From 3bc1409befecf0f387ed85b9a031371265f100ed Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 10:47:20 -0800
Subject: [PATCH 01/10] Speculative fix for stream backpressure

Part of microsoft/vscode#283056
Part of microsoft/vscode#246204
---
 src/unix/pty.cc     | 16 ++++++++--------
 src/unixTerminal.ts | 11 ++++++++++-
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/src/unix/pty.cc b/src/unix/pty.cc
index ec5524f5..7b4b9e1f 100644
--- a/src/unix/pty.cc
+++ b/src/unix/pty.cc
@@ -448,10 +448,10 @@ Napi::Value PtyFork(const Napi::CallbackInfo& info) {
   }
 #endif
 
-  Napi::Object obj = Napi::Object::New(napiEnv);  
-  obj.Set("fd", Napi::Number::New(napiEnv, master));  
-  obj.Set("pid", Napi::Number::New(napiEnv, pid));  
-  obj.Set("pty", Napi::String::New(napiEnv, ptsname(master)));  
+  Napi::Object obj = Napi::Object::New(napiEnv);
+  obj.Set("fd", Napi::Number::New(napiEnv, master));
+  obj.Set("pid", Napi::Number::New(napiEnv, pid));
+  obj.Set("pty", Napi::String::New(napiEnv, ptsname(master)));
 
   // Set up process exit callback.
   Napi::Function cb = info[10].As<Napi::Function>();
@@ -492,10 +492,10 @@ Napi::Value PtyOpen(const Napi::CallbackInfo& info) {
     throw Napi::Error::New(env, "Could not set slave fd to nonblocking.");
   }
 
-  Napi::Object obj = Napi::Object::New(env);  
-  obj.Set("master", Napi::Number::New(env, master));  
-  obj.Set("slave", Napi::Number::New(env, slave));  
-  obj.Set("pty", Napi::String::New(env, ptsname(master)));  
+  Napi::Object obj = Napi::Object::New(env);
+  obj.Set("master", Napi::Number::New(env, master));
+  obj.Set("slave", Napi::Number::New(env, slave));
+  obj.Set("pty", Napi::String::New(env, ptsname(master)));
 
   return obj;
 }
diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index 211aadcb..9c65986a 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -3,6 +3,7 @@
  * Copyright (c) 2016, Daniel Imms (MIT License).
  * Copyright (c) 2018, Microsoft Corporation (MIT License).
  */
+import * as fs from 'fs';
 import * as net from 'net';
 import * as path from 'path';
 import * as tty from 'tty';
@@ -162,7 +163,15 @@ export class UnixTerminal extends Terminal {
   }
 
   protected _write(data: string | Buffer): void {
-    this._socket.write(data);
+    // Use the underlying file descriptor to avoid issues with backpressure
+    // handling in net.Socket/tty.*Stream which can result in data loss with
+    // ~1-4kb of data.
+    // Context: https://github.com/microsoft/vscode/issues/283056
+    fs.write(this.fd, data, (err, written) => {
+      if (err) {
+        console.log('pty write error', err);
+      }
+    });
   }
 
   /* Accessors */

From bb8a1af7f0b7c65f3bc745640202dca14b94efcb Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 10:55:42 -0800
Subject: [PATCH 02/10] Add additional change needed to fix it

---
 src/unixTerminal.ts | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index 9c65986a..763e92f8 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -26,6 +26,7 @@ const DESTROY_SOCKET_TIMEOUT_MS = 200;
 export class UnixTerminal extends Terminal {
   protected _fd: number;
   protected _pty: string;
+  private _writeSocket!: net.Socket; // HACK: This is unsafe
 
   protected _file: string;
   protected _name: string;
@@ -103,6 +104,8 @@ export class UnixTerminal extends Terminal {
     const term = pty.fork(file, args, parsedEnv, cwd, this._cols, this._rows, uid, gid, (encoding === 'utf8'), helperPath, onexit);
 
     this._socket = new tty.ReadStream(term.fd);
+    // HACK: This needs to be created or all some data may get lost
+    this._writeSocket = new tty.WriteStream(term.fd);
     if (encoding !== null) {
       this._socket.setEncoding(encoding);
     }
@@ -171,7 +174,12 @@ export class UnixTerminal extends Terminal {
       if (err) {
         console.log('pty write error', err);
       }
+      console.log('written', written);
     });
+    // TODO: Understand why this hangs the process
+    // this._writeSocket.write(data, (err) => {
+    //   console.log('err', err);
+    // });
   }
 
   /* Accessors */

From b8ecd5eefb7c7848f6fa22ae25f19a4c2820bacf Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 19:13:02 -0800
Subject: [PATCH 03/10] Use raw write stream, handle partial writes/kernel
 backpressure

