Prefix file magic detection

maven-resolver-impl/src/main/java/org/eclipse/aether/internal/impl/filter/PrefixesRemoteRepositoryFilterSource.java

@@ -22,6 +22,7 @@
 import javax.inject.Named;
 import javax.inject.Singleton;
 
+import java.io.BufferedReader;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -85,6 +86,11 @@ public final class PrefixesRemoteRepositoryFilterSource extends RemoteRepository
 
     private static final String PREFIX_FILE_PATH = ".meta/prefixes.txt";
 
+    /**
+     * Visible for UT.
+     */
+    static final String PREFIX_FIRST_LINE = "## repository-prefixes/2.0";
+
     /**
      * Is filter enabled?
      *
@@ -183,7 +189,7 @@ private PrefixTree loadPrefixTree(
             if (filePath == null) {
                 filePath = resolvePrefixesFromRemoteRepository(session, remoteRepository);
             }
-            if (filePath != null) {
+            if (isPrefixFile(filePath)) {
                 logger.debug(
                         "Loading prefixes for remote repository {} from file '{}'", remoteRepository.getId(), filePath);
                 try (Stream<String> lines = Files.lines(filePath, StandardCharsets.UTF_8)) {
@@ -204,6 +210,19 @@ private PrefixTree loadPrefixTree(
         return PrefixTree.SENTINEL;
     }
 
+    private boolean isPrefixFile(Path path) {
+        if (path == null || !Files.isRegularFile(path)) {
+            return false;
+        }
+        try (BufferedReader reader = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
+            return PREFIX_FIRST_LINE.equals(reader.readLine());
+        } catch (FileNotFoundException e) {
+            return false;
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+    }
+
     private Path resolvePrefixesFromLocalConfiguration(
             RepositorySystemSession session, Path baseDir, RemoteRepository remoteRepository) {
         Path filePath = baseDir.resolve(PREFIXES_FILE_PREFIX + remoteRepository.getId() + PREFIXES_FILE_SUFFIX);

maven-resolver-impl/src/test/java/org/eclipse/aether/internal/impl/filter/PrefixesRemoteRepositoryFilterSourceTest.java

@@ -76,11 +76,15 @@ protected void allowArtifact(
             Path baseDir = session.getLocalRepository()
                     .getBasePath()
                     .resolve(PrefixesRemoteRepositoryFilterSource.LOCAL_REPO_PREFIX_DIR);
-            Path groupId = baseDir.resolve(PrefixesRemoteRepositoryFilterSource.PREFIXES_FILE_PREFIX
+            Path prefixes = baseDir.resolve(PrefixesRemoteRepositoryFilterSource.PREFIXES_FILE_PREFIX
                     + remoteRepository.getId()
                     + PrefixesRemoteRepositoryFilterSource.PREFIXES_FILE_SUFFIX);
-            Files.createDirectories(groupId.getParent());
-            Files.write(groupId, artifact.getGroupId().replaceAll("\\.", "/").getBytes(StandardCharsets.UTF_8));
+            Files.createDirectories(prefixes.getParent());
+            Files.write(
+                    prefixes,
+                    (PrefixesRemoteRepositoryFilterSource.PREFIX_FIRST_LINE + "\n"
+                                    + artifact.getGroupId().replaceAll("\\.", "/"))
+                            .getBytes(StandardCharsets.UTF_8));
         } catch (IOException e) {
             throw new UncheckedIOException(e);
         }

src/site/markdown/remote-repository-filtering.md

@@ -126,6 +126,9 @@ Many MRMs and Maven Central itself publish these files. Some prefixes file examp
 The user provided prefixes files are expected in the following location by default: 
 `${filterBasedir}/prefixes-${remoteRepository.id}.txt`.
 
+**Important**: Valid prefix files start with following "magic" on their very first line: `## repository-prefixes/2.0`.
+If the first line in file is not this string, the prefix file is discarded.
+
 To disable prefixes filter, use the following setting: `-Daether.remoteRepositoryFilter.prefixes=false`.
 To disable for single repository filtering, append to key `.repoId`.