scan mountinfo when hardcoded cgroupv1 mountpoints don't work

library/std/src/sys/unix/thread.rs

@@ -381,19 +381,27 @@ pub fn available_parallelism() -> io::Result<NonZeroUsize> {
 
 #[cfg(any(target_os = "android", target_os = "linux"))]
 mod cgroups {
+    //! Currently not covered
+    //! * cgroup v2 in non-standard mountpoints
+    //! * paths containing control characters or spaces, since those would be escaped in procfs
+    //!   output and we don't unescape
+    use crate::borrow::Cow;
     use crate::ffi::OsString;
     use crate::fs::{try_exists, File};
     use crate::io::Read;
+    use crate::io::{BufRead, BufReader};
     use crate::os::unix::ffi::OsStringExt;
+    use crate::path::Path;
     use crate::path::PathBuf;
     use crate::str::from_utf8;
 
+    #[derive(PartialEq)]
     enum Cgroup {
         V1,
         V2,
     }
 
-    /// Returns cgroup CPU quota in core-equivalents, rounded down, or usize::MAX if the quota cannot
+    /// Returns cgroup CPU quota in core-equivalents, rounded down or usize::MAX if the quota cannot
     /// be determined or is not set.
     pub(super) fn quota() -> usize {
         let mut quota = usize::MAX;
@@ -407,27 +415,30 @@ mod cgroups {
             let mut buf = Vec::with_capacity(128);
             // find our place in the cgroup hierarchy
             File::open("/proc/self/cgroup").ok()?.read_to_end(&mut buf).ok()?;
-            let (cgroup_path, version) = buf
-                .split(|&c| c == b'\n')
-                .filter_map(|line| {
+            let (cgroup_path, version) =
+                buf.split(|&c| c == b'\n').fold(None, |previous, line| {
                     let mut fields = line.splitn(3, |&c| c == b':');
                     // 2nd field is a list of controllers for v1 or empty for v2
                     let version = match fields.nth(1) {
-                        Some(b"") => Some(Cgroup::V2),
+                        Some(b"") => Cgroup::V2,
                         Some(controllers)
                             if from_utf8(controllers)
                                 .is_ok_and(|c| c.split(",").any(|c| c == "cpu")) =>
                         {
-                            Some(Cgroup::V1)
+                            Cgroup::V1
                         }
-                        _ => None,
-                    }?;
+                        _ => return previous,
+                    };
+
+                    // already-found v1 trumps v2 since it explicitly specifies its controllers
+                    if previous.is_some() && version == Cgroup::V2 {
+                        return previous;
+                    }
 
                     let path = fields.last()?;
                     // skip leading slash
                     Some((path[1..].to_owned(), version))
-                })
-                .next()?;
+                })?;
             let cgroup_path = PathBuf::from(OsString::from_vec(cgroup_path));
 
             quota = match version {
@@ -493,31 +504,38 @@ mod cgroups {
         let mut read_buf = String::with_capacity(20);
 
         // Hardcode commonly used locations mentioned in the cgroups(7) manpage
-        // since scanning mountinfo can be expensive on some systems.
-        // This isn't exactly standardized since cgroupv1 was meant to allow flexibly
-        // mixing and matching controller hierarchies.
-        let mounts = ["/sys/fs/cgroup/cpu", "/sys/fs/cgroup/cpu,cpuacct"];
+        // if that doesn't work scan mountinfo and adjust `group_path` for bind-mounts
+        let mounts: &[fn(&Path) -> Option<(_, &Path)>] = &[
+            |p| Some((Cow::Borrowed("/sys/fs/cgroup/cpu"), p)),
+            |p| Some((Cow::Borrowed("/sys/fs/cgroup/cpu,cpuacct"), p)),
+            // this can be expensive on systems with tons of mountpoints
+            // but we only get to this point when /proc/self/cgroups explicitly indicated
+            // this process belongs to a cpu-controller cgroup v1 and the defaults didn't work
+            find_mountpoint,
+        ];
 
         for mount in mounts {
+            let Some((mount, group_path)) = mount(&group_path) else { continue };
+
             path.clear();
-            path.push(mount);
+            path.push(mount.as_ref());
             path.push(&group_path);
 
             // skip if we guessed the mount incorrectly
             if matches!(try_exists(&path), Err(_) | Ok(false)) {
                 continue;
             }
 
-            while path.starts_with(mount) {
+            while path.starts_with(mount.as_ref()) {
                 let mut parse_file = |name| {
                     path.push(name);
                     read_buf.clear();
 
-                    let mut f = File::open(&path).ok()?;
-                    f.read_to_string(&mut read_buf).ok()?;
+                    let f = File::open(&path);
+                    path.pop(); // restore buffer before any early returns
+                    f.ok()?.read_to_string(&mut read_buf).ok()?;
                     let parsed = read_buf.trim().parse::<usize>().ok()?;
 
-                    path.pop();
                     Some(parsed)
                 };
 
@@ -531,10 +549,56 @@ mod cgroups {
 
                 path.pop();
             }
+
+            // we passed the try_exists above so we should have traversed the correct hierarchy
+            // when reaching this line
+            break;
         }
 
         quota
     }
+
+    /// Scan mountinfo for cgroup v1 mountpoint with a cpu controller
+    ///
+    /// If the cgroupfs is a bind mount then `group_path` is adjusted to skip
+    /// over the already-included prefix
+    fn find_mountpoint(group_path: &Path) -> Option<(Cow<'static, str>, &Path)> {
+        let mut reader = BufReader::new(File::open("/proc/self/mountinfo").ok()?);
+        let mut line = String::with_capacity(256);
+        loop {
+            line.clear();
+            if reader.read_line(&mut line).ok()? == 0 {
+                break;
+            }
+
+            let line = line.trim();
+            let mut items = line.split(' ');
+
+            let sub_path = items.nth(3)?;
+            let mount_point = items.next()?;
+            let mount_opts = items.next_back()?;
+            let filesystem_type = items.nth_back(1)?;
+
+            if filesystem_type != "cgroup" || !mount_opts.split(',').any(|opt| opt == "cpu") {
+                // not a cgroup / not a cpu-controller
+                continue;
+            }
+
+            let sub_path = Path::new(sub_path).strip_prefix("/").ok()?;
+
+            if !group_path.starts_with(sub_path) {
+                // this is a bind-mount and the bound subdirectory
+                // does not contain the cgroup this process belongs to
+                continue;
+            }
+
+            let trimmed_group_path = group_path.strip_prefix(sub_path).ok()?;
+
+            return Some((Cow::Owned(mount_point.to_owned()), trimmed_group_path));
+        }
+
+        None
+    }
 }
 
 #[cfg(all(