ipn/local: log OS-specific diagnostic information as JSON (#11700)

There is an undocumented 16KiB limit for text log messages.
However, the limit for JSON messages is 256KiB.
Even worse, logging JSON as text results in significant overhead
since each double quote needs to be escaped.

Instead, use logger.Logf.JSON to explicitly log the info as JSON.

We also modify osdiag to return the information as structured data
rather than implicitly have the package log on our behalf.
This gives more control to the caller on how to log.

Updates #7802

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/osdiag/osdiag_windows.go

@@ -5,10 +5,8 @@ package osdiag
 
 import (
 	"encoding/binary"
-	"encoding/json"
 	"errors"
 	"fmt"
-	"io"
 	"path/filepath"
 	"strings"
 	"unicode/utf16"
@@ -18,7 +16,6 @@ import (
 	"github.com/dblohm7/wingoes/pe"
 	"golang.org/x/sys/windows"
 	"golang.org/x/sys/windows/registry"
-	"tailscale.com/types/logger"
 	"tailscale.com/util/osdiag/internal/wsc"
 	"tailscale.com/util/winutil"
 	"tailscale.com/util/winutil/authenticode"
@@ -34,15 +31,6 @@ const (
 	initialValueBufLen = 80    // large enough to contain a stringified GUID encoded as UTF-16
 )
 
-func logSupportInfo(logf logger.Logf, reason LogSupportInfoReason) {
-	var b strings.Builder
-	if err := getSupportInfo(&b, reason); err != nil {
-		logf("error encoding support info: %v", err)
-		return
-	}
-	logf("%s", b.String())
-}
-
 const (
 	supportInfoKeyModules    = "modules"
 	supportInfoKeyPageFile   = "pageFile"
@@ -51,7 +39,7 @@ const (
 	supportInfoKeyWinsockLSP = "winsockLSP"
 )
 
-func getSupportInfo(w io.Writer, reason LogSupportInfoReason) error {
+func supportInfo(reason LogSupportInfoReason) map[string]any {
 	output := make(map[string]any)
 
 	regInfo, err := getRegistrySupportInfo(registry.LOCAL_MACHINE, []string{winutil.RegPolicyBase, winutil.RegBase})
@@ -86,8 +74,7 @@ func getSupportInfo(w io.Writer, reason LogSupportInfoReason) error {
 		}
 	}
 
-	enc := json.NewEncoder(w)
-	return enc.Encode(output)
+	return output
 }
 
 type getRegistrySupportInfoBufs struct {