cmd/go: add -C flag

The -C flag is like tar -C or make -C: it changes to the named directory
early in command startup, before anything else happens.

Fixes #50332.

Change-Id: I8e4546f69044cb3a028d4d26dfba482b08cb845d
Reviewed-on: https://go-review.googlesource.com/c/go/+/421436
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

24 files changed, 136 insertions, 9 deletions

diff --git a/src/cmd/doc/main.go b/src/cmd/doc/main.go
index 3c45dd76df..ae1b7575e8 100644
--- a/src/cmd/doc/main.go
+++ b/src/cmd/doc/main.go
@@ -57,12 +57,13 @@ import (
 )
 
 var (
-	unexported bool // -u flag
-	matchCase  bool // -c flag
-	showAll    bool // -all flag
-	showCmd    bool // -cmd flag
-	showSrc    bool // -src flag
-	short      bool // -short flag
+	unexported bool   // -u flag
+	matchCase  bool   // -c flag
+	chdir      string // -C flag
+	showAll    bool   // -all flag
+	showCmd    bool   // -cmd flag
+	showSrc    bool   // -src flag
+	short      bool   // -short flag
 )
 
 // usage is a replacement usage function for the flags package.
@@ -96,6 +97,7 @@ func do(writer io.Writer, flagSet *flag.FlagSet, args []string) (err error) {
 	flagSet.Usage = usage
 	unexported = false
 	matchCase = false
+	flagSet.StringVar(&chdir, "C", "", "change to `dir` before running command")
 	flagSet.BoolVar(&unexported, "u", false, "show unexported symbols as well as exported")
 	flagSet.BoolVar(&matchCase, "c", false, "symbol matching honors case (paths not affected)")
 	flagSet.BoolVar(&showAll, "all", false, "show all documentation for package")
@@ -103,6 +105,11 @@ func do(writer io.Writer, flagSet *flag.FlagSet, args []string) (err error) {
 	flagSet.BoolVar(&showSrc, "src", false, "show source code for symbol")
 	flagSet.BoolVar(&short, "short", false, "one-line representation for each symbol")
 	flagSet.Parse(args)
+	if chdir != "" {
+		if err := os.Chdir(chdir); err != nil {
+			return err
+		}
+	}
 	var paths []string
 	var symbol, method string
 	// Loop until something is printed.
diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index a8206c475c..051cf25996 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -100,6 +100,10 @@
 // The build flags are shared by the build, clean, get, install, list, run,
 // and test commands:
 //
+//	-C dir
+//		Change to dir before running the command.
+//		Any files named on the command line are interpreted after
+//		changing directories.
 //	-a
 //		force rebuilding of packages that are already up-to-date.
 //	-n
@@ -1233,6 +1237,8 @@
 // referred to indirectly. For the full set of modules available to a build,
 // use 'go list -m -json all'.
 //
+// Edit also provides the -C, -n, and -x build flags.
+//
 // See https://golang.org/ref/mod#go-mod-edit for more about 'go mod edit'.
 //
 // # Print module requirement graph
@@ -1797,7 +1803,7 @@
 //
 // Usage:
 //
-//	go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
+//	go vet [-C dir] [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
 //
 // Vet runs the Go vet command on the packages named by the import paths.
 //
@@ -1806,6 +1812,7 @@
 // For a list of checkers and their flags, see 'go tool vet help'.
 // For details of a specific checker such as 'printf', see 'go tool vet help printf'.
 //
+// The -C flag changes to dir before running the 'go vet' command.
 // The -n flag prints commands that would be executed.
 // The -x flag prints commands as they are executed.
 //
diff --git a/src/cmd/go/chdir_test.go b/src/cmd/go/chdir_test.go
new file mode 100644
index 0000000000..44cbb9c3f7
--- /dev/null
+++ b/src/cmd/go/chdir_test.go
@@ -0,0 +1,49 @@
+// Copyright 2022 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.
+
+package main
+
+import (
+	"cmd/go/internal/base"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestChdir(t *testing.T) {
+	// We want -C to apply to every go subcommand.
+	// Test that every command either has a -C flag registered
+	// or has CustomFlags set. In the latter case, the command
+	// must be explicitly tested in TestScript/chdir.
+	script, err := os.ReadFile("testdata/script/chdir.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var walk func(string, *base.Command)
+	walk = func(name string, cmd *base.Command) {
+		if len(cmd.Commands) > 0 {
+			for _, sub := range cmd.Commands {
+				walk(name+" "+sub.Name(), sub)
+			}
+			return
+		}
+		if !cmd.Runnable() {
+			return
+		}
+		if cmd.CustomFlags {
+			if !strings.Contains(string(script), "# "+name+"\n") {
+				t.Errorf("%s has custom flags, not tested in testdata/script/chdir.txt", name)
+			}
+			return
+		}
+		f := cmd.Flag.Lookup("C")
+		if f == nil {
+			t.Errorf("%s has no -C flag", name)
+		} else if f.Usage != "AddChdirFlag" {
+			t.Errorf("%s has -C flag but not from AddChdirFlag", name)
+		}
+	}
+	walk("go", base.Go)
+}
diff --git a/src/cmd/go/internal/base/flag.go b/src/cmd/go/internal/base/flag.go
index 120420a126..9d8d1c0c8d 100644
--- a/src/cmd/go/internal/base/flag.go
+++ b/src/cmd/go/internal/base/flag.go
@@ -6,6 +6,7 @@ package base
 
