Keep ACP terminal output from stalling daemon requests (#7953)

* fix(acp): trim terminal output in linear time

* test(acp): reproduce terminal output stalls with a larger workload

The 128 KiB workload completes inside the regression deadline on faster
machines even with quadratic trimming. One MiB exceeds that deadline on
the old implementation while the bounded UTF-8 suffix implementation
finishes promptly.

Co-authored-by: liyang <liyang@msh.team>

---------

Co-authored-by: liyang <liyang@msh.team>
Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>

packages/server/src/server/agent/providers/acp-agent.test.ts

@@ -811,6 +811,46 @@ describe("ACPAgentSession terminal tools", () => {
       truncated: false,
     });
   });
+
+  test("trims large terminal output without rescanning the retained suffix", async () => {
+    const session = createSession();
+    const terminal = await session.createTerminal({
+      sessionId: "session-1",
+      command: process.execPath,
+      args: ["-e", 'process.stdout.write("x".repeat(1024 * 1024))'],
+      cwd: process.cwd(),
+      outputByteLimit: 1024,
+    });
+    await session.waitForTerminalExit({ sessionId: "session-1", terminalId: terminal.terminalId });
+
+    await expect(
+      session.terminalOutput({ sessionId: "session-1", terminalId: terminal.terminalId }),
+    ).resolves.toEqual({
+      output: "x".repeat(1024),
+      truncated: true,
+      exitStatus: { exitCode: 0, signal: null },
+    });
+  }, 1000);
+
+  test("trims terminal output at a UTF-8 character boundary", async () => {
+    const session = createSession();
+    const terminal = await session.createTerminal({
+      sessionId: "session-1",
+      command: process.execPath,
+      args: ["-e", 'process.stdout.write("😀ab")'],
+      cwd: process.cwd(),
+      outputByteLimit: 3,
+    });
+    await session.waitForTerminalExit({ sessionId: "session-1", terminalId: terminal.terminalId });
+
+    await expect(
+      session.terminalOutput({ sessionId: "session-1", terminalId: terminal.terminalId }),
+    ).resolves.toEqual({
+      output: "ab",
+      truncated: true,
+      exitStatus: { exitCode: 0, signal: null },
+    });
+  });
 });
 
 describe("mapACPUsage", () => {

packages/server/src/server/agent/providers/acp-agent.ts

@@ -4093,10 +4093,16 @@ function appendTerminalOutput(entry: TerminalEntry, chunk: string): void {
   if (!limit) {
     return;
   }
-  while (Buffer.byteLength(entry.output, "utf8") > limit && entry.output.length > 0) {
-    entry.output = entry.output.slice(1);
-    entry.truncated = true;
+  const output = Buffer.from(entry.output, "utf8");
+  if (output.length <= limit) {
+    return;
   }
+  let start = output.length - limit;
+  while (start < output.length && (output[start] & 0xc0) === 0x80) {
+    start += 1;
+  }
+  entry.output = output.subarray(start).toString("utf8");
+  entry.truncated = true;
 }
 
 function readRecord(value: unknown): Record<string, unknown> | null {