From e152ebaeadfd9f364c771a121d6a5bd97bc7b891 Mon Sep 17 00:00:00 2001
From: Daniel Hofstetter <daniel.hofstetter@42dh.com>
Date: Fri, 25 Mar 2022 10:46:13 +0100
Subject: [PATCH] df: fix calculation of Use% column

Change formula from: "Used/Size * 100" to "Used/(Used + Avail) * 100".
This formula also works if "Used" and "Avail" do not add up to "Size",
which is the case if there are reserved disk blocks.
---
 src/uu/df/src/table.rs   | 10 +++++++---
 tests/by-util/test_df.rs | 25 ++++++++-----------------
 2 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/src/uu/df/src/table.rs b/src/uu/df/src/table.rs
index a99adaa6c..39c762711 100644
--- a/src/uu/df/src/table.rs
+++ b/src/uu/df/src/table.rs
@@ -149,24 +149,28 @@ impl From<Filesystem> for Row {
             ffree,
             ..
         } = fs.usage;
+        let bused = blocks - bfree;
         Self {
             file: fs.file,
             fs_device: dev_name,
             fs_type,
             fs_mount: mount_dir,
             bytes: blocksize * blocks,
-            bytes_used: blocksize * (blocks - bfree),
+            bytes_used: blocksize * bused,
             bytes_avail: blocksize * bavail,
             bytes_usage: if blocks == 0 {
                 None
             } else {
-                Some(((blocks - bfree) as f64) / blocks as f64)
+                // We use "(bused + bavail)" instead of "blocks" because on some filesystems (e.g.
+                // ext4) "blocks" also includes reserved blocks we ignore for the usage calculation.
+                // https://www.gnu.org/software/coreutils/faq/coreutils-faq.html#df-Size-and-Used-and-Available-do-not-add-up
+                Some((bused as f64) / (bused + bavail) as f64)
             },
             #[cfg(target_os = "macos")]
             bytes_capacity: if bavail == 0 {
                 None
             } else {
-                Some(bavail as f64 / ((blocks - bfree + bavail) as f64))
+                Some(bavail as f64 / ((bused + bavail) as f64))
             },
             inodes: files,
             inodes_used: files - ffree,
diff --git a/tests/by-util/test_df.rs b/tests/by-util/test_df.rs
index b178c40c0..87fd04312 100644
--- a/tests/by-util/test_df.rs
+++ b/tests/by-util/test_df.rs
@@ -1,4 +1,4 @@
-// spell-checker:ignore udev
+// spell-checker:ignore udev pcent
 use crate::common::util::*;
 
 #[test]
@@ -139,33 +139,24 @@ fn test_total() {
 
 #[test]
 fn test_use_percentage() {
-    // Example output:
-    //
-    //     Filesystem            1K-blocks     Used Available Use% Mounted on
-    //     udev                    3858016        0   3858016   0% /dev
-    //     ...
-    //     /dev/loop14               63488    63488         0 100% /snap/core20/1361
-    let output = new_ucmd!().succeeds().stdout_move_str();
+    let output = new_ucmd!()
+        .args(&["--output=used,avail,pcent"])
+        .succeeds()
+        .stdout_move_str();
 
     // Skip the header line.
     let lines: Vec<&str> = output.lines().skip(1).collect();
 
     for line in lines {
         let mut iter = line.split_whitespace();
-        iter.next();
-        let reported_size = iter.next().unwrap().parse::<f64>().unwrap();
         let reported_used = iter.next().unwrap().parse::<f64>().unwrap();
-        // Skip "Available" column
-        iter.next();
-        if cfg!(target_os = "macos") {
-            // Skip "Capacity" column
-            iter.next();
-        }
+        let reported_avail = iter.next().unwrap().parse::<f64>().unwrap();
         let reported_percentage = iter.next().unwrap();
         let reported_percentage = reported_percentage[..reported_percentage.len() - 1]
             .parse::<u8>()
             .unwrap();
-        let computed_percentage = (100.0 * (reported_used / reported_size)).ceil() as u8;
+        let computed_percentage =
+            (100.0 * (reported_used / (reported_used + reported_avail))).ceil() as u8;
 
         assert_eq!(computed_percentage, reported_percentage);
     }