cmd/go: only generate a go.mod file during 'go mod init'

In the general case, we do not know the correct module path for a new
module unless we have checked its VCS tags for a major version. If we
do not know the correct path, then we should not synthesize a go.mod
file automatically from it.

On the other hand, we don't want to run VCS commands in the working
directory without an explicit request by the user to do so: 'go mod
init' can reasonably invoke a VCS command, but 'go build' should not.

Therefore, we should only create a go.mod file during 'go mod init'.

This change removes the previous behavior of synthesizing a file
automatically, and instead suggests a command that the user can opt to
run explicitly.

Updates golang#29433
Updates golang#27009
Updates golang#30228

Change-Id: I8c4554969db17156e97428df220b129a4d361040
Reviewed-on: https://go-review.googlesource.com/c/162699
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/init.go

@@ -154,7 +154,7 @@ func Init() {
 			die() // Don't init a module that we're just going to ignore.
 		}
 		// No automatic enabling in GOPATH.
-		if root, _ := FindModuleRoot(cwd, "", false); root != "" {
+		if root := findModuleRoot(cwd); root != "" {
 			cfg.GoModInGOPATH = filepath.Join(root, "go.mod")
 		}
 		return
@@ -164,7 +164,7 @@ func Init() {
 		// Running 'go mod init': go.mod will be created in current directory.
 		modRoot = cwd
 	} else {
-		modRoot, _ = FindModuleRoot(cwd, "", MustUseModules)
+		modRoot = findModuleRoot(cwd)
 		if modRoot == "" {
 			if !MustUseModules {
 				// GO111MODULE is 'auto' (or unset), and we can't find a module root.
@@ -302,6 +302,19 @@ func die() {
 	if inGOPATH && !MustUseModules {
 		base.Fatalf("go: modules disabled inside GOPATH/src by GO111MODULE=auto; see 'go help modules'")
 	}
+	if cwd != "" {
+		if dir, name := findAltConfig(cwd); dir != "" {
+			rel, err := filepath.Rel(cwd, dir)
+			if err != nil {
+				rel = dir
+			}
+			cdCmd := ""
+			if rel != "." {
+				cdCmd = fmt.Sprintf("cd %s && ", rel)
+			}
+			base.Fatalf("go: cannot find main module, but found %s in %s\n\tto create a module there, run:\n\t%sgo mod init", name, dir, cdCmd)
+		}
+	}
 	base.Fatalf("go: cannot find main module; see 'go help modules'")
 }
 
@@ -330,12 +343,6 @@ func InitMod() {
 	gomod := filepath.Join(modRoot, "go.mod")
 	data, err := ioutil.ReadFile(gomod)
 	if err != nil {
-		if os.IsNotExist(err) {
-			legacyModInit()
-			modFileToBuildList()
-			WriteGoMod()
-			return
-		}
 		base.Fatalf("go: %v", err)
 	}
 
@@ -349,7 +356,7 @@ func InitMod() {
 
 	if len(f.Syntax.Stmt) == 0 || f.Module == nil {
 		// Empty mod file. Must add module path.
-		path, err := FindModulePath(modRoot)
+		path, err := findModulePath(modRoot)
 		if err != nil {
 			base.Fatalf("go: %v", err)
 		}
@@ -387,7 +394,7 @@ func Allowed(m module.Version) bool {
 
 func legacyModInit() {
 	if modFile == nil {
-		path, err := FindModulePath(modRoot)
+		path, err := findModulePath(modRoot)
 		if err != nil {
 			base.Fatalf("go: %v", err)
 		}
@@ -454,57 +461,55 @@ var altConfigs = []string{
 	".git/config",
 }
 
-// Exported only for testing.
-func FindModuleRoot(dir, limit string, legacyConfigOK bool) (root, file string) {
+func findModuleRoot(dir string) (root string) {
 	dir = filepath.Clean(dir)
-	dir1 := dir
-	limit = filepath.Clean(limit)
 
 	// Look for enclosing go.mod.
 	for {
 		if fi, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil && !fi.IsDir() {
-			return dir, "go.mod"
-		}
-		if dir == limit {
-			break
+			return dir
 		}
 		d := filepath.Dir(dir)
 		if d == dir {
 			break
 		}
 		dir = d
 	}
+	return ""
+}
 
