Addressed review comments.

Author: nvharikrishna

server/src/main/java/org/apache/cassandra/sidecar/exceptions/LiveMigrationExceptions.java

@@ -72,6 +72,22 @@ public LiveMigrationTaskNotFoundException(String message)
         }
     }
 
+    /**
+     * Thrown when a live-migration file URL is well-formed but matches no configured directory prefix
+     * on this instance, i.e. it addresses no resource here. Extends {@link IllegalArgumentException} so
+     * that destination-side callers - which map file URLs obtained from the source's file-list API onto
+     * local paths - keep treating an unmatched prefix (a source/destination directory-configuration
+     * mismatch) as a bad argument, while the source-side request handler can catch this specific type to
+     * distinguish "no such resource" (HTTP 404) from a malformed URL (HTTP 400).
+     */
+    public static class UnknownMigrationPrefixException extends IllegalArgumentException
+    {
+        public UnknownMigrationPrefixException(String message)
+        {
+            super(message);
+        }
+    }
+
     /**
      * Exception thrown when file verification fails during live migration.
      */

server/src/main/java/org/apache/cassandra/sidecar/handlers/livemigration/LiveMigrationFileResolveHandler.java

@@ -47,6 +47,7 @@
 import org.apache.cassandra.sidecar.concurrent.ExecutorPools;
 import org.apache.cassandra.sidecar.config.LiveMigrationConfiguration;
 import org.apache.cassandra.sidecar.config.SidecarConfiguration;
+import org.apache.cassandra.sidecar.exceptions.LiveMigrationExceptions.UnknownMigrationPrefixException;
 import org.apache.cassandra.sidecar.handlers.AbstractHandler;
 import org.apache.cassandra.sidecar.handlers.AccessProtected;
 import org.apache.cassandra.sidecar.handlers.FileStreamHandler;
@@ -151,8 +152,16 @@ protected void handleInternal(RoutingContext rc,
         {
             resolved = LiveMigrationInstanceMetadataUtil.resolveLexically(normalizedPath, instanceMeta);
         }
+        catch (UnknownMigrationPrefixException e)
+        {
+            // The URL is well-formed but matches no configured live-migration directory on this
+            // instance, so it addresses no resource here - report it as not found.
+            rc.fail(wrapHttpException(HttpResponseStatus.NOT_FOUND, e.getMessage(), e));
+            return;
+        }
         catch (IllegalArgumentException e)
         {
+            // URL must have been malformed, report it as bad request
             rc.fail(wrapHttpException(HttpResponseStatus.BAD_REQUEST, e.getMessage(), e));
             return;
         }

server/src/main/java/org/apache/cassandra/sidecar/livemigration/LiveMigrationInstanceMetadataUtil.java

@@ -39,6 +39,7 @@
 
 import org.apache.cassandra.sidecar.cluster.instance.InstanceMetadata;
 import org.apache.cassandra.sidecar.common.ApiEndpointsV1;
+import org.apache.cassandra.sidecar.exceptions.LiveMigrationExceptions.UnknownMigrationPrefixException;
 import org.apache.cassandra.sidecar.handlers.livemigration.LiveMigrationDirType;
 import org.jetbrains.annotations.NotNull;
 
@@ -306,6 +307,20 @@ private static Path canonicalBaseDir(Path baseDir) throws IOException
         return existing != null ? existing : canonical;
     }
 
+    /**
+     * Lexically resolves a live migration file download URL to a {@link ResolvedPath}. Performs only
+     * string-level validation; the filesystem is not touched and the file is not required to exist.
+     *
+     * @param fileUrl  Live migration file download URL
+     * @param metadata Cassandra instance metadata
+     * @return the lexically-resolved {@link ResolvedPath}
+     * @throws IllegalArgumentException        if the URL is malformed - it contains a relative traversal
+     *                                         segment ({@code /../}) or lexically escapes the configured base directory
+     * @throws UnknownMigrationPrefixException if the URL does not match any configured live-migration directory
+     *                                         prefix; the URL is well-formed but addresses no resource on this
+     *                                         instance. This is a subtype of {@link IllegalArgumentException}, so
+     *                                         callers that only care about "bad URL" need not distinguish it
+     */
     public static ResolvedPath resolveLexically(@NotNull String fileUrl,
                                                 @NotNull InstanceMetadata metadata)
     {
@@ -341,7 +356,7 @@ public static ResolvedPath resolveLexically(@NotNull String fileUrl,
         }
 
