cmd/go: default to mod=readonly when the go.mod file is read-only

Updates #30185
Updates #33326
Updates #34822

Change-Id: Ie13651585898d1bbbf4f779b97ee50b6c7e7ad50
Reviewed-on: https://go-review.googlesource.com/c/go/+/204521
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/cfg/cfg.go

@@ -27,6 +27,7 @@ var (
 	BuildBuildmode         string // -buildmode flag
 	BuildContext           = defaultContext()
 	BuildMod               string             // -mod flag
+	BuildModReason         string             // reason -mod flag is set, if set by default
 	BuildI                 bool               // -i flag
 	BuildLinkshared        bool               // -linkshared flag
 	BuildMSan              bool               // -msan flag

src/cmd/go/internal/modfetch/repo.go

@@ -5,7 +5,6 @@
 package modfetch
 
 import (
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -215,7 +214,7 @@ func Lookup(proxy, path string) (Repo, error) {
 // lookup returns the module with the given module path.
 func lookup(proxy, path string) (r Repo, err error) {
 	if cfg.BuildMod == "vendor" {
-		return nil, errModVendor
+		return nil, errLookupDisabled
 	}
 
 	if str.GlobsMatchPath(cfg.GONOPROXY, path) {
@@ -239,11 +238,21 @@ func lookup(proxy, path string) (r Repo, err error) {
 	}
 }
 
+type lookupDisabledError struct{}
+
+func (lookupDisabledError) Error() string {
+	if cfg.BuildModReason == "" {
+		return fmt.Sprintf("module lookup disabled by -mod=%s", cfg.BuildMod)
+	}
+	return fmt.Sprintf("module lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+}
+
+var errLookupDisabled error = lookupDisabledError{}
+
 var (
-	errModVendor       = errors.New("module lookup disabled by -mod=vendor")
-	errProxyOff        = notExistError("module lookup disabled by GOPROXY=off")
-	errNoproxy   error = notExistError("disabled by GOPRIVATE/GONOPROXY")
-	errUseProxy  error = notExistError("path does not match GOPRIVATE/GONOPROXY")
+	errProxyOff       = notExistError("module lookup disabled by GOPROXY=off")
+	errNoproxy  error = notExistError("disabled by GOPRIVATE/GONOPROXY")
+	errUseProxy error = notExistError("path does not match GOPRIVATE/GONOPROXY")
 )
 
 func lookupDirect(path string) (Repo, error) {

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

@@ -460,25 +460,36 @@ func setDefaultBuildMod() {
 		// manipulate the build list.
 		return
 	}
-	if modRoot != "" {
-		if fi, err := os.Stat(filepath.Join(modRoot, "vendor")); err == nil && fi.IsDir() {
-			modGo := "unspecified"
-			if modFile.Go != nil {
-				if semver.Compare("v"+modFile.Go.Version, "v1.14") >= 0 {
-					// The Go version is at least 1.14, and a vendor directory exists.
-					// Set -mod=vendor by default.
-					cfg.BuildMod = "vendor"
-					return
-				} else {
-					modGo = modFile.Go.Version
-				}
+	if modRoot == "" {
+		return
+	}
+
+	if fi, err := os.Stat(filepath.Join(modRoot, "vendor")); err == nil && fi.IsDir() {
+		modGo := "unspecified"
+		if modFile.Go != nil {
+			if semver.Compare("v"+modFile.Go.Version, "v1.14") >= 0 {
+				// The Go version is at least 1.14, and a vendor directory exists.
+				// Set -mod=vendor by default.
+				cfg.BuildMod = "vendor"
+				cfg.BuildModReason = "Go version in go.mod is at least 1.14 and vendor directory exists."
+				return
+			} else {
+				modGo = modFile.Go.Version
 			}
-			fmt.Fprintf(os.Stderr, "go: not defaulting to -mod=vendor because go.mod 'go' version is %s\n", modGo)
 		}
+
+		// Since a vendor directory exists, we have a non-trivial reason for
+		// choosing -mod=mod, although it probably won't be used for anything.
+		// Record the reason anyway for consistency.
+		// It may be overridden if we switch to mod=readonly below.
+		cfg.BuildModReason = fmt.Sprintf("Go version in go.mod is %s.", modGo)
 	}
 
-	// TODO(golang.org/issue/33326): set -mod=readonly implicitly if the go.mod
-	// file is itself read-only?
+	p := ModFilePath()
+	if fi, err := os.Stat(p); err == nil && !hasWritePerm(p, fi) {
+		cfg.BuildMod = "readonly"
+		cfg.BuildModReason = "go.mod file is read-only."
+	}
 }
 
