cmd/go: add source file contents to plugin hash

It is common to have multiple plugins built from ephemeral
source files all with the same name:

	# generate main.go
	go build -buildmode=plugin -o=p1.so main.go
	# rm main.go, generate new main.go
	go build -buildmode=plugin -o=p2.so main.go
	...

These different plugins currently have the same build ID,
and hence the same package path. This means only one can be
loaded.

To remove this restriction, this commit adds the contents of the
main package source files to the plugin hash.

Fixes #19358

Change-Id: Icd42024b085feb29c09c2771aaecb85f8b528dd3
Reviewed-on: https://go-review.googlesource.com/61170
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/test.bash

@@ -27,8 +27,8 @@ GOPATH=$(pwd) go build -buildmode=plugin plugin1
 GOPATH=$(pwd) go build -buildmode=plugin plugin2
 GOPATH=$(pwd)/altpath go build -buildmode=plugin plugin-mismatch
 GOPATH=$(pwd) go build -buildmode=plugin -o=sub/plugin1.so sub/plugin1
-GOPATH=$(pwd) go build -buildmode=plugin unnamed1.go
-GOPATH=$(pwd) go build -buildmode=plugin unnamed2.go
+GOPATH=$(pwd) go build -buildmode=plugin -o=unnamed1.so unnamed1/main.go
+GOPATH=$(pwd) go build -buildmode=plugin -o=unnamed2.so unnamed2/main.go
 GOPATH=$(pwd) go build host
 
 LD_LIBRARY_PATH=$(pwd) ./host

src/cmd/go/internal/load/pkg.go

@@ -1671,15 +1671,8 @@ func isStale(p *Package) (bool, string) {
 	return false, ""
 }
 
-// computeBuildID computes the build ID for p, leaving it in p.Internal.BuildID.
-// Build ID is a hash of the information we want to detect changes in.
-// See the long comment in isStale for details.
-func computeBuildID(p *Package) {
-	h := sha1.New()
-
-	// Include the list of files compiled as part of the package.
-	// This lets us detect removed files. See issue 3895.
-	inputFiles := str.StringList(
+func pkgInputFiles(p *Package) []string {
+	return str.StringList(
 		p.GoFiles,
 		p.CgoFiles,
 		p.CFiles,
@@ -1692,6 +1685,40 @@ func computeBuildID(p *Package) {
 		p.SwigFiles,
 		p.SwigCXXFiles,
 	)
+}
+
+// PluginPath computes the package path for a plugin main package.
+//
+// This is typically the import path of the main package p, unless the
+// plugin is being built directly from source files. In that case we
+// combine the package build ID with the contents of the main package
+// source files. This allows us to identify two different plugins
+// built from two source files with the same name.
+func PluginPath(p *Package) string {
+	if p.ImportPath != "command-line-arguments" {
+		return p.ImportPath
+	}
+	h := sha1.New()
+	fmt.Fprintf(h, "build ID: %s\n", p.Internal.BuildID)
+	for _, file := range str.StringList(p.GoFiles, p.CgoFiles, p.SFiles) {
+		data, err := ioutil.ReadFile(filepath.Join(p.Dir, file))
+		if err != nil {
+			base.Fatalf("go: %s", err)
+		}
+		h.Write(data)
+	}
+	return fmt.Sprintf("plugin/unnamed-%x", h.Sum(nil))
+}
+
+// computeBuildID computes the build ID for p, leaving it in p.Internal.BuildID.
+// Build ID is a hash of the information we want to detect changes in.
+// See the long comment in isStale for details.
+func computeBuildID(p *Package) {
+	h := sha1.New()
+
+	// Include the list of files compiled as part of the package.
+	// This lets us detect removed files. See issue 3895.
+	inputFiles := pkgInputFiles(p)
 	for _, file := range inputFiles {
 		fmt.Fprintf(h, "file %s\n", file)
 	}

src/cmd/go/internal/work/build.go

@@ -2200,9 +2200,7 @@ func (gcToolchain) gc(b *Builder, p *load.Package, archive, objdir string, asmhd
 
 	pkgpath := p.ImportPath
 	if cfg.BuildBuildmode == "plugin" {
-		if pkgpath == "command-line-arguments" {
-			pkgpath = "plugin/unnamed-" + p.Internal.BuildID
-		}
+		pkgpath = load.PluginPath(p)
 	} else if p.Name == "main" {
 		pkgpath = "main"
 	}
@@ -2536,11 +2534,7 @@ func (gcToolchain) ld(b *Builder, root *Action, out string, allactions []*Action
 		ldflags = append(ldflags, "-s", "-w")
 	}
 	if cfg.BuildBuildmode == "plugin" {
-		pluginpath := root.Package.ImportPath
-		if pluginpath == "command-line-arguments" {
-			pluginpath = "plugin/unnamed-" + root.Package.Internal.BuildID
-		}
-		ldflags = append(ldflags, "-pluginpath", pluginpath)
+		ldflags = append(ldflags, "-pluginpath", load.PluginPath(root.Package))
 	}
 
 	// If the user has not specified the -extld option, then specify the