cmd/go: fix 'go test' and 'go fmt' with files outside a module

Use the actual loader result in findModule instead of making
assumptions about nesting in the build list.
As a side-effect, this produces much clearer error messages for
packages that (for one reason or another) failed to load.

Adjust the package and module path outside a module to
"command-line-arguments". That string already appears in the output of
a number of (module-mode and GOPATH-mode) commands for file arguments,
and as far as I can tell operation outside a module is currently the
only case in which the module path of a package is not actually a
prefix of the import path.

Fixes #28011
Fixes #27099
Fixes #28943
Updates #27102
Updates #28459
Updates #27063

Change-Id: I61d5556df7b1b7d1efdaffa892f0e3e95b612d87
Reviewed-on: https://go-review.googlesource.com/c/153459
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

7 files changed, 91 insertions, 32 deletions

diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index 616adcc57a..228be07f24 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -997,10 +997,12 @@ func disallowInternal(srcDir string, importer *Package, importerPath string, p *
 	} else {
 		// p is in a module, so make it available based on the importer's import path instead
 		// of the file path (https://golang.org/issue/23970).
-		if importerPath == "." {
+		if importer.Internal.CmdlineFiles {
 			// The importer is a list of command-line files.
 			// Pretend that the import path is the import path of the
 			// directory containing them.
+			// If the directory is outside the main module, this will resolve to ".",
+			// which is not a prefix of any valid module.
 			importerPath = ModDirImportPath(importer.Dir)
 		}
 		parentOfInternal := p.ImportPath[:i]
@@ -1515,11 +1517,13 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	}
 
 	if cfg.ModulesEnabled {
-		if !p.Internal.CmdlineFiles {
-			p.Module = ModPackageModuleInfo(p.ImportPath)
+		mainPath := p.ImportPath
+		if p.Internal.CmdlineFiles {
+			mainPath = "command-line-arguments"
 		}
+		p.Module = ModPackageModuleInfo(mainPath)
 		if p.Name == "main" {
-			p.Internal.BuildInfo = ModPackageBuildInfo(p.ImportPath, p.Deps)
+			p.Internal.BuildInfo = ModPackageBuildInfo(mainPath, p.Deps)
 		}
 	}
 }
@@ -1986,11 +1990,6 @@ func GoFilesPackage(gofiles []string) *Package {
 	}
 
 	bp, err := ctxt.ImportDir(dir, 0)
-	if ModDirImportPath != nil {
-		// Use the effective import path of the directory
-		// for deciding visibility during pkg.load.
-		bp.ImportPath = ModDirImportPath(dir)
-	}
 	pkg := new(Package)
 	pkg.Internal.Local = true
 	pkg.Internal.CmdlineFiles = true
diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
index b4856a9419..efeb7a5fd5 100644
--- a/src/cmd/go/internal/modload/build.go
+++ b/src/cmd/go/internal/modload/build.go
@@ -17,6 +17,7 @@ import (
 	"internal/goroot"
 	"os"
 	"path/filepath"
+	"runtime/debug"
 	"strings"
 )
 
@@ -98,11 +99,13 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 			Path:    m.Path,
 			Version: m.Version,
 			Main:    true,
-			Dir:     ModRoot(),
-			GoMod:   filepath.Join(ModRoot(), "go.mod"),
 		}
-		if modFile.Go != nil {
-			info.GoVersion = modFile.Go.Version
+		if HasModRoot() {
+			info.Dir = ModRoot()
+			info.GoMod = filepath.Join(info.Dir, "go.mod")
+			if modFile.Go != nil {
+				info.GoVersion = modFile.Go.Version
+			}
 		}
 		return info
 	}
@@ -184,6 +187,7 @@ func PackageBuildInfo(path string, deps []string) string {
 	if isStandardImportPath(path) || !Enabled() {
 		return ""
 	}
+
 	target := findModule(path, path)
 	mdeps := make(map[module.Version]bool)
 	for _, dep := range deps {
@@ -223,19 +227,23 @@ func PackageBuildInfo(path string, deps []string) string {
 	return buf.String()
 }
 
+// findModule returns the module containing the package at path,
+// needed to build the package at target.
 func findModule(target, path string) module.Version {
-	// TODO: This should use loaded.
-	if path == "." {
-		return buildList[0]
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if ok {
+		if pkg.err != nil {
+			base.Fatalf("build %v: cannot load %v: %v", target, path, pkg.err)
+		}
+		return pkg.mod
 	}
-	if cfg.BuildMod == "vendor" {
-		readVendorList()
-		return vendorMap[path]
+
+	if path == "command-line-arguments" {
+		return Target
 	}
-	for _, mod := range buildList {
-		if maybeInModule(path, mod.Path) {
-			return mod
-		}
+
+	if printStackInDie {
+		debug.PrintStack()
 	}
 	base.Fatalf("build %v: cannot find module for path %v", target, path)
 	panic("unreachable")
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 97c48be00e..22d14ccce7 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -314,7 +314,7 @@ func InitMod() {
 
