envknob: add new package for all the strconv.ParseBool(os.Getenv(..))

A new package can also later record/report which knobs are checked and
set. It also makes the code cleaner & easier to grep for env knobs.

Change-Id: Id8a123ab7539f1fadbd27e0cbeac79c2e4f09751
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/file.go

@@ -16,7 +16,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"strconv"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -26,6 +25,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/client/tailscale/apitype"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
@@ -148,7 +148,7 @@ func runCp(ctx context.Context, args []string) error {
 				name = filepath.Base(fileArg)
 			}
 
-			if slow, _ := strconv.ParseBool(os.Getenv("TS_DEBUG_SLOW_PUSH")); slow {
+			if envknob.Bool("TS_DEBUG_SLOW_PUSH") {
 				fileContents = &slowReader{r: fileContents}
 			}
 		}

cmd/tailscale/cli/netcheck.go

@@ -13,13 +13,13 @@ import (
 	"io/ioutil"
 	"log"
 	"net/http"
-	"os"
 	"sort"
 	"strings"
 	"time"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"tailscale.com/client/tailscale"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/net/netcheck"
 	"tailscale.com/net/portmapper"
@@ -49,7 +49,7 @@ var netcheckArgs struct {
 
 func runNetcheck(ctx context.Context, args []string) error {
 	c := &netcheck.Client{
-		UDPBindAddr: os.Getenv("TS_DEBUG_NETCHECK_UDP_BIND"),
+		UDPBindAddr: envknob.String("TS_DEBUG_NETCHECK_UDP_BIND"),
 		PortMapper:  portmapper.NewClient(logger.WithPrefix(log.Printf, "portmap: "), nil),
 	}
 	if netcheckArgs.verbose {

cmd/tailscale/depaware.txt

@@ -38,6 +38,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/derp/derphttp                                  from tailscale.com/net/netcheck
    L    tailscale.com/derp/wsconn                                    from tailscale.com/derp/derphttp
         tailscale.com/disco                                          from tailscale.com/derp
+        tailscale.com/envknob                                        from tailscale.com/cmd/tailscale/cli+
         tailscale.com/hostinfo                                       from tailscale.com/net/interfaces
         tailscale.com/ipn                                            from tailscale.com/cmd/tailscale/cli+
         tailscale.com/ipn/ipnstate                                   from tailscale.com/cmd/tailscale/cli+