 // checkVendorConsistency verifies that the vendor/modules.txt file matches (if
@@ -858,7 +869,11 @@ func WriteGoMod() {
 	if dirty && cfg.BuildMod == "readonly" {
 		// If we're about to fail due to -mod=readonly,
 		// prefer to report a dirty go.mod over a dirty go.sum
-		base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly")
+		if cfg.BuildModReason != "" {
+			base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly\n\t(%s)", cfg.BuildModReason)
+		} else {
+			base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly")
+		}
 	}
 	// Always update go.sum, even if we didn't change go.mod: we may have
 	// downloaded modules that we didn't have before.

src/cmd/go/internal/modload/stat_openfile.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix js,wasm plan9
+
+// On plan9, per http://9p.io/magic/man2html/2/access: “Since file permissions
+// are checked by the server and group information is not known to the client,
+// access must open the file to check permissions.”
+//
+// aix and js,wasm are similar, in that they do not define syscall.Access.
+
+package modload
+
+import (
+	"os"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(path string, _ os.FileInfo) bool {
+	if f, err := os.OpenFile(path, os.O_WRONLY, 0); err == nil {
+		f.Close()
+		return true
+	}
+	return false
+}

src/cmd/go/internal/modload/stat_unix.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package modload
+
+import (
+	"os"
+	"syscall"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+//
+// Although the root user on most Unix systems can write to files even without
+// permission, hasWritePerm reports false if no appropriate permission bit is
+// set even if the current user is root.
+func hasWritePerm(path string, fi os.FileInfo) bool {
+	if os.Getuid() == 0 {
+		// The root user can access any file, but we still want to default to
+		// read-only mode if the go.mod file is marked as globally non-writable.
+		// (If the user really intends not to be in readonly mode, they can
+		// pass -mod=mod explicitly.)
+		return fi.Mode()&0222 != 0
+	}
+
+	const W_OK = 0x2
+	return syscall.Access(path, W_OK) == nil
+}

src/cmd/go/internal/modload/stat_windows.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+package modload
+
+import (
+	"os"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(_ string, fi os.FileInfo) bool {
+	// Windows has a read-only attribute independent of ACLs, so use that to
+	// determine whether the file is intended to be overwritten.
+	//
+	// Per https://golang.org/pkg/os/#Chmod:
+	// “On Windows, only the 0200 bit (owner writable) of mode is used; it
+	// controls whether the file's read-only attribute is set or cleared.”
+	return fi.Mode()&0200 != 0
+}

src/cmd/go/script_test.go

@@ -431,7 +431,11 @@ func (ts *testScript) cmdChmod(neg bool, args []string) {
 	if err != nil || perm&uint64(os.ModePerm) != perm {
 		ts.fatalf("invalid mode: %s", args[0])
 	}
-	for _, path := range args[1:] {
+	for _, arg := range args[1:] {
+		path := arg
+		if !filepath.IsAbs(path) {
+			path = filepath.Join(ts.cd, arg)
+		}
 		err := os.Chmod(path, os.FileMode(perm))
 		ts.check(err)
 	}

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

@@ -12,6 +12,14 @@ cp go.mod go.mod.empty
 stderr 'import lookup disabled by -mod=readonly'
 cmp go.mod go.mod.empty
 
+# -mod=readonly should be set implicitly if the go.mod file is read-only
+chmod 0400 go.mod
+env GOFLAGS=
+! go list all
+
+chmod 0600 go.mod
+env GOFLAGS=-mod=readonly
+
 # update go.mod - go get allowed
 go get rsc.io/quote
 grep rsc.io/quote go.mod
@@ -21,11 +29,11 @@ cp go.mod.empty go.mod
 go mod tidy
 
 # -mod=readonly must succeed once go.mod is up-to-date...
-go list
+go list all
 
 # ... even if it needs downloads
 go clean -modcache
-go list
+go list all
 
 # -mod=readonly should reject inconsistent go.mod files
 # (ones that would be rewritten).

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

@@ -48,7 +48,6 @@ go list -f {{.Dir}} -tags tools all
 stdout '^'$WORK'[/\\]auto$'
 stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
 stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
-stderr '^go: not defaulting to -mod=vendor because go.mod .go. version is 1.13$'
 
 go list -m -f '{{.Dir}}' all
 stdout '^'$WORK'[/\\]auto$'
@@ -146,7 +145,6 @@ go list -mod=vendor -f {{.Dir}} -tags tools all
 stdout '^'$WORK'[/\\]auto$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
 stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
-! stderr 'not defaulting to -mod=vendor'
 
 # ...but a version mismatch for an explicit dependency should be noticed.
 cp $WORK/modules-bad-1.13.txt vendor/modules.txt