         LOGGER.warn("File url {} does not match any configured live-migration directory prefix.", fileUrl);
-        throw new IllegalArgumentException("File url " + fileUrl + " is unknown.");
+        throw new UnknownMigrationPrefixException("File url " + fileUrl + " is unknown.");
     }
 
     /**

server/src/test/java/org/apache/cassandra/sidecar/handlers/livemigration/LiveMigrationFileDigestHandlerTest.java

@@ -185,7 +185,7 @@ public void testRouteFailsForInvalidDir(VertxTestContext context) throws IOExcep
         createFile(dummyText, firstInstanceMeta.commitlogDir() + filePath);
 
         String testRoute = LIVE_MIGRATION_FILES_ROUTE + "/commitlog/1" + filePath + "?digestAlgorithm=xxhash32";
-        shouldFail(context, testRoute, FIRST_INSTANCE_IP, SECOND_INSTANCE_IP, FIRST_INSTANCE_IP, 400);
+        shouldFail(context, testRoute, FIRST_INSTANCE_IP, SECOND_INSTANCE_IP, FIRST_INSTANCE_IP, 404);
     }
 
     @Test

server/src/test/java/org/apache/cassandra/sidecar/handlers/livemigration/LiveMigrationFileStreamTest.java

@@ -316,10 +316,10 @@ public void testRouteHavingDirIndexThatDoesNotExist(VertxTestContext context) th
         createFile(DUMMY_CONTENT, firstInstanceDataDirs.get(0) + filePath);
 
         // dirIndex 100 is greater than the configured data dir count, so the URL doesn't match
-        // any configured prefix — resolveLexically rejects it as a malformed URL (400), not a
-        // missing file (404). The request can never resolve to a file on this server.
+        // any configured prefix - reports it as a missing resource (404), since
+        // the URL is well-formed but addresses no directory configured on this instance.
         shouldThrowError(context, LIVE_MIGRATION_DATA_FILE_DIR_PATH + "/100" + filePath,
-                         FIRST_INSTANCE_IP, THIRD_INSTANCE_IP, FIRST_INSTANCE_IP, 400);
+                         FIRST_INSTANCE_IP, THIRD_INSTANCE_IP, FIRST_INSTANCE_IP, 404);
     }
 
     @Test

server/src/test/java/org/apache/cassandra/sidecar/livemigration/LiveMigrationInstanceMetadataUtilTest.java

@@ -36,6 +36,7 @@
 import org.junit.jupiter.api.io.TempDir;
 
 import org.apache.cassandra.sidecar.cluster.instance.InstanceMetadata;
+import org.apache.cassandra.sidecar.exceptions.LiveMigrationExceptions.UnknownMigrationPrefixException;
 import org.mockito.Mockito;
 
 import static org.apache.cassandra.sidecar.handlers.livemigration.InstanceMetadataTestUtil.LIVE_MIGRATION_CDC_RAW_DIR_PATH;
@@ -411,6 +412,22 @@ public void testRealPathRejectsLexicalTraversal()
                           LIVE_MIGRATION_DATA_FILE_DIR_PATH + "/0/../../etc/passwd", instanceMetadata));
     }
 
+    @Test
+    public void testResolveLexicallyThrowsForUnknownPrefix()
+    {
+        // A well-formed URL whose prefix matches no configured directory is reported via
+        // UnknownMigrationPrefixException. It is a subtype of IllegalArgumentException (so destination-side
+        // localPath callers keep treating it as a bad argument), while the source-side handler catches this
+        // specific type to answer 404 - "no such resource" - instead of 400 for a malformed URL.
+        String cassandraHomeDir = tempDir.resolve("unknownPrefix").toString();
+        InstanceMetadata instanceMetadata = getInstanceMetadata(cassandraHomeDir);
+        when(instanceMetadata.cdcDir()).thenReturn(null);
+
+        assertThatExceptionOfType(UnknownMigrationPrefixException.class)
+        .isThrownBy(() -> resolveLexically(LIVE_MIGRATION_CDC_RAW_DIR_PATH + "/0/" + FILE_NAME, instanceMetadata))
+        .isInstanceOf(IllegalArgumentException.class);
+    }
+
     @Test
     public void testLocalPathNonExistingDirs()
     {