---
 src/unixTerminal.ts | 94 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 77 insertions(+), 17 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index 763e92f8..5d871acc 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -26,7 +26,6 @@ const DESTROY_SOCKET_TIMEOUT_MS = 200;
 export class UnixTerminal extends Terminal {
   protected _fd: number;
   protected _pty: string;
-  private _writeSocket!: net.Socket; // HACK: This is unsafe
 
   protected _file: string;
   protected _name: string;
@@ -36,6 +35,8 @@ export class UnixTerminal extends Terminal {
 
   private _boundClose: boolean = false;
   private _emittedClose: boolean = false;
+
+  private readonly _writeStream: fs.WriteStream;
   private _master: net.Socket | undefined;
   private _slave: net.Socket | undefined;
 
@@ -104,11 +105,11 @@ export class UnixTerminal extends Terminal {
     const term = pty.fork(file, args, parsedEnv, cwd, this._cols, this._rows, uid, gid, (encoding === 'utf8'), helperPath, onexit);
 
     this._socket = new tty.ReadStream(term.fd);
-    // HACK: This needs to be created or all some data may get lost
-    this._writeSocket = new tty.WriteStream(term.fd);
-    if (encoding !== null) {
-      this._socket.setEncoding(encoding);
-    }
+    this._writeStream = fs.createWriteStream('', {
+      fd: term.fd,
+      encoding: encoding ?? undefined,
+      autoClose: false
+    });
 
     // setup
     this._socket.on('error', (err: any) => {
@@ -165,21 +166,77 @@ export class UnixTerminal extends Terminal {
     this._forwardEvents();
   }
 
+  private _writeQueue: (string | Buffer)[] = [];
+
   protected _write(data: string | Buffer): void {
-    // Use the underlying file descriptor to avoid issues with backpressure
-    // handling in net.Socket/tty.*Stream which can result in data loss with
-    // ~1-4kb of data.
-    // Context: https://github.com/microsoft/vscode/issues/283056
-    fs.write(this.fd, data, (err, written) => {
+    // Writes are put in a queue and processed asynchronously in order to handle
+    // backpressure from the kernel buffer.
+    this._writeQueue.push(data);
+    if (this._writeInProgress) {
+      return;
+    }
+    this._writeInProgress = true;
+    this._processWriteQueue();
+  }
+
+  private _writeInProgress: boolean = false;
+  private _writeTimeout: NodeJS.Timeout | undefined;
+
+  private async _processWriteQueue(): Promise<void> {
+    const data = this._writeQueue.shift();
+    if (!data) {
+      this._writeInProgress = false;
+      return;
+    }
+
+    // Write to the underlying file descriptor and handle it directly, rather
+    // than using the `net.Socket`/`tty.WriteStream` wrappers which swallow the
+    // errors and cause the thread to block indefinitely.
+    fs.write(this._fd, data, (err, written) => {
+      console.log('written', written);
       if (err) {
-        console.log('pty write error', err);
+        const errno = (err as any).errno;
+        switch (errno) {
+          case -35: // EAGAIN (macOS)
+          case -11: // EAGAIN (Linux(
+            // This error appears to get swallowed and translated into
+            // `ERR_SYSTEM_ERROR` when using tty.WriteStream and not fs.write
+            // directly.
+            // This can happen during a regular partial write, but the most
+            // reliable way to test this is to run `sleep 10` in the shell and
+            // paste enough data to fill the kernel-level buffer. Once the sleep
+            // ends, the pty should accept the data again. Re-process after a
+            // short break.
+            this._writeTimeout = setTimeout(() => this._processWriteQueue(), 5);
+            return;
+          case -5:  // macOS+Linux EIO
+          case -32: // EPIPE
+            // Stop processing writes immediately as the pty is closed.
+            this._writeInProgress = false;
+            return;
+          default:
+            console.error('Unhandled pty write error', errno, err);
+            // Fall through as it's important to finish processing the queue
+            break;
+        }
       }
-      console.log('written', written);
+
+      // Requeue any partial writes
+      if (written < data.length) {
+        console.log('requeueing unwritten', data.slice(written).length);
+        this._writeQueue.unshift(data.slice(written));
+      }
+
+      // Using `setImmediate` here appears to corrupt the data, this may be what
+      // the interleaving/dropped data comment is about in Node.js' tty module:
+      // https://github.com/nodejs/node/blob/4cac2b94bed4bf02810be054e8f63c0048c66564/lib/tty.js#L106C1-L111C34
+      //
+      // Yielding via `setImmediate` also doesn't seem to drain the buffer much
+      // anyway, so use a short timeout when this happens instead. Note that the `drain`
+      // event does not appear to happen on `net.Socket`/`tty.WriteStream` when
+      // writing to ptys.
+      this._writeTimeout = setTimeout(() => this._processWriteQueue(), 5);
     });
-    // TODO: Understand why this hangs the process
-    // this._writeSocket.write(data, (err) => {
-    //   console.log('err', err);
-    // });
   }
 
