net: context plumbing, add Dialer.DialContext

For #12580 (http.Transport tracing/analytics)
Updates #13021

Change-Id: I126e494a7bd872e42c388ecb58499ecbf0f014cc
Reviewed-on: https://go-review.googlesource.com/22101
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

1 files changed, 15 insertions, 26 deletions

diff --git a/src/net/lookup.go b/src/net/lookup.go
index ab6886ddff..0d3ef79bab 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -5,8 +5,8 @@
 package net
 
 import (
+	"context"
 	"internal/singleflight"
-	"time"
 )
 
 // protocols contains minimal mappings between internet protocol
@@ -33,7 +33,7 @@ func LookupHost(host string) (addrs []string, err error) {
 	if ip := ParseIP(host); ip != nil {
 		return []string{host}, nil
 	}
-	return lookupHost(host)
+	return lookupHost(context.Background(), host)
 }
 
 // LookupIP looks up host using the local resolver.
@@ -47,7 +47,7 @@ func LookupIP(host string) (ips []IP, err error) {
 	if ip := ParseIP(host); ip != nil {
 		return []IP{ip}, nil
 	}
-	addrs, err := lookupIPMerge(host)
+	addrs, err := lookupIPMerge(context.Background(), host)
 	if err != nil {
 		return
 	}
@@ -63,9 +63,9 @@ var lookupGroup singleflight.Group
 // lookupIPMerge wraps lookupIP, but makes sure that for any given
 // host, only one lookup is in-flight at a time. The returned memory
 // is always owned by the caller.
-func lookupIPMerge(host string) (addrs []IPAddr, err error) {
+func lookupIPMerge(ctx context.Context, host string) (addrs []IPAddr, err error) {
 	addrsi, err, shared := lookupGroup.Do(host, func() (interface{}, error) {
-		return testHookLookupIP(lookupIP, host)
+		return testHookLookupIP(ctx, lookupIP, host)
 	})
 	return lookupIPReturn(addrsi, err, shared)
 }
@@ -85,37 +85,26 @@ func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error
 	return addrs, nil
 }
 
-// lookupIPDeadline looks up a hostname with a deadline.
-func lookupIPDeadline(host string, deadline time.Time) (addrs []IPAddr, err error) {
-	if deadline.IsZero() {
-		return lookupIPMerge(host)
-	}
-
-	// We could push the deadline down into the name resolution
-	// functions. However, the most commonly used implementation
-	// calls getaddrinfo, which has no timeout.
-
-	timeout := deadline.Sub(time.Now())
-	if timeout <= 0 {
-		return nil, errTimeout
-	}
-	t := time.NewTimer(timeout)
-	defer t.Stop()
+// lookupIPContext looks up a hostname with a context.
+func lookupIPContext(ctx context.Context, host string) (addrs []IPAddr, err error) {
+	// TODO(bradfitz): when adding trace hooks later here, make
+	// sure the tracing is done outside of the singleflight
+	// merging. Both callers should see the DNS lookup delay, even
+	// if it's only being done once. The r.Shared bit can be
+	// included in the trace for callers who need it.
 
 	ch := lookupGroup.DoChan(host, func() (interface{}, error) {
-		return testHookLookupIP(lookupIP, host)
+		return testHookLookupIP(ctx, lookupIP, host)
 	})
 
 	select {
-	case <-t.C:
+	case <-ctx.Done():
 		// The DNS lookup timed out for some reason. Force
 		// future requests to start the DNS lookup again
 		// rather than waiting for the current lookup to
 		// complete. See issue 8602.
 		lookupGroup.Forget(host)
-
-		return nil, errTimeout
-
+		return nil, mapErr(ctx.Err())
 	case r := <-ch:
 		return lookupIPReturn(r.Val, r.Err, r.Shared)
 	}