Fix io.stat parsing

Summary:
Don't fail `io.stat` parsing due to lines without kv pairs.
Example:
```
$ cat /sys/fs/cgroup/io.stat
7:7
7:6
7:5
7:4
7:3
7:2
7:1
7:0
253:0 rbytes=40137493143552 wbytes=11685175186432 rios=172066882 wios=73548256 dbytes=17052408471552 dios=36700731
```
Kernel logic: https://github.com/torvalds/linux/blob/075dbe9f6e3c21596c5245826a4ee1f1c1676eb8/block/blk-cgroup.c#L1171

Reviewed By: dschatzberg

Differential Revision: D63488396

fbshipit-source-id: a5421e1105ef984c6f4a88cf79773d4f01af7bde

below/cgroupfs/src/lib.rs

@@ -777,28 +777,32 @@ macro_rules! name_key_equal_value_format {
                         r.io_error(file_name.clone(), e)
                     })?;
                     // as an example, io.stat looks like:
+                    // 7:0
                     // 253:0 rbytes=531745786880 wbytes=1623798909952 ...
-                    let mut items = line.split_ascii_whitespace();
+                    let mut items = line.split_terminator(' ').peekable();
+                    let Some(first_item) = items.next() else {
+                      continue
+                    };
+                    // Skip lines with no kv pairs
+                    if items.peek().is_none() {
+                      continue
+                    }
                     let mut s = $struct::default();
-                    if let Some(first_item) = items.next() {
-                        for item in items {
-                            let mut kv = item.splitn(2, "=");
-                            let key = kv.next().ok_or_else(|| r.invalid_file_format(file_name.clone()))?;
-                            let value = kv.next().ok_or_else(|| r.invalid_file_format(file_name.clone()))?;
-                            // Certain keys such as cost.usage cannot be struct fields so must use cost_usage
-                            let key = key.replace(".", "_");
-                            parse_and_set_fields!(
-                                s;
-                                key.as_ref();
-                                value.parse().map_err(|_| r.unexpected_line(file_name.clone(), line.clone()))?;
-                                [ $($field,)* ]
-                            )
-                        }
-                        if s == $struct::default() {
-                            return Err(r.invalid_file_format(file_name));
-                        }
-                        map.insert(first_item.to_string(), s);
+                    for item in items {
+                        let (key, value) = item.split_once('=').ok_or_else(|| r.invalid_file_format(file_name.clone()))?;
+                        // Certain keys such as cost.usage cannot be struct fields so must use cost_usage
+                        let key = key.replace(".", "_");
+                        parse_and_set_fields!(
+                            s;
+                            key.as_ref();
+                            value.parse().map_err(|_| r.unexpected_line(file_name.clone(), line.clone()))?;
+                            [ $($field,)* ]
+                        )
+                    }
+                    if s == $struct::default() {
+                        return Err(r.invalid_file_format(file_name));
                     }
+                    map.insert(first_item.to_string(), s);
                 }
                 if !$allows_empty.0 && map.is_empty() {
                     Err(r.invalid_file_format(file_name))

below/cgroupfs/src/test.rs

@@ -436,7 +436,7 @@ fn test_cpu_stat_invalid_format() {
 #[test]
 fn test_io_stat_success() {
     let cgroup = TestCgroup::new();
-    cgroup.create_file_with_content("io.stat", b"253:0 rbytes=531 wbytes=162379 rios=61 wios=81 dbytes=0 dios=0\n13:0 rbytes=135 wbytes=162379 rios=61 wios=81 dbytes=0 dios=0 cost.usage=25 cost.wait=38 cost.indebt=64 cost.indelay=0\n");
+    cgroup.create_file_with_content("io.stat", b"7:0 \n253:0 rbytes=531 wbytes=162379 rios=61 wios=81 dbytes=0 dios=0\n13:0 rbytes=135 wbytes=162379 rios=61 wios=81 dbytes=0 dios=0 cost.usage=25 cost.wait=38 cost.indebt=64 cost.indelay=0\n");
 
     let cgroup_reader = cgroup.get_reader();
     let val = cgroup_reader