 import (
 	"flag"
+	"os"
 
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
@@ -57,6 +58,13 @@ func AddBuildFlagsNX(flags *flag.FlagSet) {
 	flags.BoolVar(&cfg.BuildX, "x", false, "")
 }
 
+// AddChdirFlag adds the -C flag to the flag set.
+func AddChdirFlag(flags *flag.FlagSet) {
+	// The usage message is never printed, but it's used in chdir_test.go
+	// to identify that the -C flag is from AddChdirFlag.
+	flags.Func("C", "AddChdirFlag", os.Chdir)
+}
+
 // AddModFlag adds the -mod build flag to the flag set.
 func AddModFlag(flags *flag.FlagSet) {
 	flags.Var(explicitStringFlag{value: &cfg.BuildMod, explicit: &cfg.BuildModExplicit}, "mod", "")
diff --git a/src/cmd/go/internal/bug/bug.go b/src/cmd/go/internal/bug/bug.go
index e667012fbb..ed1813605e 100644
--- a/src/cmd/go/internal/bug/bug.go
+++ b/src/cmd/go/internal/bug/bug.go
@@ -37,6 +37,7 @@ The report includes useful system information.
 
 func init() {
 	CmdBug.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	base.AddChdirFlag(&CmdBug.Flag)
 }
 
 func runBug(ctx context.Context, cmd *base.Command, args []string) {
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index f7f065529d..10499c2d3e 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -57,6 +57,7 @@ For more about environment variables, see 'go help environment'.
 
 func init() {
 	CmdEnv.Run = runEnv // break init cycle
+	base.AddChdirFlag(&CmdEnv.Flag)
 }
 
 var (
diff --git a/src/cmd/go/internal/fmtcmd/fmt.go b/src/cmd/go/internal/fmtcmd/fmt.go
index f6a8d207cd..62b22f6bcf 100644
--- a/src/cmd/go/internal/fmtcmd/fmt.go
+++ b/src/cmd/go/internal/fmtcmd/fmt.go
@@ -21,6 +21,7 @@ import (
 
 func init() {
 	base.AddBuildFlagsNX(&CmdFmt.Flag)
+	base.AddChdirFlag(&CmdFmt.Flag)
 	base.AddModFlag(&CmdFmt.Flag)
 	base.AddModCommonFlags(&CmdFmt.Flag)
 }
diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
index 0b50afb668..f0b62e8b4b 100644
--- a/src/cmd/go/internal/modcmd/download.go
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -84,6 +84,7 @@ func init() {
 
 	// TODO(jayconrod): https://golang.org/issue/35849 Apply -x to other 'go mod' commands.
 	cmdDownload.Flag.BoolVar(&cfg.BuildX, "x", false, "")
+	base.AddChdirFlag(&cmdDownload.Flag)
 	base.AddModCommonFlags(&cmdDownload.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/edit.go b/src/cmd/go/internal/modcmd/edit.go
index edc1b19877..5fd13f2627 100644
--- a/src/cmd/go/internal/modcmd/edit.go
+++ b/src/cmd/go/internal/modcmd/edit.go
@@ -127,6 +127,8 @@ Note that this only describes the go.mod file itself, not other modules
 referred to indirectly. For the full set of modules available to a build,
 use 'go list -m -json all'.
 
