net: report a name that does not exist as a not-found lookup error

[yohimik]

net: report a name that does not exist as a not-found lookup error

Repository tinygo-org/net. Branch upstream-pr/dns-not-found, base main.

What this does

The resolver learns from an NXDOMAIN reply that the name does not exist, but
both layers above it lost that fact.

• dnsLookup in netdev_native.go replaced the error with a generic "no
  address found".
• The Resolver methods in lookup.go built the DNSError from the message
  alone.

A caller that reads DNSError.IsNotFound, to tell "no such host" from "the
resolver did not answer", thus never saw a not-found lookup. A command line
program that must tell a typo in a host name from an offline network cannot do
so.

The change carries the flag out through both layers.

• dnsLookup reports not-found when the A query and the AAAA query both said
  so.
• The Resolver methods copy IsNotFound, IsTimeout and IsTemporary from
  the DNSError of the netdev, keep it as the unwrap target, and take its
  description and not its Error(), which repeats the "lookup " prefix.

Evidence

There is no CI in this repository. Checked by hand on macOS 26.6 arm64 with a
TinyGo build that carries the matching toolchain change. A lookup of a name that
does not exist now returns a DNSError with IsNotFound true, and a lookup
with no reachable resolver returns one with IsNotFound false.

A downstream product ships binaries built with these changes in a production
release. dispat v1.4.0 is published and is not a prerelease. It carries
dispat-tiny-linux-amd64 and dispat-tiny-linux-arm64, built by the fork
release v0.42.0-net.4 from sha256-pinned tarballs and smoke-executed under
binfmt before upload, beside six binaries from the gc toolchain.
https://github.com/yohimik/dispat/releases/tag/services%2Fdispat%2Fv1.4.0

The acceptance record of that repository is committed at
packages/docs/docs/internals/tinygo.md. It reports the net.2 to net.4
acceptance history, an integration suite of 694 rows that passes with 0 failures
and 1 documented skip on darwin, and a size table of 0.58x to 0.63x against the
gc equivalents with TinyGo -opt=z -no-debug against go build -trimpath -ldflags "-s -w". Those figures come from that document. They are not a
measurement of this branch.

The suite of that repository has rows that read IsNotFound to give the user
the right message.

Scope

• Two files. No API change. The behaviour for a lookup that succeeds is the
  same.
• The change is independent of the darwin netdev work and applies to linux as
  it stands.

Related

• Support for x86_64 Linux #28, "Support for x86_64 Linux".

Related pull requests

This change is part of one body of work. Together the changes make programs that use the network and child processes work on hosted linux and macOS. A full CLI was tested end to end with all of them and ships binaries built this way, see dispat v1.4.0 in the evidence section.

In tinygo-org/tinygo

• sync: hand the RWMutex over instead of a re-test of the reader count tinygo#5630 sync, RWMutex hand-over. Bug fix, standalone.
• runtime: deliver os/signal notifications under the threads scheduler tinygo#5631 runtime, os/signal delivery with the threads scheduler. Bug fix, standalone.
• compiler,runtime: call fcntl through a C wrapper on darwin tinygo#5632 compiler and runtime, fcntl on darwin. Bug fix, standalone.
• runtime: implement weak.runtime_makeStrongFromWeak tinygo#5633 runtime, weak.runtime_makeStrongFromWeak. Needed by real crypto/tls.
• os: start processes with posix_spawn tinygo#5634 os, start processes with posix_spawn.
• loader: use the real crypto/tls on hosted linux and darwin tinygo#5635 loader, real crypto/tls on hosted linux and darwin.
• builder: declare the BSD socket API in the darwin libSystem stub tinygo#5636 builder, BSD socket symbols in the darwin libSystem stub.

In this repository

• net/http: follow redirects in the client again #72 net/http, follow redirects in the client again.
• net: report a name that does not exist as a not-found lookup error #73 net, report a name that does not exist as a not-found error.
• net: extend the host netdev to darwin #74 net, extend the host netdev to darwin.
• net/http: give the HTTPS client trust roots on darwin #75 net/http, give the HTTPS client trust roots on darwin.

A merge order that works. The three bug fixes are independent. tinygo-org/tinygo#5633 goes before tinygo-org/tinygo#5635. HTTPS on linux needs only tinygo-org/tinygo#5633 and tinygo-org/tinygo#5635. Full darwin support also needs tinygo-org/tinygo#5636, the net changes and a new src/net submodule pin.

[yohimik]

tinygo-org/net main has not moved since this branch was opened. It is still
70037cf, so the branch needs no rebase and it is current. v0.42.0 of the
toolchain pins src/net at that same commit, so the change applies to the
released toolchain.