os: add linux API for getting cgroup by pid

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);
+}

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");