Implement a Simplified DNS Resolver

Implement a simplified DNS resolver in Python. You are given an in-memory DNS zone and must complete a small resolver step by step. The goal is not to build a real network DNS server, but to correctly handle name normalization, aliases, fallback behavior, errors, cycles, caching, and safe concurrent access. ### Data model A DNS zone is represented as a dictionary from domain name to a record object: ```python zone = { "example.com": {"A": ["93.184.216.34"]}, "www.example.com": {"CNAME": "example.com"}, "api.example.com": {"A": ["10.0.0.5", "10.0.0.6"]}, } ``` Each record may contain: - `A`: a list of IPv4 address strings. - `CNAME`: a single alias target domain. Assume a record will not intentionally contain both `A` and `CNAME`, but your code should handle malformed records gracefully. ### Required API Implement: ```python class DNSResolver: def __init__(self, zone: dict, fallback=None): pass def resolve(self, domain: str) -> list[str]: pass ``` `fallback`, if provided, is another object with a compatible `resolve(domain: str) -> list[str]` method. ### Requirements 1. **Normalize domain names** - Domain lookup should be case-insensitive. - Ignore leading and trailing whitespace. - Treat a trailing dot as optional. - For example, `" WWW.Example.COM. "` should resolve the same way as `"www.example.com"`. 2. **Resolve direct A records** - If a domain has an `A` record, return its list of IP addresses in stored order. - Return a new list so callers cannot mutate internal resolver state. 3. **Resolve aliases** - If a domain has a `CNAME`, follow the alias chain until an `A` record is found. - For example, if `www.example.com -> example.com` and `example.com` has an `A` record, resolving `www.example.com` should return the IPs for `example.com`. 4. **Handle missing records** - If the domain is not found locally and no fallback resolver exists, raise `LookupError`. - If a fallback resolver exists, delegate to it. 5. **Handle invalid records** - If a record has neither `A` nor `CNAME`, raise `ValueError`. - If a record has a malformed `A` value, such as a string instead of a list, raise `ValueError`. - If a record has a malformed `CNAME` value, raise `ValueError`. 6. **Detect alias cycles** - If aliases form a cycle, raise `RuntimeError`. - Example: `a.com -> b.com -> c.com -> a.com`. 7. **Add caching** - Cache successful local resolutions so repeated calls avoid recomputing the same alias chain. - The cache key should use the normalized domain name. - Do not cache failures unless explicitly documented. - Returned cached lists must still be safe from caller mutation. 8. **Support concurrent calls** - Multiple threads may call `resolve` at the same time. - Ensure the cache and other mutable internal state remain consistent. - Avoid holding locks while calling the fallback resolver if possible. ### Examples ```python zone = { "example.com": {"A": ["93.184.216.34"]}, "www.example.com": {"CNAME": "example.com"}, "loop1.com": {"CNAME": "loop2.com"}, "loop2.com": {"CNAME": "loop1.com"}, } resolver = DNSResolver(zone) resolver.resolve(" example.COM. ") # returns ["93.184.216.34"] resolver.resolve("www.example.com") # returns ["93.184.216.34"] resolver.resolve("missing.com") # raises LookupError resolver.resolve("loop1.com") # raises RuntimeError ``` Design the implementation so that it can be tested incrementally with unit tests for each requirement.