Merge pull request #618 from debugmcp/codex/569-bounded-log-tail

fix: bound proxy diagnostic log reads (#569)

Author: debugmcpdev

changelog.d/569.changed.md

@@ -0,0 +1 @@
+Bound proxy-failure log reads to the final 64 KiB before redacting and trimming them to 80 lines, and remove redundant raw-error serialization from diagnostic log metadata.

packages/adapter-rust/tests/rust-adapter.test.ts

@@ -43,6 +43,7 @@ import { deriveSourceMapFromBinary } from '@debugmcp/codelldb-common';
 const mockDependencies: AdapterDependencies = {
   fileSystem: {
     readFile: vi.fn(),
+    readTail: vi.fn(),
     writeFile: vi.fn(),
     outputFile: vi.fn(),
     exists: vi.fn(),

packages/adapter-rust/tests/rust-debug-adapter.toolchain.test.ts

@@ -50,6 +50,7 @@ import { resolveCodeLLDBExecutable } from '../src/utils/codelldb-resolver.js';
 const createDependencies = (): AdapterDependencies => ({
   fileSystem: {
     readFile: vi.fn(),
+    readTail: vi.fn(),
     writeFile: vi.fn(),
     outputFile: vi.fn(),
     exists: vi.fn(),

packages/shared/src/interfaces/external-dependencies.ts

@@ -21,6 +21,8 @@ export interface IProxyManager {
 export interface IFileSystem {
   // Basic fs operations
   readFile(path: string, encoding?: BufferEncoding): Promise<string>;
+  /** Read at most `maxBytes` from the end of a file as UTF-8 text. */
+  readTail(path: string, maxBytes: number): Promise<string>;
   writeFile(path: string, data: string | Buffer): Promise<void>;
   exists(path: string): Promise<boolean>;
   mkdir(path: string, options?: { recursive?: boolean }): Promise<void>;

src/implementations/file-system-impl.ts

@@ -3,6 +3,7 @@
  */
 import fs from 'fs-extra';
 import { Stats } from 'fs';
+import { open } from 'node:fs/promises';
 import { IFileSystem } from '@debugmcp/shared';
 
 export class FileSystemImpl implements IFileSystem {
@@ -11,6 +12,26 @@ export class FileSystemImpl implements IFileSystem {
     return fs.readFile(path, encoding || 'utf-8');
   }
 
+  async readTail(path: string, maxBytes: number): Promise<string> {
+    if (!Number.isSafeInteger(maxBytes) || maxBytes < 0) {
+      throw new RangeError('maxBytes must be a non-negative safe integer');
+    }
+    if (maxBytes === 0) {
+      return '';
+    }
+
+    const file = await open(path, 'r');
+    try {
+      const { size } = await file.stat();
+      const length = Math.min(size, maxBytes);
+      const buffer = Buffer.alloc(length);
+      const { bytesRead } = await file.read(buffer, 0, length, size - length);
+      return buffer.subarray(0, bytesRead).toString('utf8');
+    } finally {
+      await file.close();
+    }
+  }
+
   async writeFile(path: string, data: string | Buffer): Promise<void> {
     return fs.writeFile(path, data);
   }

src/interfaces/external-dependencies.ts

@@ -17,6 +17,8 @@ import type { IDebugAdapter } from '@debugmcp/shared';
 export interface IFileSystem {
   // Basic fs operations
   readFile(path: string, encoding?: BufferEncoding): Promise<string>;
+  /** Read at most `maxBytes` from the end of a file as UTF-8 text. */
+  readTail(path: string, maxBytes: number): Promise<string>;
   writeFile(path: string, data: string | Buffer): Promise<void>;
   exists(path: string): Promise<boolean>;
   mkdir(path: string, options?: { recursive?: boolean }): Promise<void>;

src/session/launch/proxy-failure-diagnostics.ts

@@ -23,15 +23,11 @@ import type { ProxyInitProgress } from '../../utils/error-messages.js';
 import { getErrorMessage, SessionNotFoundError } from '../../errors/debug-errors.js';
 import { proxyLogPathFor } from '../../proxy/session-log-layout.js';
 
-/** How many trailing proxy-log lines are worth reading after a failure. */
+/** How many trailing proxy-log lines are worth reporting after a failure. */
 const PROXY_LOG_TAIL_LINES = 80;
 
