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, 91 insertions, 2 deletions

diff --git a/rsnix/src/fs.rs b/rsnix/src/fs.rs
index 5fc6d7a..24d8fcd 100644
--- a/rsnix/src/fs.rs
+++ b/rsnix/src/fs.rs
@@ -464,9 +464,98 @@ pub fn write(h: usize, buf: &[u8], pos: u64) -> Result<usize, ()> {
     }
 }
 
+/// Handle of `crash.log` on the crash-log volume, if it exists.  The
+/// crash-log volume is the one labeled CRASHLOG (a dedicated stick that
+/// survives reboot writes), else the first mounted volume.  Resolves the
+/// FAT path (`/disk/<uid>/crash.log`) because Disk nodes hold no cached
+/// children.
+pub fn read_crash_log() -> Option<usize> {
+    let disk_dir = lookup(0, "disk")?;
+    let children = match &nodes()[disk_dir].data {
+        NodeData::Dir(v) => v.clone(),
+        _ => return None,
+    };
+    // Prefer a disk node whose name starts with "CRASHLOG-".
+    let mut first = None;
+    for &c in &children {
+        if nodes()[c].removed || !matches!(nodes()[c].data, NodeData::Disk(_)) {
+            continue;
+        }
+        let name = &nodes()[c].name;
+        let label = name.split_once('-').map(|(l, _)| l).unwrap_or(name);
+        if label.eq_ignore_ascii_case("CRASHLOG") {
+            return resolve(&alloc::format!("/disk/{}/crash.log", name));
+        }
+        if first.is_none() {
+            first = Some(c);
+        }
+    }
+    first.and_then(|disk| {
+        let name = nodes()[disk].name.clone();
+        resolve(&alloc::format!("/disk/{}/crash.log", name))
+    })
+}
+
+/// Persist a crash report to the mounted FAT volume so it survives a reboot
+/// (real hardware has no serial console).  Best-effort: any failure is
+/// ignored — the caller (fault handler) must never fault here; the
+/// in-dump ISR guard catches that case and halts.
+///
+/// Goes straight at the FAT layer (the crash-log volume + its root
+/// cluster) rather than through the fs node table: the fault may have
+/// interrupted the fs mid-operation, and a stale DiskInfo/ACTIVE_VOL
+/// would make node-based writes fail with "no volume at slot N".  The
+/// crash-log volume is the one labeled CRASHLOG (a dedicated stick that
+/// survives reboot writes), else slot 0 (first mounted) as before.
+pub fn write_crash_log(lines: &[&str]) {
+    // Assemble the report on the stack (heap-free — a corrupted heap is
+    // exactly the situation that produces crash reports).
+    let mut text = [0u8; 640];
+    let mut n = 0usize;
+    for line in lines {
+        for &b in line.as_bytes() {
+            if n < text.len() {
+                text[n] = b;
+                n += 1;
+            }
+        }
+        if n < text.len() {
+            text[n] = b'\n';
+            n += 1;
+        }
+    }
+    let Some(vol) = crate::fat::crash_vol() else {
+        crate::serial::write_raw("crashlog: no volume\r\n");
+        return;
+    };
+    // Trace the target so a test can verify the right stick got the log.
+    let label = crate::fat::volume_label(vol).unwrap_or("?");
+    let mut tb = [0u8; 48];
+    let tn = {
+        let mut w = crate::strbuf::FmtBuf::new(&mut tb);
+        let _ = core::fmt::write(&mut w, core::format_args!("crashlog: vol {} ({})\r\n", vol, label));
+        w.len()
+    };
+    crate::serial::write_raw(core::str::from_utf8(&tb[..tn]).unwrap_or("?"));
+    let root = crate::fat::root_cluster(vol);
+    if crate::fat::create(vol, root, "crash.log", false).is_err()
+        && crate::fat::find(vol, root, "crash.log").is_err()
+    {
+        crate::serial::write_raw("crashlog: create failed\r\n");
+        return;
+    }
+    if crate::fat::set_len(vol, root, "crash.log", 0).is_err() {
+        crate::serial::write_raw("crashlog: set_len failed\r\n");
+        return;
+    }
+    match crate::fat::write_file(vol, root, "crash.log", &text[..n], 0) {
+        Ok(_) => crate::serial::write_raw("crashlog: wrote\r\n"),
+        Err(_) => crate::serial::write_raw("crashlog: write failed\r\n"),
+    }
+}
+
 /// Truncate/resize a regular file.
-pub fn set_len(h: usize, len: u64) -> Result<(), ()> {
-    match &mut nodes()[h].data {
+pub fn set_len(h: usize, len: u64) -> Result<(), ()> {    match &mut nodes()[h].data {
         NodeData::Regular(v) => {
             v.resize(len as usize, 0);
             Ok(())