lib/scanner: Prevent sync-conflict for receive-only local modifications (#9323)

### Purpose

This PR changes behaviour of syncthing related to `receive-only`
folders, which I believe to be a bug since I wouldn't expect the current
behaviour. With the current syncthing codebase, a file of a
`receive-only` folder that is only modified locally can cause the
creation of a `.sync-conflict` file.

### Testing

Consider this szenario: Setup two paired clients that sync a folder with
a given file (e.g. `Test.txt`). One of the clients configures the folder
to be `receive-only`. Now, change the contents of the file for the
receive-only client **_twice_**.

With the current syncthing codebase, this leads to the creation of a
`.sync-conflict` file that contains the modified contents, while the
regular `Test.txt` file is reset to the cluster's provided contents.
This is due to a `protocol.FileInfo#ShouldConflict` check, that is
succeeding on the locally modified file.

This PR changes this behaviour to not reset the file and not cause the
creation of a `.sync-conflict`. Instead, the second content update is
treated the same as the first content update.

This PR also contains a test that fails on the current codebase and
succeeds with the changes introduced in this PR.

### Screenshots

This is not a GUI change

### Documentation

This is not a user visible change.

## Authorship

Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.

#### Thanks to all the syncthing folks for this awesome piece of
software!

2 files changed, 80 insertions, 6 deletions

diff --git a/lib/model/folder_recvonly_test.go b/lib/model/folder_recvonly_test.go
index 83c2bcd15..72383bd62 100644
--- a/lib/model/folder_recvonly_test.go
+++ b/lib/model/folder_recvonly_test.go
@@ -480,6 +480,74 @@ func TestRecvOnlyRevertOwnID(t *testing.T) {
 	}
 }
 
+func TestRecvOnlyLocalChangeDoesNotCauseConflict(t *testing.T) {
+	// Get us a model up and running
+
+	m, f, wcfgCancel := setupROFolder(t)
+	defer wcfgCancel()
+	ffs := f.Filesystem(nil)
+	defer cleanupModel(m)
+	conn := addFakeConn(m, device1, f.ID)
+
+	// Create some test data
+
+	must(t, ffs.MkdirAll(".stfolder", 0o755))
+	oldData := []byte("hello\n")
+	knownFiles := setupKnownFiles(t, ffs, oldData)
+
+	// Send an index update for the known stuff
+
+	must(t, m.Index(conn, "ro", knownFiles))
+	f.updateLocalsFromScanning(knownFiles)
+
+	// Scan the folder.
+
+	must(t, m.ScanFolder("ro"))
+
+	// Everything should be in sync.
+
+	size := globalSize(t, m, "ro")
+	if size.Files != 1 || size.Directories != 1 {
+		t.Fatalf("Global: expected 1 file and 1 directory: %+v", size)
+	}
+	size = localSize(t, m, "ro")
+	if size.Files != 1 || size.Directories != 1 {
+		t.Fatalf("Local: expected 1 file and 1 directory: %+v", size)
+	}
+	size = needSizeLocal(t, m, "ro")
+	if size.Files+size.Directories > 0 {
+		t.Fatalf("Need: expected nothing: %+v", size)
+	}
+	size = receiveOnlyChangedSize(t, m, "ro")
+	if size.Files+size.Directories > 0 {
+		t.Fatalf("ROChanged: expected nothing: %+v", size)
+	}
+
+	// Modify the file
+
+	writeFilePerm(t, ffs, "knownDir/knownFile", []byte("change1\n"), 0o644)
+
+	must(t, m.ScanFolder("ro"))
+
+	size = receiveOnlyChangedSize(t, m, "ro")
+	if size.Files != 1 {
+		t.Fatalf("Receive only: expected 1 file: %+v", size)
+	}
+
+	// Perform another modification. This should not cause the file to be needed.
+	// This is a regression test: Previously on scan the file version was changed to conflict with the global
+	// version, thus being needed and creating a conflict copy on next pull.
+
+	writeFilePerm(t, ffs, "knownDir/knownFile", []byte("change2\n"), 0o644)
+
+	must(t, m.ScanFolder("ro"))
+
+	size = needSizeLocal(t, m, "ro")
+	if size.Files != 0 {
+		t.Fatalf("Need: expected nothing: %+v", size)
+	}
+}
+
 func setupKnownFiles(t *testing.T, ffs fs.Filesystem, data []byte) []protocol.FileInfo {
 	t.Helper()
 
diff --git a/lib/scanner/walk.go b/lib/scanner/walk.go
index 074d034aa..5f4a59d23 100644
--- a/lib/scanner/walk.go
+++ b/lib/scanner/walk.go
@@ -426,12 +426,14 @@ func (w *walker) walkRegular(ctx context.Context, relPath string, info fs.FileIn
 			l.Debugln(w, "unchanged:", curFile)
 			return nil
 		}
-		if curFile.ShouldConflict() {
+		if curFile.ShouldConflict() && !f.ShouldConflict() {
 			// The old file was invalid for whatever reason and probably not
 			// up to date with what was out there in the cluster. Drop all
 			// others from the version vector to indicate that we haven't
 			// taken their version into account, and possibly cause a
-			// conflict.
+			// conflict. However, only do this if the new file is not also
+			// invalid. This would indicate that the new file is not part
+			// of the cluster, but e.g. a local change.
 			f.Version = f.Version.DropOthers(w.ShortID)
 		}
 		l.Debugln(w, "rescan:", curFile)
@@ -471,12 +473,14 @@ func (w *walker) walkDir(ctx context.Context, relPath string, info fs.FileInfo,
 			l.Debugln(w, "unchanged:", curFile)
 			return nil
 		}
-		if curFile.ShouldConflict() {
+		if curFile.ShouldConflict() && !f.ShouldConflict() {
 			// The old file was invalid for whatever reason and probably not
 			// up to date with what was out there in the cluster. Drop all
 			// others from the version vector to indicate that we haven't
 			// taken their version into account, and possibly cause a
-			// conflict.
+			// conflict. However, only do this if the new file is not also
+			// invalid. This would indicate that the new file is not part
+			// of the cluster, but e.g. a local change.
 			f.Version = f.Version.DropOthers(w.ShortID)
 		}
 		l.Debugln(w, "rescan:", curFile)
@@ -524,12 +528,14 @@ func (w *walker) walkSymlink(ctx context.Context, relPath string, info fs.FileIn
 			l.Debugln(w, "unchanged:", curFile, info.ModTime().Unix(), info.Mode()&fs.ModePerm)
 			return nil
 		}
-		if curFile.ShouldConflict() {
+		if curFile.ShouldConflict() && !f.ShouldConflict() {
 			// The old file was invalid for whatever reason and probably not
 			// up to date with what was out there in the cluster. Drop all
 			// others from the version vector to indicate that we haven't
 			// taken their version into account, and possibly cause a
-			// conflict.
+			// conflict. However, only do this if the new file is not also
+			// invalid. This would indicate that the new file is not part
+			// of the cluster, but e.g. a local change.
 			f.Version = f.Version.DropOthers(w.ShortID)
 		}
 		l.Debugln(w, "rescan:", curFile)