acpid: allow PCI fd replacement; eagerly init AML symbols on sendfd

Closes the v4.0 plan P3 'acpid pci_fd is not registered' item.

Two related defects:

1. scheme.rs::on_sendfd had a one-shot EINVAL when self.pci_fd was
   already Some. Combined with the lazy AML context init in
   AcpiContext::aml_symbols, this created a permanent failure mode:
   - pcid starts AFTER acpid (per the requires_weak chain)
   - if a /scheme/acpi/symbols request arrives before pcid's sendfd
     reaches acpid, the aml init runs with pci_fd = None and logs
     'pci_fd is not registered' at error level
   - the resulting aml_context state makes subsequent retries fail
     the same way; the next pcid sendfd hits the one-shot EINVAL and
     is rejected
   - aml stays broken for the entire boot

   Fix: allow replacement (warn-and-replace on subsequent sendfd).
   Document the bug history inline so a future agent does not 'clean
   up' the warn log.

2. aml_physmem.rs::AmlPhysMemHandler::new logged the missing fd at
   log::error. Downgrade to log::warn -- the deferred-init path is
   normal during the acpid-then-pcid ordering window and an error
   level is misleading.

3. scheme.rs::on_sendfd now eagerly calls self.ctx.aml_symbols()
   after setting pci_fd, so the symbol cache is built immediately
   rather than on the first consumer request. The error path logs
   at error level (with Debug-formatted detail because AmlError
   does not implement Display), making the failure observable
   rather than silently deferred.

Test note: cargo test -p acpid fails to link on the host because
libredox's redox_munmap_v1 is target-only; this is pre-existing
and unrelated to this change. cargo check -p acpid is clean.

drivers/acpid/src/aml_physmem.rs

@@ -178,7 +178,9 @@ impl AmlPhysMemHandler {
         let pci_fd = if let Some(pci_fd) = pci_fd_opt {
             Some(libredox::Fd::new(pci_fd.raw()))
         } else {
-            log::error!("pci_fd is not registered");
+            log::warn!(
+                "acpid: AmlPhysMemHandler created without PCI fd; AML init will retry once pcid sends it via scheme:acpid sendfd"
+            );
             None
         };
         Self {

drivers/acpid/src/scheme.rs

@@ -1070,10 +1070,27 @@ impl SchemeSync for AcpiScheme<'_, '_> {
         }
         let new_fd = libredox::Fd::new(new_fd);
 
+        // Allow replacement: pcid may resend the fd after a restart,
+        // and the previous one-shot EINVAL left aml init permanently
+        // broken if the first sendfd raced with an early aml request.
         if self.pci_fd.is_some() {
-            return Err(Error::new(EINVAL));
-        } else {
-            self.pci_fd = Some(new_fd);
+            log::warn!(
+                "acpid: replacing previously-registered PCI fd; AML symbol cache will rebuild on next request"
+            );
+        }
+        self.pci_fd = Some(new_fd);
+
+        // Kick aml symbol init now that pci_fd is registered. The
+        // next aml request will either find a working cache or get a
+        // fresh error log here; either way the failure mode is
+        // observable rather than silently deferred to "the next
+        // caller retries".
+        match self.ctx.aml_symbols(self.pci_fd.as_ref()) {
+            Ok(_) => log::info!("acpid: AML symbols initialized on PCI fd registration"),
+            Err(err) => {
+                log::error!("acpid: AML symbol init failed after PCI fd registration");
+                log::error!("acpid: AML error detail: {:?}", err);
+            }
         }
 
         Ok(num_fds)