   /* Accessors */
@@ -255,6 +312,9 @@ export class UnixTerminal extends Terminal {
     });
 
     this._socket.destroy();
+
+    this._writeStream?.destroy();
+    clearTimeout(this._writeTimeout);
   }
 
   public kill(signal?: string): void {

From 8b2b77bd7959c3eefcee82f6878ffd4f5e40c356 Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 19:36:00 -0800
Subject: [PATCH 04/10] Remove unneeded write stream

---
 src/unixTerminal.ts | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index 5d871acc..f771ceb1 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -36,7 +36,6 @@ export class UnixTerminal extends Terminal {
   private _boundClose: boolean = false;
   private _emittedClose: boolean = false;
 
-  private readonly _writeStream: fs.WriteStream;
   private _master: net.Socket | undefined;
   private _slave: net.Socket | undefined;
 
@@ -105,11 +104,6 @@ export class UnixTerminal extends Terminal {
     const term = pty.fork(file, args, parsedEnv, cwd, this._cols, this._rows, uid, gid, (encoding === 'utf8'), helperPath, onexit);
 
     this._socket = new tty.ReadStream(term.fd);
-    this._writeStream = fs.createWriteStream('', {
-      fd: term.fd,
-      encoding: encoding ?? undefined,
-      autoClose: false
-    });
 
     // setup
     this._socket.on('error', (err: any) => {
@@ -313,7 +307,6 @@ export class UnixTerminal extends Terminal {
 
     this._socket.destroy();
 
-    this._writeStream?.destroy();
     clearTimeout(this._writeTimeout);
   }
 

From 3538d165fc32c2e4e8f23d383606ee50914fec2a Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 19:45:47 -0800
Subject: [PATCH 05/10] Remove unwanted logs

---
 src/unixTerminal.ts | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index f771ceb1..3a516b26 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -187,7 +187,6 @@ export class UnixTerminal extends Terminal {
     // than using the `net.Socket`/`tty.WriteStream` wrappers which swallow the
     // errors and cause the thread to block indefinitely.
     fs.write(this._fd, data, (err, written) => {
-      console.log('written', written);
       if (err) {
         const errno = (err as any).errno;
         switch (errno) {
@@ -217,7 +216,6 @@ export class UnixTerminal extends Terminal {
 
       // Requeue any partial writes
       if (written < data.length) {
-        console.log('requeueing unwritten', data.slice(written).length);
         this._writeQueue.unshift(data.slice(written));
       }
 

From 80b2af767675caa849afd80deabd3c2eb809578d Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Fri, 12 Dec 2025 19:47:12 -0800
Subject: [PATCH 06/10] Tidy up comments

---
 src/unixTerminal.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index 3a516b26..ed9d8a8e 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -191,7 +191,7 @@ export class UnixTerminal extends Terminal {
         const errno = (err as any).errno;
         switch (errno) {
           case -35: // EAGAIN (macOS)
-          case -11: // EAGAIN (Linux(
+          case -11: // EAGAIN (Linux)
             // This error appears to get swallowed and translated into
             // `ERR_SYSTEM_ERROR` when using tty.WriteStream and not fs.write
             // directly.
@@ -202,7 +202,7 @@ export class UnixTerminal extends Terminal {
             // short break.
             this._writeTimeout = setTimeout(() => this._processWriteQueue(), 5);
             return;
-          case -5:  // macOS+Linux EIO
+          case -5:  // EIO
           case -32: // EPIPE
             // Stop processing writes immediately as the pty is closed.
             this._writeInProgress = false;

From 872ff8ec4a7628554cd58c1085797e6aedaaa595 Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Sat, 13 Dec 2025 06:27:48 -0800
Subject: [PATCH 07/10] Add setting of encoding on the read socket back

---
 src/unixTerminal.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index ed9d8a8e..da3ff72a 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -104,6 +104,9 @@ export class UnixTerminal extends Terminal {
     const term = pty.fork(file, args, parsedEnv, cwd, this._cols, this._rows, uid, gid, (encoding === 'utf8'), helperPath, onexit);
 
     this._socket = new tty.ReadStream(term.fd);
+    if (encoding !== null) {
+      this._socket.setEncoding(encoding);
+    }
 
