.Out("/tmp/deps.txt")))
     assert(parseArgs(List("--browse")) == List(Arg.Browse))

sbt-app/src/sbt-test/dependency-graph/append-output/a/src/main/scala/A.scala

@@ -0,0 +1,2 @@
+object A:
+  def a: Int = 1

sbt-app/src/sbt-test/dependency-graph/append-output/b/src/main/scala/B.scala

@@ -0,0 +1,2 @@
+object B:
+  def b: Int = 2

sbt-app/src/sbt-test/dependency-graph/append-output/build.sbt

@@ -0,0 +1,26 @@
+scalaVersion := "2.12.21"
+organization := "org.example"
+version := "0.1"
+
+lazy val root = (project in file("."))
+  .aggregate(a, b)
+  .settings(
+    publish / skip := true,
+  )
+
+lazy val a = (project in file("a")).settings(
+  name := "a",
+  libraryDependencies += "org.typelevel" %% "cats-core" % "2.10.0",
+)
+
+lazy val b = (project in file("b")).settings(
+  name := "b",
+  libraryDependencies += "com.lihaoyi" %% "pprint" % "0.9.0",
+)
+
+TaskKey[Unit]("check") := {
+  val f = new File("target/tree.txt")
+  val content = IO.read(f)
+  assert(content.contains("org.typelevel:cats-core_2.12:2.10.0"), s"Missing cats-core in output:\n$content")
+  assert(content.contains("com.lihaoyi:pprint_2.12:0.9.0"), s"Missing pprint in output:\n$content")
+}

sbt-app/src/sbt-test/dependency-graph/append-output/test

@@ -0,0 +1,3 @@
+> a/dependencyTree --out target/tree.txt
+> b/dependencyTree --out target/tree.txt --append
+> check