 	Init()
 	if modRoot == "" {
-		Target = module.Version{Path: "main"}
+		Target = module.Version{Path: "command-line-arguments"}
 		buildList = []module.Version{Target}
 		return
 	}
diff --git a/src/cmd/go/testdata/script/mod_enabled.txt b/src/cmd/go/testdata/script/mod_enabled.txt
index 1de4719d53..ab5ee3d6df 100644
--- a/src/cmd/go/testdata/script/mod_enabled.txt
+++ b/src/cmd/go/testdata/script/mod_enabled.txt
@@ -39,8 +39,8 @@ stdout z[/\\]go.mod
 cd $GOPATH/src/x/y
 go env GOMOD
 stdout 'NUL|/dev/null'
-! go list -m
-stderr 'cannot find main module'
+go list -m
+stdout '^command-line-arguments$'
 
 cd $GOPATH/foo
 go env GOMOD
diff --git a/src/cmd/go/testdata/script/mod_get_svn.txt b/src/cmd/go/testdata/script/mod_get_svn.txt
index ad96fa1357..b3436284af 100644
--- a/src/cmd/go/testdata/script/mod_get_svn.txt
+++ b/src/cmd/go/testdata/script/mod_get_svn.txt
@@ -10,8 +10,7 @@ env GOPROXY=direct # obtain llvm.org directory, not via svn.
 ! go get -d llvm.org/llvm/bindings/go/llvm
 stderr 'ReadZip not implemented for svn'
 ! go install .
-# TODO(bcmills): The error message here should mention ReadZip.
-stderr 'cannot find module for path llvm.org'
+stderr 'ReadZip not implemented for svn'
 
 -- go.mod --
 module golang/go/issues/28943/main
diff --git a/src/cmd/go/testdata/script/mod_outside.txt b/src/cmd/go/testdata/script/mod_outside.txt
index 25013b6271..db994a1656 100644
--- a/src/cmd/go/testdata/script/mod_outside.txt
+++ b/src/cmd/go/testdata/script/mod_outside.txt
@@ -12,8 +12,8 @@ stdout 'NUL|/dev/null'
 # which is not in a module.
 ! go list
 stderr 'cannot find main module'
-! go list -m
-stderr 'cannot find main module'
+go list -m
+stdout '^command-line-arguments$'
 # 'go list' in the working directory should fail even if there is a a 'package
 # main' present: without a main module, we do not know its package path.
 ! go list ./foo
@@ -148,6 +148,10 @@ stderr 'no such package'
 stderr 'can only use path@version syntax with'
 
 
+# 'go fmt' should be able to format files outside of a module.
+go fmt foo/foo.go
+
+
 # The remainder of the test checks dependencies by linking and running binaries.
 [short] stop
 
@@ -185,8 +189,8 @@ stdout 'using example.com/version v1.0.0'
 
 # 'go run' should use 'main' as the effective module and import path.
 go run ./foo/foo.go
-stdout 'path is \.$'
-stdout 'main is main \(devel\)'
+stdout 'path is command-line-arguments$'
+stdout 'main is command-line-arguments \(devel\)'
 stdout 'using example.com/version v1.1.0'
 
 # 'go generate' should work with file arguments.
diff --git a/src/cmd/go/testdata/script/mod_test_files.txt b/src/cmd/go/testdata/script/mod_test_files.txt
new file mode 100644
index 0000000000..87aecb44f6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_test_files.txt
@@ -0,0 +1,49 @@
+env GO111MODULE=on
+
+cd foo
+
+# Testing an explicit source file should use the same import visibility as the
+# package in the same directory.
+go list -test -deps
+go list -test -deps foo_test.go
+
+# If the file is inside the main module's vendor directory, it should have
+# visibility based on the vendor-relative import path.
+mkdir vendor/example.com/foo
+cp foo_test.go vendor/example.com/foo
+go list -test -deps vendor/example.com/foo/foo_test.go
+
+# If the file is outside the main module entirely, it should be treated as outside.
+cp foo_test.go ../foo_test.go
+! go list -test -deps ../foo_test.go
+stderr 'use of internal package'
+
+-- foo/go.mod --
+module example.com/foo
+require example.com/internal v0.0.0
+replace example.com/internal => ../internal
+
+-- foo/internal.go --
+package foo
+import _ "example.com/internal"
+
+-- foo/foo_test.go --
+package foo_test
+
+import (
+	"testing"
+	"example.com/internal"
+)
+
+func TestHacksEnabled(t *testing.T) {
+	if !internal.Hacks {
+		t.Fatal("hacks not enabled")
+	}
+}
+
+-- internal/go.mod --
+module example.com/internal
+
+-- internal/internal.go --
+package internal
+const Hacks = true