fix(debug): handle mapping edges, stray traps and dying debuggees

Assisted-by: GLM 5.3

debug/disasm_linux_riscv64.go

@@ -14,17 +14,25 @@ import (
 )
 
 // Disassemble decodes the instruction at the given address in the debuggee's
-// memory and returns its text representation and length in bytes.
+// memory and returns its text representation and length in bytes.  The read
+// shrinks from 4 to 2 bytes when the full word crosses into unmapped memory:
+// a compressed instruction at the mapping's end still fits the shorter
+// window, and an instruction can never extend past the mapping that holds it.
 func (s *Session) Disassemble(addr uint64) (string, int, error) {
-	mem, err := s.ReadMemory(addr, 4)
-	if err != nil {
-		return "", 0, err
+	var lastErr error
+	for _, n := range []int{4, 2} {
+		mem, err := s.ReadMemory(addr, n)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		ins, derr := disasm.Decode(arch.RISCV, mem, addr)
+		if derr != nil {
+			return "", 0, derr
+		}
+		return ins.Text, ins.Len, nil
 	}
-	ins, err := disasm.Decode(arch.RISCV, mem, addr)
-	if err != nil {
-		return "", 0, err
-	}
-	return ins.Text, ins.Len, nil
+	return "", 0, lastErr
 }
 
 // DisassembleN decodes up to n instructions starting at addr.