internal/godebug: define more efficient API

We have been expanding our use of GODEBUG for compatibility,
and the current implementation forces a tradeoff between
freshness and efficiency. It parses the environment variable
in full each time it is called, which is expensive. But if clients
cache the result, they won't respond to run-time GODEBUG
changes, as happened with x509sha1 (#56436).

This CL changes the GODEBUG API to provide efficient,
up-to-date results. Instead of a single Get function,
New returns a *godebug.Setting that itself has a Get method.
Clients can save the result of New, which is no more expensive
than errors.New, in a global variable, and then call that
variable's Get method to get the value. Get costs only two
atomic loads in the case where the variable hasn't changed
since the last call.

Unfortunately, these changes do require importing sync
from godebug, which will mean that sync itself will never
be able to use a GODEBUG setting. That doesn't seem like
such a hardship. If it was really necessary, the runtime could
pass a setting to package sync itself at startup, with the
caveat that that setting, like the ones used by runtime itself,
would not respond to run-time GODEBUG changes.

Change-Id: I99a3acfa24fb2a692610af26a5d14bbc62c966ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/449504
Run-TryBot: Russ Cox <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

src/cmd/go/go_test.go

@@ -2348,9 +2348,11 @@ func TestUpxCompression(t *testing.T) {
 	}
 }
 
+var gocacheverify = godebug.New("gocacheverify")
+
 func TestCacheListStale(t *testing.T) {
 	tooSlow(t)
-	if godebug.Get("gocacheverify") == "1" {
+	if gocacheverify.Value() == "1" {
 		t.Skip("GODEBUG gocacheverify")
 	}
 	tg := testgo(t)
@@ -2373,7 +2375,7 @@ func TestCacheListStale(t *testing.T) {
 func TestCacheCoverage(t *testing.T) {
 	tooSlow(t)
 
-	if godebug.Get("gocacheverify") == "1" {
+	if gocacheverify.Value() == "1" {
 		t.Skip("GODEBUG gocacheverify")
 	}
 
@@ -2407,7 +2409,7 @@ func TestIssue22588(t *testing.T) {
 
 func TestIssue22531(t *testing.T) {
 	tooSlow(t)
-	if godebug.Get("gocacheverify") == "1" {
+	if gocacheverify.Value() == "1" {
 		t.Skip("GODEBUG gocacheverify")
 	}
 	tg := testgo(t)
@@ -2436,7 +2438,7 @@ func TestIssue22531(t *testing.T) {
 
 func TestIssue22596(t *testing.T) {
 	tooSlow(t)
-	if godebug.Get("gocacheverify") == "1" {
+	if gocacheverify.Value() == "1" {
 		t.Skip("GODEBUG gocacheverify")
 	}
 	tg := testgo(t)
@@ -2466,7 +2468,7 @@ func TestIssue22596(t *testing.T) {
 func TestTestCache(t *testing.T) {
 	tooSlow(t)
 
-	if godebug.Get("gocacheverify") == "1" {
+	if gocacheverify.Value() == "1" {
 		t.Skip("GODEBUG gocacheverify")
 	}
 	tg := testgo(t)

src/cmd/go/internal/fsys/fsys.go

@@ -36,27 +36,29 @@ func Trace(op, path string) {
 	traceMu.Lock()
 	defer traceMu.Unlock()
 	fmt.Fprintf(traceFile, "%d gofsystrace %s %s\n", os.Getpid(), op, path)
-	if traceStack != "" {
-		if match, _ := pathpkg.Match(traceStack, path); match {
+	if pattern := gofsystracestack.Value(); pattern != "" {
+		if match, _ := pathpkg.Match(pattern, path); match {
 			traceFile.Write(debug.Stack())
 		}
 	}
 }
 
 var (
-	doTrace    bool
-	traceStack string
-	traceFile  *os.File
-	traceMu    sync.Mutex
+	doTrace   bool
+	traceFile *os.File
+	traceMu   sync.Mutex
+
+	gofsystrace      = godebug.New("gofsystrace")
+	gofsystracelog   = godebug.New("gofsystracelog")
+	gofsystracestack = godebug.New("gofsystracestack")
 )
 
 func init() {
-	if godebug.Get("gofsystrace") != "1" {
+	if gofsystrace.Value() != "1" {
 		return
 	}
 	doTrace = true
-	traceStack = godebug.Get("gofsystracestack")
-	if f := godebug.Get("gofsystracelog"); f != "" {
+	if f := gofsystracelog.Value(); f != "" {
 		// Note: No buffering on writes to this file, so no need to worry about closing it at exit.
 		var err error
 		traceFile, err = os.OpenFile(f, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)

src/cmd/go/internal/modindex/read.go

@@ -37,7 +37,7 @@ import (
 // It will be removed before the release.
 // TODO(matloob): Remove enabled once we have more confidence on the
 // module index.
-var enabled bool = godebug.Get("goindex") != "0"
+var enabled = godebug.New("goindex").Value() != "0"
 