+Edit also provides the -C, -n, and -x build flags.
+
 See https://golang.org/ref/mod#go-mod-edit for more about 'go mod edit'.
 	`,
 }
@@ -157,8 +159,9 @@ func init() {
 	cmdEdit.Flag.Var(flagFunc(flagRetract), "retract", "")
 	cmdEdit.Flag.Var(flagFunc(flagDropRetract), "dropretract", "")
 
-	base.AddModCommonFlags(&cmdEdit.Flag)
 	base.AddBuildFlagsNX(&cmdEdit.Flag)
+	base.AddChdirFlag(&cmdEdit.Flag)
+	base.AddModCommonFlags(&cmdEdit.Flag)
 }
 
 func runEdit(ctx context.Context, cmd *base.Command, args []string) {
diff --git a/src/cmd/go/internal/modcmd/graph.go b/src/cmd/go/internal/modcmd/graph.go
index 9568c65740..feed6a0005 100644
--- a/src/cmd/go/internal/modcmd/graph.go
+++ b/src/cmd/go/internal/modcmd/graph.go
@@ -41,6 +41,7 @@ var (
 
 func init() {
 	cmdGraph.Flag.Var(&graphGo, "go", "")
+	base.AddChdirFlag(&cmdGraph.Flag)
 	base.AddModCommonFlags(&cmdGraph.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/init.go b/src/cmd/go/internal/modcmd/init.go
index bc4620a2a8..e4be73fab0 100644
--- a/src/cmd/go/internal/modcmd/init.go
+++ b/src/cmd/go/internal/modcmd/init.go
@@ -34,6 +34,7 @@ See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
 }
 
 func init() {
+	base.AddChdirFlag(&cmdInit.Flag)
 	base.AddModCommonFlags(&cmdInit.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
index d35476eb53..27889941c7 100644
--- a/src/cmd/go/internal/modcmd/tidy.go
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -64,6 +64,7 @@ func init() {
 	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
 	cmdTidy.Flag.Var(&tidyGo, "go", "")
 	cmdTidy.Flag.Var(&tidyCompat, "compat", "")
+	base.AddChdirFlag(&cmdTidy.Flag)
 	base.AddModCommonFlags(&cmdTidy.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index a93c52dbb3..4f820eb13e 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -61,6 +61,7 @@ func init() {
 	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
 	cmdVendor.Flag.BoolVar(&vendorE, "e", false, "")
 	cmdVendor.Flag.StringVar(&vendorO, "o", "", "")
+	base.AddChdirFlag(&cmdVendor.Flag)
 	base.AddModCommonFlags(&cmdVendor.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/verify.go b/src/cmd/go/internal/modcmd/verify.go
index 459bf5d070..a5f7f24563 100644
--- a/src/cmd/go/internal/modcmd/verify.go
+++ b/src/cmd/go/internal/modcmd/verify.go
@@ -38,6 +38,7 @@ See https://golang.org/ref/mod#go-mod-verify for more about 'go mod verify'.
 }
 
 func init() {
+	base.AddChdirFlag(&cmdVerify.Flag)
 	base.AddModCommonFlags(&cmdVerify.Flag)
 }
 
diff --git a/src/cmd/go/internal/modcmd/why.go b/src/cmd/go/internal/modcmd/why.go
index 8e929a0001..729c88f3f1 100644
--- a/src/cmd/go/internal/modcmd/why.go
+++ b/src/cmd/go/internal/modcmd/why.go
@@ -58,6 +58,7 @@ var (
 
 func init() {
 	cmdWhy.Run = runWhy // break init cycle
+	base.AddChdirFlag(&cmdWhy.Flag)
 	base.AddModCommonFlags(&cmdWhy.Flag)
 }
 
diff --git a/src/cmd/go/internal/tool/tool.go b/src/cmd/go/internal/tool/tool.go
index afa3ac404f..069968b1b6 100644
--- a/src/cmd/go/internal/tool/tool.go
+++ b/src/cmd/go/internal/tool/tool.go
@@ -48,6 +48,7 @@ func isGccgoTool(tool string) bool {
 }
 
 func init() {
+	base.AddChdirFlag(&CmdTool.Flag)
 	CmdTool.Flag.BoolVar(&toolN, "n", false, "")
 }
 
diff --git a/src/cmd/go/internal/version/version.go b/src/cmd/go/internal/version/version.go
index 6bbd48c6e6..a0f6123149 100644
--- a/src/cmd/go/internal/version/version.go
+++ b/src/cmd/go/internal/version/version.go
@@ -44,6 +44,7 @@ See also: go doc runtime/debug.BuildInfo.
 }
 
 func init() {
+	base.AddChdirFlag(&CmdVersion.Flag)
 	CmdVersion.Run = runVersion // break init cycle
 }
 
diff --git a/src/cmd/go/internal/vet/vet.go b/src/cmd/go/internal/vet/vet.go
index e5f8af1c37..c73fa5b424 100644
--- a/src/cmd/go/internal/vet/vet.go
+++ b/src/cmd/go/internal/vet/vet.go
@@ -25,7 +25,7 @@ func init() {
 
 var CmdVet = &base.Command{
 	CustomFlags: true,
-	UsageLine:   "go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]",
+	UsageLine:   "go vet [-C dir] [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]",
 	Short:       "report likely mistakes in packages",
 	Long: `
 Vet runs the Go vet command on the packages named by the import paths.
@@ -35,6 +35,7 @@ For more about specifying packages, see 'go help packages'.
 For a list of checkers and their flags, see 'go tool vet help'.
 For details of a specific checker such as 'printf', see 'go tool vet help printf'.
 