-	// Failing that, look for enclosing alternate version config.
-	if legacyConfigOK {
-		dir = dir1
-		for {
-			for _, name := range altConfigs {
-				if fi, err := os.Stat(filepath.Join(dir, name)); err == nil && !fi.IsDir() {
-					return dir, name
+func findAltConfig(dir string) (root, name string) {
+	dir = filepath.Clean(dir)
+	for {
+		for _, name := range altConfigs {
+			if fi, err := os.Stat(filepath.Join(dir, name)); err == nil && !fi.IsDir() {
+				if rel := search.InDir(dir, cfg.BuildContext.GOROOT); rel == "." {
+					// Don't suggest creating a module from $GOROOT/.git/config.
+					return "", ""
 				}
+				return dir, name
 			}
-			if dir == limit {
-				break
-			}
-			d := filepath.Dir(dir)
-			if d == dir {
-				break
-			}
-			dir = d
 		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
 	}
-
 	return "", ""
 }
 
-// Exported only for testing.
-func FindModulePath(dir string) (string, error) {
+func findModulePath(dir string) (string, error) {
 	if CmdModModule != "" {
 		// Running go mod init x/y/z; return x/y/z.
 		return CmdModModule, nil
 	}
 
+	// TODO(bcmills): once we have located a plausible module path, we should
+	// query version control (if available) to verify that it matches the major
+	// version of the most recent tag.
+	// See https://golang.org/issue/29433 and https://golang.org/issue/27009.
+
 	// Cast about for import comments,
 	// first in top-level directory, then in subdirectories.
 	list, _ := ioutil.ReadDir(dir)

src/cmd/go/internal/modload/load.go

@@ -111,7 +111,7 @@ func ImportPaths(patterns []string) []*search.Match {
 						} else {
 							pkg = Target.Path + suffix
 						}
-					} else if sub := search.InDir(dir, cfg.GOROOTsrc); sub != "" && !strings.Contains(sub, "@") {
+					} else if sub := search.InDir(dir, cfg.GOROOTsrc); sub != "" && sub != "." && !strings.Contains(sub, "@") {
 						pkg = filepath.ToSlash(sub)
 					} else if path := pathInModuleCache(dir); path != "" {
 						pkg = path

src/cmd/go/testdata/script/mod_convert_dep.txt

@@ -1,9 +1,31 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found Gopkg.lock in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 
+# In Plan 9, directories are automatically created in /n.
+# For example, /n/Gopkg.lock always exists, but it's a directory.
+# Test that we ignore directories when trying to find alternate config files.
+cd $WORK/gopkgdir/x
+! go list .
+stderr 'cannot find main module'
+! stderr 'Gopkg.lock'
+! stderr 'go mod init'
+
 -- $WORK/test/Gopkg.lock --
 -- $WORK/test/x/x.go --
 package x // import "m/x"
+-- $WORK/gopkgdir/Gopkg.lock/README.txt --
+../Gopkg.lock is a directory, not a file.
+-- $WORK/gopkgdir/x/x.go --
+package x // import "m/x"

src/cmd/go/testdata/script/mod_convert_git.txt

@@ -1,10 +1,23 @@
 env GO111MODULE=on
 
-# detect root of module tree as root of enclosing git repo
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found .git/config in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 
+# We should not suggest creating a go.mod file in $GOROOT, even though there may be a .git/config there.
+cd $GOROOT
+! go list .
+! stderr 'go mod init'
+
 -- $WORK/test/.git/config --
 -- $WORK/test/x/x.go --
 package x // import "m/x"

src/cmd/go/testdata/script/mod_convert_glide.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found glide.lock in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_glockfile.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found GLOCKFILE in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_godeps.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found Godeps/Godeps.json in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_tsv.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found dependencies.tsv in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_vendor_conf.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found vendor.conf in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_vendor_json.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found vendor/vendor.json in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_vendor_manifest.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found vendor/manifest in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'
 

src/cmd/go/testdata/script/mod_convert_vendor_yml.txt

@@ -1,6 +1,15 @@
 env GO111MODULE=on
 
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
 cd $WORK/test/x
+! go list .
+stderr 'found vendor.yml in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
 go list -m all
 stdout '^m$'