internal/buildcfg: extract logic specific to cmd/go

cmd/go/internal/cfg duplicates many of the fields of
internal/buildcfg, but initializes them from a Go environment file in
addition to the usual process environment.

internal/buildcfg doesn't (and shouldn't) know or care about that
environment file, but prior to this CL it exposed hooks for
cmd/go/internal/cfg to write data back to internal/buildcfg to
incorporate information from the file. It also produced quirky
GOEXPERIMENT strings when a non-trivial default was overridden,
seemingly so that 'go env' would produce those same quirky strings in
edge-cases where they are needed.

This change reverses that information flow: internal/buildcfg now
exports a structured type with methods — instead of top-level
functions communicating through global state — so that cmd/go can
utilize its marshaling and unmarshaling functionality without also
needing to write results back into buildcfg package state.

The quirks specific to 'go env' have been eliminated by distinguishing
between the raw GOEXPERIMENT value set by the user (which is what we
should report from 'go env') and the cleaned, canonical equivalent
(which is what we should use in the build cache key).

For #51461.

Change-Id: I4ef5b7c58b1fb3468497649a6d2fb6c19aa06c70
Reviewed-on: https://go-review.googlesource.com/c/go/+/393574
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

10 files changed, 81 insertions, 50 deletions

diff --git a/src/cmd/asm/internal/lex/input.go b/src/cmd/asm/internal/lex/input.go
index e373ae817e..276b4b0dcd 100644
--- a/src/cmd/asm/internal/lex/input.go
+++ b/src/cmd/asm/internal/lex/input.go
@@ -50,7 +50,7 @@ func predefine(defines flags.MultiFlag) map[string]*Macro {
 	// Set macros for GOEXPERIMENTs so we can easily switch
 	// runtime assembly code based on them.
 	if *flags.CompilingRuntime {
-		for _, exp := range buildcfg.EnabledExperiments() {
+		for _, exp := range buildcfg.Experiment.Enabled() {
 			// Define macro.
 			name := "GOEXPERIMENT_" + exp
 			macros[name] = &Macro{
diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index deab3dddd0..77c0e229e5 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -22,6 +22,26 @@ import (
 	"cmd/go/internal/fsys"
 )
 
+// Global build parameters (used during package load)
+var (
+	Goos   = envOr("GOOS", build.Default.GOOS)
+	Goarch = envOr("GOARCH", build.Default.GOARCH)
+
+	ExeSuffix = exeSuffix()
+
+	// ModulesEnabled specifies whether the go command is running
+	// in module-aware mode (as opposed to GOPATH mode).
+	// It is equal to modload.Enabled, but not all packages can import modload.
+	ModulesEnabled bool
+)
+
+func exeSuffix() string {
+	if Goos == "windows" {
+		return ".exe"
+	}
+	return ""
+}
+
 // These are general "build flags" used by build and other commands.
 var (
 	BuildA                 bool   // -a flag
@@ -60,8 +80,6 @@ var (
 	// GoPathError is set when GOPATH is not set. it contains an
 	// explanation why GOPATH is unset.
 	GoPathError string
-
-	GOEXPERIMENT = envOr("GOEXPERIMENT", buildcfg.DefaultGOEXPERIMENT)
 )
 
 func defaultContext() build.Context {
@@ -79,20 +97,15 @@ func defaultContext() build.Context {
 		build.ToolDir = filepath.Join(ctxt.GOROOT, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
 	}
 
-	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
-
 	// Override defaults computed in go/build with defaults
 	// from go environment configuration file, if known.
-	ctxt.GOOS = envOr("GOOS", ctxt.GOOS)
-	ctxt.GOARCH = envOr("GOARCH", ctxt.GOARCH)
+	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
+	ctxt.GOOS = Goos
+	ctxt.GOARCH = Goarch
 
-	// The experiments flags are based on GOARCH, so they may
-	// need to change.  TODO: This should be cleaned up.
-	buildcfg.UpdateExperiments(ctxt.GOOS, ctxt.GOARCH, GOEXPERIMENT)
+	// ToolTags are based on GOEXPERIMENT, which we will parse and
+	// initialize later.
 	ctxt.ToolTags = nil
-	for _, exp := range buildcfg.EnabledExperiments() {
-		ctxt.ToolTags = append(ctxt.ToolTags, "goexperiment."+exp)
-	}
 
 	// The go/build rule for whether cgo is enabled is:
 	//	1. If $CGO_ENABLED is set, respect it.
@@ -137,6 +150,33 @@ func init() {
 	BuildToolchainLinker = func() string { return "missing-linker" }
 }
 
