net: report completed when context is done in cgoLookupIP and cgoLook…

…upPTR

All the Lookup* methods that resolve hostnames eventually call lookupIP
or lookupHost method. When the order is selected to be hostLookupCGO
then lookupHost calls cgoLookupHost which internally calls cgoLookupIP
(the lookupIP directly calls cgoLookupIP).
When we provide a context that is cancelled after cgo call, then the
cgoLookupIP returns completed  == false, which caues the
lookupIP/lookupHost to fallback to the go resolver.
This fallback is unnecessary because our context is already cancelled.

The same thing can happen to LookupAddr.

Change-Id: Ifff7716c461f05d954ef43b5205865103558b410
GitHub-Last-Rev: 2ef2023e8c51cdd251986f79e94aba86a0722230
GitHub-Pull-Request: golang/go#57042
Reviewed-on: https://go-review.googlesource.com/c/go/+/454696
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/net/cgo_unix.go

@@ -222,7 +222,7 @@ func cgoLookupIP(ctx context.Context, network, name string) (addrs []IPAddr, err
  case r := <-result:
  return r.addrs, r.err, true
  case <-ctx.Done():
- return nil, mapErr(ctx.Err()), false
+ return nil, mapErr(ctx.Err()), true
  }
 }
 
@@ -262,7 +262,7 @@ func cgoLookupPTR(ctx context.Context, addr string) (names []string, err error,
  case r := <-result:
  return r.names, r.err, true
  case <-ctx.Done():
- return nil, mapErr(ctx.Err()), false
+ return nil, mapErr(ctx.Err()), true
  }
 }