 // Module represents and encoded module index file. It is used to
 // do the equivalent of build.Import of packages in the module and answer other
@@ -368,6 +368,8 @@ func relPath(path, modroot string) string {
 	return str.TrimFilePathPrefix(filepath.Clean(path), filepath.Clean(modroot))
 }
 
+var installgorootAll = godebug.New("installgoroot").Value() == "all"
+
 // Import is the equivalent of build.Import given the information in Module.
 func (rp *IndexPackage) Import(bctxt build.Context, mode build.ImportMode) (p *build.Package, err error) {
 	defer unprotect(protect(), &err)
@@ -436,7 +438,7 @@ func (rp *IndexPackage) Import(bctxt build.Context, mode build.ImportMode) (p *b
 				p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
 
 				// Set the install target if applicable.
-				if !p.Goroot || (strings.EqualFold(godebug.Get("installgoroot"), "all") && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
+				if !p.Goroot || (installgorootAll && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
 					p.PkgObj = ctxt.joinPath(p.Root, pkga)
 				}
 			}

src/crypto/x509/x509.go

@@ -813,6 +813,8 @@ func signaturePublicKeyAlgoMismatchError(expectedPubKeyAlgo PublicKeyAlgorithm,
 	return fmt.Errorf("x509: signature algorithm specifies an %s public key, but have public key of type %T", expectedPubKeyAlgo.String(), pubKey)
 }
 
+var x509sha1 = godebug.New("x509sha1")
+
 // checkSignature verifies that signature is a valid signature over signed from
 // a crypto.PublicKey.
 func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey, allowSHA1 bool) (err error) {
@@ -835,7 +837,7 @@ func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey
 		return InsecureAlgorithmError(algo)
 	case crypto.SHA1:
 		// SHA-1 signatures are mostly disabled. See go.dev/issue/41682.
-		if !allowSHA1 && godebug.Get("x509sha1") != "1" {
+		if !allowSHA1 && x509sha1.Value() != "1" {
 			return InsecureAlgorithmError(algo)
 		}
 		fallthrough

src/go/build/build.go

@@ -521,6 +521,8 @@ func nameExt(name string) string {
 	return name[i:]
 }
 
+var installgoroot = godebug.New("installgoroot")
+
 // Import returns details about the Go package named by the import path,
 // interpreting local import paths relative to the srcDir directory.
 // If the path is a local import path naming a package that can be imported
@@ -783,7 +785,7 @@ Found:
 			p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
 
 			// Set the install target if applicable.
-			if !p.Goroot || (strings.EqualFold(godebug.Get("installgoroot"), "all") && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
+			if !p.Goroot || (installgoroot.Value() == "all" && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
 				p.PkgObj = ctxt.joinPath(p.Root, pkga)
 			}
 		}

src/go/build/deps_test.go

@@ -52,13 +52,10 @@ var depsRules = `
 	internal/goarch, unsafe
 	< internal/abi;
 
-	unsafe
-	< internal/godebug;
-
 	# RUNTIME is the core runtime group of packages, all of them very light-weight.
 	internal/abi, internal/cpu, internal/goarch,
 	internal/coverage/rtcov, internal/goexperiment,
-	internal/goos, internal/godebug, unsafe
+	internal/goos, unsafe
 	< internal/bytealg
 	< internal/itoa
 	< internal/unsafeheader
@@ -70,6 +67,7 @@ var depsRules = `
 	< sync/atomic
 	< internal/race
 	< sync
+	< internal/godebug
 	< internal/reflectlite
 	< errors
 	< internal/oserror, math/bits

src/internal/cpu/cpu_test.go

@@ -48,7 +48,7 @@ func TestDisableAllCapabilities(t *testing.T) {
 func TestAllCapabilitiesDisabled(t *testing.T) {
 	MustHaveDebugOptionsSupport(t)
 
-	if godebug.Get("cpu.all") != "off" {
+	if godebug.New("cpu.all").Value() != "off" {
 		t.Skipf("skipping test: GODEBUG=cpu.all=off not set")
 	}
 

src/internal/cpu/cpu_x86_test.go

@@ -28,7 +28,7 @@ func TestDisableSSE3(t *testing.T) {
 func TestSSE3DebugOption(t *testing.T) {
 	MustHaveDebugOptionsSupport(t)
 
-	if godebug.Get("cpu.sse3") != "off" {
+	if godebug.New("cpu.sse3").Value() != "off" {
 		t.Skipf("skipping test: GODEBUG=cpu.sse3=off not set")
 	}
 

src/internal/fuzz/fuzz.go

@@ -21,7 +21,6 @@ import (
 	"reflect"
 	"runtime"
 	"strings"
-	"sync"
 	"time"
 )
 
@@ -1077,14 +1076,8 @@ var zeroVals []any = []any{
 	uint64(0),
 }
 
-var (
-	debugInfo     bool
-	debugInfoOnce sync.Once
-)
+var debugInfo = godebug.New("fuzzdebug").Value() == "1"
 
 func shouldPrintDebugInfo() bool {
-	debugInfoOnce.Do(func() {
-		debugInfo = godebug.Get("fuzzdebug") == "1"
-	})
 	return debugInfo
 }