+// Experiment configuration.
+var (
+	// RawGOEXPERIMENT is the GOEXPERIMENT value set by the user.
+	RawGOEXPERIMENT = envOr("GOEXPERIMENT", buildcfg.DefaultGOEXPERIMENT)
+	// CleanGOEXPERIMENT is the minimal GOEXPERIMENT value needed to reproduce the
+	// experiments enabled by RawGOEXPERIMENT.
+	CleanGOEXPERIMENT = RawGOEXPERIMENT
+
+	Experiment    *buildcfg.ExperimentFlags
+	ExperimentErr error
+)
+
+func init() {
+	Experiment, ExperimentErr = buildcfg.ParseGOEXPERIMENT(Goos, Goarch, RawGOEXPERIMENT)
+	if ExperimentErr != nil {
+		return
+	}
+
+	// GOEXPERIMENT is valid, so convert it to canonical form.
+	CleanGOEXPERIMENT = Experiment.String()
+
+	// Add build tags based on the experiments in effect.
+	for _, exp := range Experiment.Enabled() {
+		BuildContext.ToolTags = append(BuildContext.ToolTags, "goexperiment."+exp)
+	}
+}
+
 // An EnvVar is an environment variable Name=Value.
 type EnvVar struct {
 	Name  string
@@ -151,26 +191,6 @@ var OrigEnv []string
 // not CmdEnv.
 var CmdEnv []EnvVar
 
-// Global build parameters (used during package load)
-var (
-	Goarch = BuildContext.GOARCH
-	Goos   = BuildContext.GOOS
-
-	ExeSuffix = exeSuffix()
-
-	// ModulesEnabled specifies whether the go command is running
-	// in module-aware mode (as opposed to GOPATH mode).
-	// It is equal to modload.Enabled, but not all packages can import modload.
-	ModulesEnabled bool
-)
-
-func exeSuffix() string {
-	if Goos == "windows" {
-		return ".exe"
-	}
-	return ""
-}
-
 var envCache struct {
 	once sync.Once
 	m    map[string]string
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index c1adf8cef4..fcabc8d1c7 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -74,7 +74,14 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GOCACHE", Value: cache.DefaultDir()},
 		{Name: "GOENV", Value: envFile},
 		{Name: "GOEXE", Value: cfg.ExeSuffix},
-		{Name: "GOEXPERIMENT", Value: buildcfg.GOEXPERIMENT()},
+
+		// List the raw value of GOEXPERIMENT, not the cleaned one.
+		// The set of default experiments may change from one release
+		// to the next, so a GOEXPERIMENT setting that is redundant
+		// with the current toolchain might actually be relevant with
+		// a different version (for example, when bisecting a regression).
+		{Name: "GOEXPERIMENT", Value: cfg.RawGOEXPERIMENT},
+
 		{Name: "GOFLAGS", Value: cfg.Getenv("GOFLAGS")},
 		{Name: "GOHOSTARCH", Value: runtime.GOARCH},
 		{Name: "GOHOSTOS", Value: runtime.GOOS},
@@ -222,6 +229,9 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 	}
 
 	buildcfg.Check()
+	if cfg.ExperimentErr != nil {
+		base.Fatalf("go: %v", cfg.ExperimentErr)
+	}
 
 	env := cfg.CmdEnv
 	env = append(env, ExtraEnvVars()...)
@@ -374,9 +384,9 @@ func checkBuildConfig(add map[string]string, del map[string]bool) error {
 		}
 	}
 
-	goexperiment, okGOEXPERIMENT := get("GOEXPERIMENT", buildcfg.GOEXPERIMENT(), "")
+	goexperiment, okGOEXPERIMENT := get("GOEXPERIMENT", cfg.RawGOEXPERIMENT, buildcfg.DefaultGOEXPERIMENT)
 	if okGOEXPERIMENT {
-		if _, _, err := buildcfg.ParseGOEXPERIMENT(goos, goarch, goexperiment); err != nil {
+		if _, err := buildcfg.ParseGOEXPERIMENT(goos, goarch, goexperiment); err != nil {
 			return err
 		}
 	}
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index fdc00f95dc..8c169d1643 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -2334,8 +2334,8 @@ func (p *Package) setBuildInfo() {
 			}
 		}
 		appendSetting("GOARCH", cfg.BuildContext.GOARCH)
-		if cfg.GOEXPERIMENT != "" {
-			appendSetting("GOEXPERIMENT", cfg.GOEXPERIMENT)
+		if cfg.RawGOEXPERIMENT != "" {
+			appendSetting("GOEXPERIMENT", cfg.RawGOEXPERIMENT)
 		}
 		appendSetting("GOOS", cfg.BuildContext.GOOS)
 		if key, val := cfg.GetArchEnv(); key != "" && val != "" {
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index 7d3a16c5f5..6d6837aa8a 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -12,7 +12,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"internal/buildcfg"
 	exec "internal/execabs"
 	"internal/lazyregexp"
 	"io"
@@ -320,8 +319,8 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 		key, val := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
-		if goexperiment := buildcfg.GOEXPERIMENT(); goexperiment != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", goexperiment)
+		if cfg.CleanGOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
 		}
 