-/**
- * Character cap on the tail. Generous on purpose: it exists so a proxy log with
- * one pathological multi-megabyte line cannot blow up the record, not to trim a
- * normal 80-line tail, which is a few kilobytes.
- */
-const PROXY_LOG_TAIL_CHARS = 64 * 1024;
+/** Hard I/O and allocation cap applied before the log is sanitized. */
+export const PROXY_LOG_TAIL_MAX_BYTES = 64 * 1024;
 
   /** The pointers a failed launch/attach returns to the caller (issue #493 / #551). */
   export interface ProxyFailureDiagnostics {
@@ -45,7 +41,7 @@ const PROXY_LOG_TAIL_CHARS = 64 * 1024;
  */
 export interface ProxyFailureLogDeps {
   logger: ILogger;
-  fileSystem: Pick<IFileSystem, 'readFile'>;
+  fileSystem: Pick<IFileSystem, 'readTail'>;
 }
 
 /** The two operations that can fail this way, named as they appear in the log. */
@@ -94,7 +90,7 @@ export function collectProxyFailureDiagnostics(
 /**
  * Read the last `tailLineCount` lines of the proxy log, if there is one.
  *
- * Reads straight through rather than asking `pathExists` first: the proxy is
+ * Reads the bounded tail rather than asking `pathExists` first: the proxy is
  * still writing (and may rotate) this file, so an exists-then-read pair can
  * report "no log" for a file that appeared a millisecond later, and spends a
  * second syscall to do it. `ENOENT` — the answer that check was buying — is
@@ -111,18 +107,18 @@ export function collectProxyFailureDiagnostics(
  * error that sent us here still reaches the log intact.
  */
 export async function readProxyLogTail(
-  fileSystem: Pick<IFileSystem, 'readFile'>,
+  fileSystem: Pick<IFileSystem, 'readTail'>,
   proxyLogPath: string | undefined,
   tailLineCount: number = PROXY_LOG_TAIL_LINES
 ): Promise<string | undefined> {
   if (!proxyLogPath) {
     return undefined;
   }
   try {
-    const logContent = await fileSystem.readFile(proxyLogPath, 'utf-8');
+    const logContent = await fileSystem.readTail(proxyLogPath, PROXY_LOG_TAIL_MAX_BYTES);
     return sanitizeStderrTail(logContent, {
       maxLines: tailLineCount,
-      maxChars: PROXY_LOG_TAIL_CHARS
+      maxChars: PROXY_LOG_TAIL_MAX_BYTES
     });
   } catch (logReadError) {
     if ((logReadError as NodeJS.ErrnoException)?.code === 'ENOENT') {
@@ -155,13 +151,6 @@ export function buildProxyFailureErrorDetails(
     proxyLogTail
   };
 
-  // Try to capture raw error object
-  try {
-    errorDetails.raw = JSON.stringify(error);
-  } catch {
-    errorDetails.raw = 'Error not JSON serializable';
-  }
-
   return errorDetails;
 }
 

tests/adapters/go/integration/go-session-smoke.test.ts

@@ -8,6 +8,7 @@ import { GoAdapterFactory } from '@debugmcp/adapter-go';
 const createDependencies = (): AdapterDependencies => ({
   fileSystem: {
     readFile: async () => '',
+    readTail: async () => '',
     writeFile: async () => {},
     exists: async () => false,
     mkdir: async () => {},

tests/adapters/go/unit/go-adapter-factory.test.ts

@@ -19,6 +19,7 @@ const mockSpawn = vi.mocked(spawn);
 const createMockDependencies = (): AdapterDependencies => ({
   fileSystem: {
     readFile: async () => '',
+    readTail: async () => '',
     writeFile: async () => {},
     exists: async () => false,
     mkdir: async () => {},

tests/adapters/go/unit/go-debug-adapter.test.ts

@@ -19,6 +19,7 @@ const mockSpawn = vi.mocked(spawn);
 const createMockDependencies = (): AdapterDependencies => ({
   fileSystem: {
     readFile: async () => '',
+    readTail: async () => '',
     writeFile: async () => {},
     exists: async () => false,
     mkdir: async () => {},