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>

net/dns/manager_windows.go

@@ -7,10 +7,8 @@ package dns
 import (
 	"errors"
 	"fmt"
-	"os"
 	"os/exec"
 	"sort"
-	"strconv"
 	"strings"
 	"syscall"
 	"time"
@@ -19,6 +17,7 @@ import (
 	"golang.org/x/sys/windows/registry"
 	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 	"inet.af/netaddr"
+	"tailscale.com/envknob"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
 )
@@ -36,7 +35,7 @@ const (
 	versionKey = `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
 )
 
-var configureWSL, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_CONFIGURE_WSL"))
+var configureWSL = envknob.Bool("TS_DEBUG_CONFIGURE_WSL")
 
 type windowsManager struct {
 	logf       logger.Logf

net/dnscache/dnscache.go

@@ -15,14 +15,13 @@ import (
 	"fmt"
 	"log"
 	"net"
-	"os"
 	"runtime"
-	"strconv"
 	"sync"
 	"time"
 
 	"golang.org/x/sync/singleflight"
 	"inet.af/netaddr"
+	"tailscale.com/envknob"
 )
 
 var single = &Resolver{
@@ -100,7 +99,7 @@ func (r *Resolver) ttl() time.Duration {
 	return 10 * time.Minute
 }
 
-var debug, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_DNS_CACHE"))
+var debug = envknob.Bool("TS_DEBUG_DNS_CACHE")
 
 // LookupIP returns the host's primary IP address (either IPv4 or
 // IPv6, but preferring IPv4) and optionally its IPv6 address, if

net/netcheck/netcheck.go

@@ -16,16 +16,15 @@ import (
 	"log"
 	"net"
 	"net/http"
-	"os"
 	"runtime"
 	"sort"
-	"strconv"
 	"sync"
 	"time"
 
 	"github.com/tcnksm/go-httpstat"
 	"inet.af/netaddr"
 	"tailscale.com/derp/derphttp"
+	"tailscale.com/envknob"
 	"tailscale.com/net/interfaces"
 	"tailscale.com/net/neterror"
 	"tailscale.com/net/netns"
@@ -40,7 +39,7 @@ import (
 
 // Debugging and experimentation tweakables.
 var (
-	debugNetcheck, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_NETCHECK"))
+	debugNetcheck = envknob.Bool("TS_DEBUG_NETCHECK")
 )
 
 // The various default timeouts for things.

net/tlsdial/tlsdial.go

@@ -17,10 +17,11 @@ import (
 	"errors"
 	"log"
 	"os"
-	"strconv"
 	"sync"
 	"sync/atomic"
 	"time"
+
+	"tailscale.com/envknob"
 )
 
 var counterFallbackOK int32 // atomic
@@ -31,7 +32,7 @@ var counterFallbackOK int32 // atomic
 // See https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format
 var sslKeyLogFile = os.Getenv("SSLKEYLOGFILE")
 
-var debug, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_TLS_DIAL"))
+var debug = envknob.Bool("TS_DEBUG_TLS_DIAL")
 
 // Config returns a tls.Config for connecting to a server.
 // If base is non-nil, it's cloned as the base config before

net/tstun/tun.go

@@ -11,13 +11,12 @@ package tstun
 
 import (
 	"errors"
-	"os"
 	"runtime"
-	"strconv"
 	"strings"
 	"time"
 
 	"golang.zx2c4.com/wireguard/tun"
+	"tailscale.com/envknob"
 	"tailscale.com/types/logger"
 )
 
@@ -32,7 +31,7 @@ import (
 var tunMTU = 1280
 
 func init() {
-	if mtu, _ := strconv.Atoi(os.Getenv("TS_DEBUG_MTU")); mtu != 0 {
+	if mtu, ok := envknob.LookupInt("TS_DEBUG_MTU"); ok {
 		tunMTU = mtu
 	}
 }