From 2cb7973162bd6b431936e56cfb3422564b6047ab Mon Sep 17 00:00:00 2001
From: Urs Fleisch <ufleisch@users.sourceforge.net>
Date: Wed, 29 Dec 2021 06:23:46 +0100
Subject: [PATCH] Remove MP4 meta atom when empty tag is saved

Currently, MP4 tags can only grow. If items are removed, they are
just replaced by padding in the form of "free" atoms. This change
will remove the whole "meta" atom when an MP4 tag without items
is saved. This will make it possible, to bring the file back to
its pristine state without metadata.
---
 taglib/mp4/mp4tag.cpp | 53 ++++++++++++++++++++++++++++++++-----------
 tests/test_mp4.cpp    | 34 +++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 13 deletions(-)

diff --git a/taglib/mp4/mp4tag.cpp b/taglib/mp4/mp4tag.cpp
index b821a440..a1e8b5ea 100644
--- a/taglib/mp4/mp4tag.cpp
+++ b/taglib/mp4/mp4tag.cpp
@@ -528,7 +528,11 @@ MP4::Tag::save()
       debug("MP4: Unknown item name \"" + name + "\"");
     }
   }
-  data = renderAtom("ilst", data);
+  // Leave data empty if there are no items. This will ensure that no meta atom
+  // is saved.
+  if (!data.isEmpty()) {
+    data = renderAtom("ilst", data);
+  }
 
   AtomList path = d->atoms->path("moov", "udta", "meta", "ilst");
   if(path.size() == 4) {
@@ -643,6 +647,9 @@ MP4::Tag::updateOffsets(long delta, long offset)
 void
 MP4::Tag::saveNew(ByteVector data)
 {
+  if(data.isEmpty())
+    return;
+
   data = renderAtom("meta", ByteVector(4, '\0') +
                     renderAtom("hdlr", ByteVector(8, '\0') + ByteVector("mdirappl") +
                                ByteVector(9, '\0')) +
@@ -699,20 +706,40 @@ MP4::Tag::saveExisting(ByteVector data, const AtomList &path)
   }
 
   long delta = data.size() - length;
-  if(delta > 0 || (delta < 0 && delta > -8)) {
-    data.append(padIlst(data));
-    delta = data.size() - length;
-  }
-  else if(delta < 0) {
-    data.append(padIlst(data, -delta - 8));
-    delta = 0;
-  }
+  if(!data.isEmpty()) {
+    if(delta > 0 || (delta < 0 && delta > -8)) {
+      data.append(padIlst(data));
+      delta = data.size() - length;
+    }
+    else if(delta < 0) {
+      data.append(padIlst(data, -delta - 8));
+      delta = 0;
+    }
 
-  d->file->insert(data, offset, length);
+    d->file->insert(data, offset, length);
 
-  if(delta) {
-    updateParents(path, delta, 1);
-    updateOffsets(delta, offset);
+    if(delta) {
+      updateParents(path, delta, 1);
+      updateOffsets(delta, offset);
+    }
+  }
+  else {
+    // Strip meta
+    MP4::Atom *udta = *(--it);
+    AtomList &udtaChildren = udta->children;
+    AtomList::Iterator metaIt = udtaChildren.find(meta);
+    if (metaIt != udtaChildren.end()) {
+      offset = meta->offset;
+      delta = - meta->length;
+      udtaChildren.erase(metaIt);
+      d->file->removeBlock(meta->offset, meta->length);
+      delete meta;
+
+      if(delta) {
+        updateParents(path, delta, 2);
+        updateOffsets(delta, offset);
+      }
+    }
   }
 }
 
diff --git a/tests/test_mp4.cpp b/tests/test_mp4.cpp
index 5f96c9c0..6e44421a 100644
--- a/tests/test_mp4.cpp
+++ b/tests/test_mp4.cpp
@@ -65,6 +65,7 @@ class TestMP4 : public CppUnit::TestFixture
   CPPUNIT_TEST(testRepeatedSave);
   CPPUNIT_TEST(testWithZeroLengthAtom);
   CPPUNIT_TEST(testEmptyValuesRemoveItems);
+  CPPUNIT_TEST(testRemoveMetadata);
   CPPUNIT_TEST_SUITE_END();
 
 public:
@@ -654,6 +655,39 @@ public:
     CPPUNIT_ASSERT_EQUAL(zeroUInt, tag->track());
     CPPUNIT_ASSERT(!tag->contains("trkn"));
   }
+
+  void testRemoveMetadata()
+  {
+    ScopedFileCopy copy("no-tags", ".m4a");
+
+    {
+      MP4::File f(copy.fileName().c_str());
+      CPPUNIT_ASSERT(f.isValid());
+      CPPUNIT_ASSERT(!f.hasMP4Tag());
+      MP4::Tag *tag = f.tag();
+      CPPUNIT_ASSERT(tag->isEmpty());
+      tag->setTitle("TITLE");
+      f.save();
+    }
+    {
+      MP4::File f(copy.fileName().c_str());
+      CPPUNIT_ASSERT(f.isValid());
+      CPPUNIT_ASSERT(f.hasMP4Tag());
+      MP4::Tag *tag = f.tag();
+      CPPUNIT_ASSERT(!tag->isEmpty());
+      tag->setTitle("");
+      f.save();
+    }
+    {
+      MP4::File f(copy.fileName().c_str());
+      CPPUNIT_ASSERT(f.isValid());
+      CPPUNIT_ASSERT(!f.hasMP4Tag());
+      CPPUNIT_ASSERT(f.tag()->isEmpty());
+      CPPUNIT_ASSERT(fileEqual(
+        copy.fileName(),
+        TEST_FILE_PATH_C("no-tags.m4a")));
+    }
+  }
 };
 
 CPPUNIT_TEST_SUITE_REGISTRATION(TestMP4);