+The -C flag changes to dir before running the 'go vet' command.
 The -n flag prints commands that would be executed.
 The -x flag prints commands as they are executed.
 
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index d8b7848071..553cd66ef3 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -58,6 +58,10 @@ will be written to that directory.
 The build flags are shared by the build, clean, get, install, list, run,
 and test commands:
 
+	-C dir
+		Change to dir before running the command.
+		Any files named on the command line are interpreted after
+		changing directories.
 	-a
 		force rebuilding of packages that are already up-to-date.
 	-n
@@ -282,6 +286,7 @@ const (
 // install, list, run, and test commands.
 func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	base.AddBuildFlagsNX(&cmd.Flag)
+	base.AddChdirFlag(&cmd.Flag)
 	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
 	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
 	if mask&OmitVFlag == 0 {
diff --git a/src/cmd/go/internal/workcmd/edit.go b/src/cmd/go/internal/workcmd/edit.go
index 8d1d38318b..f5e3304025 100644
--- a/src/cmd/go/internal/workcmd/edit.go
+++ b/src/cmd/go/internal/workcmd/edit.go
@@ -109,6 +109,7 @@ func init() {
 	cmdEdit.Flag.Var(flagFunc(flagEditworkDropUse), "dropuse", "")
 	cmdEdit.Flag.Var(flagFunc(flagEditworkReplace), "replace", "")
 	cmdEdit.Flag.Var(flagFunc(flagEditworkDropReplace), "dropreplace", "")
+	base.AddChdirFlag(&cmdEdit.Flag)
 }
 
 func runEditwork(ctx context.Context, cmd *base.Command, args []string) {
diff --git a/src/cmd/go/internal/workcmd/init.go b/src/cmd/go/internal/workcmd/init.go
index c2513bac35..6fb033ee29 100644
--- a/src/cmd/go/internal/workcmd/init.go
+++ b/src/cmd/go/internal/workcmd/init.go
@@ -34,6 +34,7 @@ for more information.
 }
 
 func init() {
+	base.AddChdirFlag(&cmdInit.Flag)
 	base.AddModCommonFlags(&cmdInit.Flag)
 }
 
diff --git a/src/cmd/go/internal/workcmd/sync.go b/src/cmd/go/internal/workcmd/sync.go
index 7712eb6b6b..9f9962709b 100644
--- a/src/cmd/go/internal/workcmd/sync.go
+++ b/src/cmd/go/internal/workcmd/sync.go
@@ -41,6 +41,7 @@ for more information.
 }
 
 func init() {
+	base.AddChdirFlag(&cmdSync.Flag)
 	base.AddModCommonFlags(&cmdSync.Flag)
 }
 
diff --git a/src/cmd/go/internal/workcmd/use.go b/src/cmd/go/internal/workcmd/use.go
index fcb4e9e5f1..be90989ddd 100644
--- a/src/cmd/go/internal/workcmd/use.go
+++ b/src/cmd/go/internal/workcmd/use.go
@@ -43,6 +43,7 @@ var useR = cmdUse.Flag.Bool("r", false, "")
 func init() {
 	cmdUse.Run = runUse // break init cycle
 
+	base.AddChdirFlag(&cmdUse.Flag)
 	base.AddModCommonFlags(&cmdUse.Flag)
 }
 
diff --git a/src/cmd/go/testdata/script/chdir.txt b/src/cmd/go/testdata/script/chdir.txt
new file mode 100644
index 0000000000..8952d18a72
--- /dev/null
+++ b/src/cmd/go/testdata/script/chdir.txt
@@ -0,0 +1,31 @@
+env OLD=$PWD
+
+# basic -C functionality
+cd $GOROOT/src/math
+go list -C ../strings
+stdout strings
+! go list -C ../nonexist
+stderr 'chdir.*nonexist'
+
+# check for -C in subcommands with custom flag parsing
+# cmd/go/chdir_test.go handles the normal ones more directly.
+
+# go doc
+go doc -C ../strings HasPrefix
+
+# go env
+go env -C $OLD/custom GOMOD
+stdout 'custom[\\/]go.mod'
+! go env -C ../nonexist
+stderr '^invalid value "../nonexist" for flag -C: chdir ../nonexist:.*$'
+
+# go test
+go test -n -C ../strings
+stderr 'strings\.test'
+
+# go vet
+go vet -n -C ../strings
+stderr strings_test
+
+-- custom/go.mod --
+module m