feat: read the available column from df

Free space is read rather than derived: size - used overstates it by the
root-reserved blocks, tens of gigabytes on a large filesystem, none of it
available to anyone but root.

Also asserts fs[3], the row whose device name ends in a digit. It sits next
to a numeric column and is the row most likely to break a parser counting
fields, and nothing covered it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/data.lua b/lib/data.lua
index d5203a8..98c6898 100644
--- a/lib/data.lua
+++ b/lib/data.lua
@@ -219,7 +219,8 @@ function M.df_parse(text)
   for line in text:gmatch('[^\n]+') do
     -- A data row ends in 'NN% /some/path'. The header ends in 'Mounted on',
     -- which fails the percent match, so it is skipped without a special case.
-    local size, used, pct, mount = line:match('(%d+)%s+(%d+)%s+%d+%s+(%d+)%%%s+(%S+)%s*$')
+    local size, used, avail, pct, mount =
+      line:match('(%d+)%s+(%d+)%s+(%d+)%s+(%d+)%%%s+(%S+)%s*$')
     if size then
       -- The device is kept so a caller can tell an NFS mount from a local one
       -- and recover its server, which is how the disks card labels the two
@@ -231,6 +232,10 @@ function M.df_parse(text)
         host  = dev and dev:match('^([^/:]+):') or nil,
         size  = tonumber(size),
         used  = tonumber(used),
+        -- Read, not derived: size - used overstates free space by the
+        -- root-reserved blocks, which is tens of gigabytes on a large
+        -- filesystem and is not available to anyone but root.
+        avail = tonumber(avail),
         pct   = tonumber(pct),
       }
     end