From 0a5f3fa0a4eebde09c0a7b37841163d812dff5d0 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Tue, 4 Jun 2024 15:15:11 -0700
Subject: [PATCH] os: add linux API for getting cgroup by pid

---
 src/os/linux.zig | 28 ++++++++++++++++++++++++++++
 src/os/main.zig  |  1 +
 2 files changed, 29 insertions(+)
 create mode 100644 src/os/linux.zig

diff --git a/src/os/linux.zig b/src/os/linux.zig
new file mode 100644
index 000000000..e399883dc
--- /dev/null
+++ b/src/os/linux.zig
@@ -0,0 +1,28 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+
+/// Returns the path to the cgroup for the given pid.
+pub fn cgroupPath(alloc: Allocator, pid: std.os.linux.pid_t) !?[]const u8 {
+    var buf: [std.fs.MAX_PATH_BYTES]u8 = undefined;
+
+    // Read our cgroup by opening /proc/<pid>/cgroup and reading the first
+    // line. The first line will look something like this:
+    // 0::/user.slice/user-1000.slice/session-1.scope
+    // The cgroup path is the third field.
+    const path = try std.fmt.bufPrint(&buf, "/proc/{}/cgroup", .{pid});
+    const file = try std.fs.cwd().openFile(path, .{});
+    defer file.close();
+
+    // Read it all into memory -- we don't expect this file to ever be that large.
+    var buf_reader = std.io.bufferedReader(file.reader());
+    const contents = try buf_reader.reader().readAllAlloc(
+        alloc,
+        1 * 1024 * 1024, // 1MB
+    );
+    defer alloc.free(contents);
+
+    // Find the last ':'
+    const idx = std.mem.lastIndexOfScalar(u8, contents, ':') orelse return null;
+    const result = std.mem.trimRight(u8, contents[idx + 1 ..], " \r\n");
+    return try alloc.dupe(u8, result);
+}
diff --git a/src/os/main.zig b/src/os/main.zig
index 1782601e0..e3c3ef595 100644
--- a/src/os/main.zig
+++ b/src/os/main.zig
@@ -13,6 +13,7 @@ pub usingnamespace @import("open.zig");
 pub usingnamespace @import("pipe.zig");
 pub usingnamespace @import("resourcesdir.zig");
 pub const TempDir = @import("TempDir.zig");
+pub const linux = @import("linux.zig");
 pub const passwd = @import("passwd.zig");
 pub const xdg = @import("xdg.zig");
 pub const windows = @import("windows.zig");