internal/buildcfg: initialize GOROOT to runtime.GOROOT

In the beginning the Go compiler was in C, and C had a function
'getgoroot' that returned GOROOT from either the environment or a
generated constant. 'getgoroot' was mechanically converted to Go
(as obj.Getgoroot) in CL 3046.

obj.Getgoroot begat obj.GOROOT. obj.GOROOT begat objabi.GOROOT,
which begat buildcfg.GOROOT.

As far as I can tell, today's buildcfg.GOROOT is functionally
identical to runtime.GOROOT(). Let's reduce some complexity by
defining it in those terms.

While we're thinking about buildcfg.GOROOT, also check whether it is
non-empty: if the toolchain is built with -trimpath, the value of
GOROOT might not be valid or meaningful if the user invokes
cmd/compile or cmd/link directly, or via a build tool other than
cmd/go that doesn't care as much about GOROOT. (As of CL 390024,
runtime.GOROOT will return the empty string instead of a bogus one
when built with -trimpath.)

For #51461.

Change-Id: I9fec020d5fa65d4aff0dd39b805f5ca93f86c36e
Reviewed-on: https://go-review.googlesource.com/c/go/+/393155
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/logopt/log_opts.go

@@ -405,7 +405,7 @@ func uriIfy(f string) DocumentURI {
 // Return filename, replacing a first occurrence of $GOROOT with the
 // actual value of the GOROOT (because LSP does not speak "$GOROOT").
 func uprootedPath(filename string) string {
-	if !strings.HasPrefix(filename, "$GOROOT/") {
+	if buildcfg.GOROOT == "" || !strings.HasPrefix(filename, "$GOROOT/") {
 		return filename
 	}
 	return buildcfg.GOROOT + filename[len("$GOROOT"):]

src/cmd/compile/internal/noder/reader.go

@@ -209,7 +209,7 @@ func (pr *pkgReader) posBaseIdx(idx int) *src.PosBase {
 	// require being more consistent about when we use native vs UNIX
 	// file paths.
 	const dollarGOROOT = "$GOROOT"
-	if strings.HasPrefix(filename, dollarGOROOT) {
+	if buildcfg.GOROOT != "" && strings.HasPrefix(filename, dollarGOROOT) {
 		filename = buildcfg.GOROOT + filename[len(dollarGOROOT):]
 	}
 

src/cmd/internal/objabi/line.go

@@ -39,7 +39,7 @@ func AbsFile(dir, file, rewrites string) string {
 	}
 
 	abs, rewritten := ApplyRewrites(abs, rewrites)
-	if !rewritten && hasPathPrefix(abs, buildcfg.GOROOT) {
+	if !rewritten && buildcfg.GOROOT != "" && hasPathPrefix(abs, buildcfg.GOROOT) {
 		abs = "$GOROOT" + abs[len(buildcfg.GOROOT):]
 	}
 

src/cmd/link/internal/ld/lib.go

@@ -390,7 +390,9 @@ func libinit(ctxt *Link) {
 		suffix = "asan"
 	}
 
-	Lflag(ctxt, filepath.Join(buildcfg.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", buildcfg.GOOS, buildcfg.GOARCH, suffixsep, suffix)))
+	if buildcfg.GOROOT != "" {
+		Lflag(ctxt, filepath.Join(buildcfg.GOROOT, "pkg", fmt.Sprintf("%s_%s%s%s", buildcfg.GOOS, buildcfg.GOARCH, suffixsep, suffix)))
+	}
 
 	mayberemoveoutfile()
 

src/cmd/link/internal/ld/main.go

@@ -121,7 +121,6 @@ func Main(arch *sys.Arch, theArch Arch) {
 
 	final := gorootFinal()
 	addstrdata1(ctxt, "runtime.defaultGOROOT="+final)
-	addstrdata1(ctxt, "internal/buildcfg.defaultGOROOT="+final)
 
 	buildVersion := buildcfg.Version
 	if goexperiment := buildcfg.Experiment.String(); goexperiment != "" {

src/internal/buildcfg/cfg.go

@@ -15,13 +15,12 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 )
 
 var (
-	defaultGOROOT string // set by linker
-
-	GOROOT   = envOr("GOROOT", defaultGOROOT)
+	GOROOT   = runtime.GOROOT() // cached for efficiency
 	GOARCH   = envOr("GOARCH", defaultGOARCH)
 	GOOS     = envOr("GOOS", defaultGOOS)
 	GO386    = envOr("GO386", defaultGO386)