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>

2 files changed, 27 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
diff --git a/test/test_data.lua b/test/test_data.lua
index 4def123..74167bf 100644
--- a/test/test_data.lua
+++ b/test/test_data.lua
@@ -110,6 +110,27 @@ assert(fs[1].pct == 84, 'root percent, got ' .. tostring(fs[1].pct))
 assert(fs[1].size == 263174213632, 'root size in bytes, got ' .. tostring(fs[1].size))
 assert(fs[1].used == 208111570944, 'root used, got ' .. tostring(fs[1].used))
 
+-- Available is READ, never derived as size - used: the two differ by the
+-- root-reserved blocks, about 5%, which on this root is some 13GB that exists
+-- but cannot be used. A card claiming that space is free would be lying.
+assert(fs[1].avail == 41746907136, 'root available, got ' .. tostring(fs[1].avail))
+assert(fs[1].size - fs[1].used ~= fs[1].avail,
+  'the fixture must exercise the reserved-block gap, or this assertion proves nothing')
+
+-- fs[3] is /dev/sda1: a device name ENDING IN A DIGIT, next to a numeric
+-- column. A parser counting fields from the left, or matching digits without
+-- anchoring, reads the partition number as a size.
+assert(fs[3].mount == '/data', 'sda1 mount, got ' .. tostring(fs[3].mount))
+assert(fs[3].dev == '/dev/sda1', 'sda1 device, got ' .. tostring(fs[3].dev))
+assert(fs[3].size == 983350091776, 'sda1 size, got ' .. tostring(fs[3].size))
+assert(fs[3].used == 547869650944, 'sda1 used, got ' .. tostring(fs[3].used))
+assert(fs[3].avail == 385453473792, 'sda1 available, got ' .. tostring(fs[3].avail))
+assert(fs[3].pct == 59, 'sda1 percent, got ' .. tostring(fs[3].pct))
+assert(fs[3].host == nil, 'a local device has no host')
+
+-- A full filesystem reports zero available, which is a real reading.
+assert(fs[6].avail == 0, 'a full mount has 0 available, got ' .. tostring(fs[6].avail))
+
 -- The NFS rows are the reason the last field matters: a colon in the device
 -- would break a parser splitting on punctuation.
 assert(fs[4].mount == '/mnt/nfs/Library', 'nfs mount, got ' .. tostring(fs[4].mount))