Tweak Maven build gen tests (#4079)

I had to update the test data of `mill.main.maven.BuildGenTests` for
#4067. The output of those tests
didn't really help.

This PR changes the helper method checking if generated files match test
data in resources, so that it prints more helpful output (removed /
added files, diff of modified files), and allows to update the test data
on disk by setting `BuildGenTests.updateSnapshots` to true.

Author: alexarchambault

Diff for: main/maven/test/src/mill/main/maven/BuildGenTests.scala

@@ -2,13 +2,19 @@ package mill.main.maven
 
 import mill.T
 import mill.api.PathRef
+import mill.main.client.OutFiles
 import mill.scalalib.scalafmt.ScalafmtModule
 import mill.testkit.{TestBaseModule, UnitTester}
 import utest.*
 import utest.framework.TestPath
 
+import java.nio.file.FileSystems
+
 object BuildGenTests extends TestSuite {
 
+  // Change this to true to update test data on disk
+  def updateSnapshots = false
+
   def tests: Tests = Tests {
     val resources = os.Path(sys.env("MILL_TEST_RESOURCE_DIR"))
     val scalafmtConfigFile = PathRef(resources / ".scalafmt.conf")
@@ -33,7 +39,7 @@ object BuildGenTests extends TestSuite {
       eval(module.reformat())
 
       // test
-      checkFiles(files, resources / expectedRel)
+      checkFiles(files.map(_.path.relativeTo(dest).asSubPath), dest, resources / expectedRel)
     }
 
     // multi level nested modules
@@ -125,15 +131,38 @@ object BuildGenTests extends TestSuite {
       .map(PathRef(_))
       .toSeq
 
-  def checkFiles(actualFiles: Seq[PathRef], expectedRoot: os.Path): Boolean = {
-    val expectedFiles = buildFiles(expectedRoot)
+  def checkFiles(actualFiles: Seq[os.SubPath], root: os.Path, expectedRoot: os.Path): Boolean = {
+    // Non *.mill files, that are not in test data, that we don't want
+    // to see in the diff
+    val toCleanUp = os.walk(root, skip = _.startsWith(root / OutFiles.defaultOut))
+      .filter(os.isFile)
+      .filter(!_.lastOpt.exists(_.endsWith(".mill")))
+    toCleanUp.foreach(os.remove)
+
+    // Try to normalize permissions while not touching those of committed test data
+    val supportsPerms = FileSystems.getDefault.supportedFileAttributeViews().contains("posix")
+    if (supportsPerms)
+      for {
+        testFile <- os.walk(expectedRoot)
+        if os.isFile(testFile)
+        targetFile = root / testFile.relativeTo(expectedRoot).asSubPath
+        if os.isFile(targetFile)
+      }
+        os.perms.set(targetFile, os.perms(testFile))
+
+    val diffExitCode = os.proc("git", "diff", "--no-index", expectedRoot, root)
+      .call(stdin = os.Inherit, stdout = os.Inherit, check = !updateSnapshots)
+      .exitCode
 
-    actualFiles.nonEmpty &&
-    actualFiles.length == expectedFiles.length &&
-    actualFiles.iterator.zip(expectedFiles.iterator).forall {
-      case (actual, expected) =>
-        actual.path.endsWith(expected.path.relativeTo(expectedRoot)) &&
-        os.read.lines(actual.path) == os.read.lines(expected.path)
+    if (updateSnapshots && diffExitCode != 0) {
+      System.err.println(
+        s"Expected and actual files differ, updating expected files in resources under $expectedRoot"
+      )
+
+      os.remove.all(expectedRoot)
+      os.copy(root, expectedRoot)
     }
+
+    diffExitCode == 0 || updateSnapshots
   }
 }