v0.5.5: crash reports go to a dedicated CRASHLOG USB stick

The fault handler now writes crash.log to the volume labeled CRASHLOG
(label-selected via fat::crash_vol), falling back to slot 0.  On real
hardware the boot USB may be unwritable, so a second stick labeled
CRASHLOG survives the write; the shell auto-prints the previous crash
report from it at next boot (read_crash_log resolves /disk/<uid>/crash.log
through the FAT path, since Disk nodes hold no cached children).

- fat.rs: volume_label() + crash_vol() (no allocation)
- fs.rs: write_crash_log targets crash_vol(); read_crash_log prefers the
  CRASHLOG disk node; drop now-unused first_disk()
- mkdisk.sh: --label option
- test-fault.sh: boots with a dedicated CRASHLOG stick (mkfs -F 32 -n
  CRASHLOG), verifies crash.log lands there and NOT on the ESP

1 files changed, 30 insertions, 1 deletions

diff --git a/rsnix/src/fat.rs b/rsnix/src/fat.rs
index eda81ed..b7e0da1 100644
--- a/rsnix/src/fat.rs
+++ b/rsnix/src/fat.rs
@@ -71,6 +71,31 @@ pub fn is_mounted(vol: usize) -> bool {
     vols().get(vol).map(|v| v.mounted).unwrap_or(false)
 }
 
+/// The volume's label — the part of its uid before the serial suffix
+/// (uid is "LABEL-XXXXXXXX", label sanitized to [A-Z0-9_], so the first
+/// '-' always terminates it).  No allocation: borrows the uid string.
+pub fn volume_label(vol: usize) -> Option<&'static str> {
+    let uid = &vols().get(vol)?.uid;
+    Some(uid.split_once('-').map(|(l, _)| l).unwrap_or(uid))
+}
+
+/// Index of the crash-log volume: the first volume whose label is
+/// "CRASHLOG" (a dedicated stick survives reboot writes that a boot
+/// device may not accept), else slot 0 (first mounted) as before.
+pub fn crash_vol() -> Option<usize> {
+    let n = volume_count();
+    if n == 0 {
+        return None;
+    }
+    for vol in 0..n {
+        if let Some(label) = volume_label(vol)
+            && label.eq_ignore_ascii_case("CRASHLOG") {
+                return Some(vol);
+            }
+    }
+    Some(0)
+}
+
 /// Re-scan after a disk arrival/removal: unmount volumes whose disk left,
 /// mount every disk that has no mounted volume (which also re-tries
 /// mounts that failed at boot).  Volume slots are tombstoned, never
@@ -175,7 +200,11 @@ fn v() -> &'static mut Volume {
     match vols().get_mut(i) {
         Some(v) => v,
         None => {
-            crate::serial_println!("fat: internal error — no volume at slot {}", i);
+            crate::serial_println!(
+                "fat: internal error — no volume at slot {} (len={})",
+                i,
+                vols().len()
+            );
             crate::interrupts::halt();
         }
     }