     // setup
     this._socket.on('error', (err: any) => {

From 1674fdfe28bce51eacac1d6f7e60efb0b248ab1c Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Sat, 13 Dec 2025 06:28:51 -0800
Subject: [PATCH 08/10] Move props to top

---
 src/unixTerminal.ts | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index da3ff72a..ce2e89ff 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -36,6 +36,10 @@ export class UnixTerminal extends Terminal {
   private _boundClose: boolean = false;
   private _emittedClose: boolean = false;
 
+  private readonly _writeQueue: (string | Buffer)[] = [];
+  private _writeInProgress: boolean = false;
+  private _writeTimeout: NodeJS.Timeout | undefined;
+
   private _master: net.Socket | undefined;
   private _slave: net.Socket | undefined;
 
@@ -163,8 +167,6 @@ export class UnixTerminal extends Terminal {
     this._forwardEvents();
   }
 
-  private _writeQueue: (string | Buffer)[] = [];
-
   protected _write(data: string | Buffer): void {
     // Writes are put in a queue and processed asynchronously in order to handle
     // backpressure from the kernel buffer.
@@ -176,9 +178,6 @@ export class UnixTerminal extends Terminal {
     this._processWriteQueue();
   }
 
-  private _writeInProgress: boolean = false;
-  private _writeTimeout: NodeJS.Timeout | undefined;
-
   private async _processWriteQueue(): Promise<void> {
     const data = this._writeQueue.shift();
     if (!data) {

From 6bb38ccb06802a7f034e7071f9ae506206274372 Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Sat, 13 Dec 2025 07:47:01 -0800
Subject: [PATCH 09/10] Use os.constants.errno

---
 src/unixTerminal.ts | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index ce2e89ff..c8d20dd6 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -4,6 +4,7 @@
  * Copyright (c) 2018, Microsoft Corporation (MIT License).
  */
 import * as fs from 'fs';
+import * as os from 'os';
 import * as net from 'net';
 import * as path from 'path';
 import * as tty from 'tty';
@@ -192,8 +193,7 @@ export class UnixTerminal extends Terminal {
       if (err) {
         const errno = (err as any).errno;
         switch (errno) {
-          case -35: // EAGAIN (macOS)
-          case -11: // EAGAIN (Linux)
+          case os.constants.errno.EAGAIN:
             // This error appears to get swallowed and translated into
             // `ERR_SYSTEM_ERROR` when using tty.WriteStream and not fs.write
             // directly.
@@ -204,8 +204,8 @@ export class UnixTerminal extends Terminal {
             // short break.
             this._writeTimeout = setTimeout(() => this._processWriteQueue(), 5);
             return;
-          case -5:  // EIO
-          case -32: // EPIPE
+          case os.constants.errno.EIO:
+          case os.constants.errno.EPIPE:
             // Stop processing writes immediately as the pty is closed.
             this._writeInProgress = false;
             return;

From c6b06ed2c21865e468ebce569018076b57f45e2a Mon Sep 17 00:00:00 2001
From: Daniel Imms <2193314+Tyriar@users.noreply.github.com>
Date: Sat, 13 Dec 2025 08:29:08 -0800
Subject: [PATCH 10/10] Revert "Use os.constants.errno"

This reverts commit 6bb38ccb06802a7f034e7071f9ae506206274372.

errno.EAGAIN is coming through as 35 (positive) on macOS, not -35.
---
 src/unixTerminal.ts | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/unixTerminal.ts b/src/unixTerminal.ts
index c8d20dd6..ce2e89ff 100644
--- a/src/unixTerminal.ts
+++ b/src/unixTerminal.ts
@@ -4,7 +4,6 @@
  * Copyright (c) 2018, Microsoft Corporation (MIT License).
  */
 import * as fs from 'fs';
-import * as os from 'os';
 import * as net from 'net';
 import * as path from 'path';
 import * as tty from 'tty';
@@ -193,7 +192,8 @@ export class UnixTerminal extends Terminal {
       if (err) {
         const errno = (err as any).errno;
         switch (errno) {
-          case os.constants.errno.EAGAIN:
+          case -35: // EAGAIN (macOS)
+          case -11: // EAGAIN (Linux)
             // This error appears to get swallowed and translated into
             // `ERR_SYSTEM_ERROR` when using tty.WriteStream and not fs.write
             // directly.
@@ -204,8 +204,8 @@ export class UnixTerminal extends Terminal {
             // short break.
             this._writeTimeout = setTimeout(() => this._processWriteQueue(), 5);
             return;
-          case os.constants.errno.EIO:
-          case os.constants.errno.EPIPE:
+          case -5:  // EIO
+          case -32: // EPIPE
             // Stop processing writes immediately as the pty is closed.
             this._writeInProgress = false;
             return;