Prefix file magic detection (#1550)

Resolver now with enabled prefix file discovery discovers even wrong files, most specifically in Maven ITs where Jetty handler is not mounted to URL and receives all sort of garbage, that is assumed to be prefix file.

From now on, prefix file must start with following line (be the very first line in prefixes.txt):
```
## repository-prefixes/2.0
```

Fixes #1549

Author: cstamas

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`.