version: use debug.ReadBuildInfo in CmdName on non-Windows

CmdName was re-opening the running executable and scanning it in
64KiB chunks for the Go modinfo markers on every call. The same
modinfo is already parsed at startup and exposed via
runtime/debug.ReadBuildInfo, so prefer that on non-Windows. Windows
still takes the scanning path because its GUI-binary override keys
off the on-disk executable name.

benchstat of BenchmarkCmdName (Linux, before vs after):

    goos: linux
    goarch: amd64
    pkg: tailscale.com/version
    cpu: Intel(R) Xeon(R) 6975P-C
               │  /tmp/old.txt  │            /tmp/new.txt             │
               │     sec/op     │   sec/op     vs base                │
    CmdName-16   556045.5n ± 1%   825.6n ± 1%  -99.85% (p=0.000 n=10)

               │ /tmp/old.txt  │             /tmp/new.txt             │
               │     B/op      │     B/op      vs base                │
    CmdName-16   64.587Ki ± 0%   1.156Ki ± 0%  -98.21% (p=0.000 n=10)

               │ /tmp/old.txt │            /tmp/new.txt            │
               │  allocs/op   │ allocs/op   vs base                │
    CmdName-16     8.000 ± 0%   7.000 ± 0%  -12.50% (p=0.000 n=10)

Fixes #19486

Change-Id: I925c5e28b64815a602459beb6c8dab8779339a6c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/cmdname.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"path"
 	"runtime"
+	"runtime/debug"
 	"strings"
 )
 
@@ -20,6 +21,15 @@ import (
 // using os.Executable. If os.Executable fails (it shouldn't), then
 // "cmd" is returned.
 func CmdName() string {
+	// On non-Windows, the modinfo embedded in the running binary is
+	// authoritative and avoids re-reading the executable from disk.
+	// Windows needs the executable-name-based GUI override in cmdName,
+	// so it still takes the slower path.
+	if runtime.GOOS != "windows" {
+		if info, ok := debug.ReadBuildInfo(); ok && info.Path != "" {
+			return path.Base(info.Path)
+		}
+	}
 	e, err := os.Executable()
 	if err != nil {
 		return "cmd"

version/version_test.go

@@ -6,6 +6,8 @@ package version_test
 import (
 	"bytes"
 	"os"
+	"path"
+	"runtime/debug"
 	"testing"
 
 	ts "tailscale.com"
@@ -49,3 +51,21 @@ func TestShortAllocs(t *testing.T) {
 		t.Errorf("allocs = %v; want 0", allocs)
 	}
 }
+
+func BenchmarkCmdName(b *testing.B) {
+	b.ReportAllocs()
+	for b.Loop() {
+		_ = version.CmdName()
+	}
+}
+
+func BenchmarkReadBuildInfo(b *testing.B) {
+	b.ReportAllocs()
+	for b.Loop() {
+		info, ok := debug.ReadBuildInfo()
+		if !ok {
+			b.Fatal("ReadBuildInfo failed")
+		}
+		_ = path.Base(info.Path)
+	}
+}