fix(process): make process.title work properly on Linux

On Linux, process.title should modify the process name visible in
tools like `ps`, `top`, and `htop`. Previously, setting process.title
only updated an internal variable but did not modify the actual process
title that external tools see.

This fix implements process.title properly on Linux by:
1. Storing the original argv buffer location during initialization
2. Overwriting the original argv memory when process.title is set
   (this is how /proc/self/cmdline is populated)
3. Using prctl(PR_SET_NAME) to set the thread name (shown in top/htop)

This matches Node.js behavior, which uses libuv's implementation that
does the same argv memory overwriting technique.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Claude Bot

src/bun.js/node/node_process.zig

@@ -21,7 +21,6 @@ pub fn getTitle(_: *JSGlobalObject, title: *bun.String) callconv(.c) void {
     title.* = bun.String.cloneUTF8(str orelse "bun");
 }
 
-// TODO: https://github.com/nodejs/node/blob/master/deps/uv/src/unix/darwin-proctitle.c
 pub fn setTitle(globalObject: *JSGlobalObject, newvalue: *bun.String) callconv(.c) void {
     defer newvalue.deref();
     title_mutex.lock();
@@ -34,6 +33,12 @@ pub fn setTitle(globalObject: *JSGlobalObject, newvalue: *bun.String) callconv(.
 
     if (bun.cli.Bun__Node__ProcessTitle) |slice| bun.default_allocator.free(slice);
     bun.cli.Bun__Node__ProcessTitle = new_title;
+
+    // On Linux, update the process title in /proc/self/cmdline so that
+    // tools like `ps` and `top` can see it.
+    if (comptime bun.Environment.isLinux) {
+        bun.process_title_info.setTitle(new_title);
+    }
 }
 
 pub fn createArgv0(globalObject: *jsc.JSGlobalObject) callconv(.c) jsc.JSValue {

src/bun.zig

@@ -1916,6 +1916,40 @@ pub const StatFS = switch (Environment.os) {
 
 pub var argv: [][:0]const u8 = &[_][:0]const u8{};
 
+/// Original argv buffer for process title on Linux.
+/// This stores the original argv memory location so we can overwrite it
+/// when setting process.title, which is how `ps` and other tools see the title.
+pub const ProcessTitleInfo = struct {
+    /// Pointer to the start of the original argv buffer
+    buffer: ?[*]u8 = null,
+    /// Total capacity of the argv buffer (all args plus the null terminators)
+    capacity: usize = 0,
+
+    pub fn setTitle(self: *ProcessTitleInfo, title: []const u8) void {
+        if (comptime !Environment.isLinux) return;
+
+        const buf = self.buffer orelse return;
+        const cap = self.capacity;
+        if (cap == 0) return;
+
+        // Copy the title into the original argv buffer
+        const title_len = @min(title.len, cap - 1);
+        @memcpy(buf[0..title_len], title[0..title_len]);
+        // Zero out the rest of the buffer
+        @memset(buf[title_len..cap], 0);
+
+        // Also set the thread name via prctl (limited to 16 bytes including null)
+        // This is what shows up in `top` and `htop` under the thread name column
+        var name_buf: [16]u8 = undefined;
+        const name_len = @min(title.len, 15);
+        @memcpy(name_buf[0..name_len], title[0..name_len]);
+        name_buf[name_len] = 0;
+        _ = std.posix.prctl(.SET_NAME, .{@intFromPtr(&name_buf)}) catch {};
+    }
+};
+
+pub var process_title_info: ProcessTitleInfo = .{};
+
 pub fn appendOptionsEnv(env: []const u8, args: *std.array_list.Managed([:0]const u8), allocator: std.mem.Allocator) !void {
     var i: usize = 0;
     var offset_in_args: usize = 1;
@@ -2032,6 +2066,23 @@ pub fn initArgv(allocator: std.mem.Allocator) !void {
         for (0..argv.len) |i| {
             argv[i] = std.mem.sliceTo(std.os.argv[i], 0);
         }
+
+        // On Linux, store the original argv buffer location so we can
+        // overwrite it when setting process.title. This is how tools
+        // like `ps` see the process title.
+        if (comptime Environment.isLinux) {
+            if (std.os.argv.len > 0) {
+                const first_arg = std.os.argv[0];
+                const last_arg = std.os.argv[std.os.argv.len - 1];
+                // Calculate the total capacity: from first arg to end of last arg
+                const last_arg_slice = std.mem.sliceTo(last_arg, 0);
+                const capacity = @intFromPtr(last_arg) - @intFromPtr(first_arg) + last_arg_slice.len + 1;
+                process_title_info = .{
+                    .buffer = @ptrCast(first_arg),
+                    .capacity = capacity,
+                };
+            }
+        }
     } else if (comptime Environment.isWindows) {
         // Zig's implementation of `std.process.argsAlloc()`on Windows platforms
         // is not reliable, specifically the way it splits the command line string.

test/js/node/process/process.test.js

@@ -102,6 +102,26 @@ it("process.title with UTF-16 characters", () => {
   expect(process.title).toBe("bun");
 });
 
+it.skipIf(!process.platform.startsWith("linux"))(
+  "process.title is visible in /proc/self/cmdline on Linux",
+  async () => {
+    // On Linux, process.title should be visible via /proc/self/cmdline
+    // This is how tools like `ps` and `top` see the process title
+    const fs = await import("fs");
+
+    const testTitle = "bun-test-title-12345";
+    process.title = testTitle;
+    expect(process.title).toBe(testTitle);
+
+    // Read /proc/self/cmdline to verify the title is set
+    const cmdline = fs.readFileSync("/proc/self/cmdline", "utf8");
+    expect(cmdline).toContain(testTitle);
+
+    // Reset title
+    process.title = "bun";
+  },
+);
+
 it("process.chdir() on root dir", () => {
   const cwd = process.cwd();
   try {