cmd/link: add -importcfg to specify import resolution

Adds the ability to specify the file location of each imported package,
like in the -importcfg added to cmd/compile in a related CL.
In effect, -importcfg is a generalization of and supersedes -installsuffix
and -L. Of course, those flags will continue to be supported, for
compatibility with other tools.

Having this flag in Go 1.9 will let us try some experiments involving
package management without needing guinea pigs to build a custom
Go toolchain.

This flag also helps with #14271 at some later point.

For #20579.

Change-Id: Ie4c171bcd3aa2faa446ac340e36516f2f9853882
Reviewed-on: https://go-review.googlesource.com/44851
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -33,6 +33,7 @@ package ld
 
 import (
 	"io/ioutil"
+	"log"
 	"os"
 	"path"
 	"path/filepath"
@@ -40,6 +41,51 @@ import (
 	"strings"
 )
 
+func (ctxt *Link) readImportCfg(file string) {
+	ctxt.PackageFile = make(map[string]string)
+	ctxt.PackageShlib = make(map[string]string)
+	data, err := ioutil.ReadFile(file)
+	if err != nil {
+		log.Fatalf("-importcfg: %v", err)
+	}
+
+	for lineNum, line := range strings.Split(string(data), "\n") {
+		lineNum++ // 1-based
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+
+		var verb, args string
+		if i := strings.Index(line, " "); i < 0 {
+			verb = line
+		} else {
+			verb, args = line[:i], strings.TrimSpace(line[i+1:])
+		}
+		var before, after string
+		if i := strings.Index(args, "="); i >= 0 {
+			before, after = args[:i], args[i+1:]
+		}
+		switch verb {
+		default:
+			log.Fatalf("%s:%d: unknown directive %q", file, lineNum, verb)
+		case "packagefile":
+			if before == "" || after == "" {
+				log.Fatalf(`%s:%d: invalid packagefile: syntax is "packagefile path=filename"`, file, lineNum)
+			}
+			ctxt.PackageFile[before] = after
+		case "packageshlib":
+			if before == "" || after == "" {
+				log.Fatalf(`%s:%d: invalid packageshlib: syntax is "packageshlib path=filename"`, file, lineNum)
+			}
+			ctxt.PackageShlib[before] = after
+		}
+	}
+}
+
 func addlib(ctxt *Link, src string, obj string, pathname string) *Library {
 	name := path.Clean(pathname)
 
@@ -56,27 +102,38 @@ func addlib(ctxt *Link, src string, obj string, pathname string) *Library {
 
 	var pname string
 	isshlib := false
-	if filepath.IsAbs(name) {
-		pname = name
+
+	if *FlagLinkshared && ctxt.PackageShlib[name] != "" {
+		pname = ctxt.PackageShlib[name]
+		isshlib = true
+	} else if ctxt.PackageFile != nil {
+		pname = ctxt.PackageFile[name]
+		if pname == "" {
+			ctxt.Logf("cannot find package %s (using -importcfg)\n", name)
+			return nil
+		}
 	} else {
-		// try dot, -L "libdir", and then goroot.
-		for _, dir := range ctxt.Libdir {
-			if *FlagLinkshared {
-				pname = dir + "/" + pkg + ".shlibname"
+		if filepath.IsAbs(name) {
+			pname = name
+		} else {
+			// try dot, -L "libdir", and then goroot.
+			for _, dir := range ctxt.Libdir {
+				if *FlagLinkshared {
+					pname = dir + "/" + pkg + ".shlibname"
+					if _, err := os.Stat(pname); err == nil {
+						isshlib = true
+						break
+					}
+				}
+				pname = dir + "/" + name
 				if _, err := os.Stat(pname); err == nil {
-					isshlib = true
 					break
 				}
 			}
-			pname = dir + "/" + name
-			if _, err := os.Stat(pname); err == nil {
-				break
-			}
 		}
+		pname = path.Clean(pname)
 	}
 
-	pname = path.Clean(pname)
-
 	if ctxt.Debugvlog > 1 {
 		ctxt.Logf("%5.2f addlib: %s %s pulls in %s isshlib %v\n", elapsed(), obj, src, pname, isshlib)
 	}

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

@@ -333,6 +333,19 @@ func errorexit() {
 }
 
 func loadinternal(ctxt *Link, name string) *Library {
+	if *FlagLinkshared && ctxt.PackageShlib != nil {
+		if shlibname := ctxt.PackageShlib[name]; shlibname != "" {
+			return addlibpath(ctxt, "internal", "internal", "", name, shlibname)
+		}
+	}
+	if ctxt.PackageFile != nil {
+		if pname := ctxt.PackageFile[name]; pname != "" {
+			return addlibpath(ctxt, "internal", "internal", pname, name, "")
+		}
+		ctxt.Logf("loadinternal: cannot find %s\n", name)
+		return nil
+	}
+
 	for i := 0; i < len(ctxt.Libdir); i++ {
 		if *FlagLinkshared {
 			shlibname := filepath.Join(ctxt.Libdir[i], name+".shlibname")

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

@@ -230,6 +230,9 @@ type Link struct {
 	Filesyms     []*Symbol
 	Moduledata   *Symbol
 
+	PackageFile  map[string]string
+	PackageShlib map[string]string
+
 	tramps []*Symbol // trampolines
 }
 

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

@@ -122,6 +122,7 @@ func Main() {
 	objabi.Flagfn0("V", "print version and exit", doversion)
 	objabi.Flagfn1("X", "add string value `definition` of the form importpath.name=value", func(s string) { addstrdata1(ctxt, s) })
 	objabi.Flagcount("v", "print link trace", &ctxt.Debugvlog)
+	objabi.Flagfn1("importcfg", "read import configuration from `file`", ctxt.readImportCfg)
 
 	objabi.Flagparse(usage)