 		// TODO(rsc): Convince compiler team not to add more magic environment variables,
@@ -1301,8 +1300,8 @@ func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
 		key, val := cfg.GetArchEnv()
 		fmt.Fprintf(h, "%s=%s\n", key, val)
 
-		if goexperiment := buildcfg.GOEXPERIMENT(); goexperiment != "" {
-			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", goexperiment)
+		if cfg.CleanGOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
 		}
 
 		// The linker writes source file paths that say GOROOT_FINAL, but
diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
index e1e2b11dd7..a6174b2ed2 100644
--- a/src/cmd/go/internal/work/gc.go
+++ b/src/cmd/go/internal/work/gc.go
@@ -8,7 +8,6 @@ import (
 	"bufio"
 	"bytes"
 	"fmt"
-	"internal/buildcfg"
 	"io"
 	"log"
 	"os"
@@ -245,7 +244,7 @@ CheckFlags:
 	}
 
 	// TODO: Test and delete these conditions.
-	if buildcfg.Experiment.FieldTrack || buildcfg.Experiment.PreemptibleLoops {
+	if cfg.ExperimentErr != nil || cfg.Experiment.FieldTrack || cfg.Experiment.PreemptibleLoops {
 		canDashC = false
 	}
 
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index c0a1d3ccfc..ed46ed822a 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -190,6 +190,9 @@ func invoke(cmd *base.Command, args []string) {
 	// 'go env' handles checking the build config
 	if cmd != envcmd.CmdEnv {
 		buildcfg.Check()
+		if cfg.ExperimentErr != nil {
+			base.Fatalf("go: %v", cfg.ExperimentErr)
+		}
 	}
 
 	// Set environment (GOOS, GOARCH, etc) explicitly.
diff --git a/src/cmd/internal/objabi/flag.go b/src/cmd/internal/objabi/flag.go
index f75c054fcb..acb2dd59ea 100644
--- a/src/cmd/internal/objabi/flag.go
+++ b/src/cmd/internal/objabi/flag.go
@@ -99,11 +99,11 @@ func (versionFlag) Set(s string) error {
 	if s == "goexperiment" {
 		// test/run.go uses this to discover the full set of
 		// experiment tags. Report everything.
-		p = " X:" + strings.Join(buildcfg.AllExperiments(), ",")
+		p = " X:" + strings.Join(buildcfg.Experiment.All(), ",")
 	} else {
-		// If the enabled experiments differ from the defaults,
+		// If the enabled experiments differ from the baseline,
 		// include that difference.
-		if goexperiment := buildcfg.GOEXPERIMENT(); goexperiment != "" {
+		if goexperiment := buildcfg.Experiment.String(); goexperiment != "" {
 			p = " X:" + goexperiment
 		}
 	}
diff --git a/src/cmd/internal/objabi/util.go b/src/cmd/internal/objabi/util.go
index 6bfa25a5ca..c2f1b204b9 100644
--- a/src/cmd/internal/objabi/util.go
+++ b/src/cmd/internal/objabi/util.go
@@ -22,5 +22,5 @@ const (
 // or link object files that are incompatible with each other. This
 // string always starts with "go object ".
 func HeaderString() string {
-	return fmt.Sprintf("go object %s %s %s X:%s\n", buildcfg.GOOS, buildcfg.GOARCH, buildcfg.Version, strings.Join(buildcfg.EnabledExperiments(), ","))
+	return fmt.Sprintf("go object %s %s %s X:%s\n", buildcfg.GOOS, buildcfg.GOARCH, buildcfg.Version, strings.Join(buildcfg.Experiment.Enabled(), ","))
 }
diff --git a/src/cmd/link/internal/ld/main.go b/src/cmd/link/internal/ld/main.go
index 26f9db8ec4..d13c3ff8b6 100644
--- a/src/cmd/link/internal/ld/main.go
+++ b/src/cmd/link/internal/ld/main.go
@@ -124,7 +124,7 @@ func Main(arch *sys.Arch, theArch Arch) {
 	addstrdata1(ctxt, "internal/buildcfg.defaultGOROOT="+final)
 
 	buildVersion := buildcfg.Version
-	if goexperiment := buildcfg.GOEXPERIMENT(); goexperiment != "" {
+	if goexperiment := buildcfg.Experiment.String(); goexperiment != "" {
 		buildVersion += " X:" + goexperiment
 	}
 	addstrdata1(ctxt, "runtime.buildVersion="+buildVersion)