cmd/go: add 'go mod vendor -o' flag

Adds a new flag to 'go mod vendor' which overrides the default
'vendor' destination directory. This can be helpful for writing the
vendor tree to a temporary location for use by other tools.
The argument can be a relative or an absolute path.
This flag has no other influence on how the command behaves.

Fixes #47327

Change-Id: I4502931127616b181dc90a2066d2fb57bfe48f96
Reviewed-on: https://go-review.googlesource.com/c/go/+/338149
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/alldocs.go

@@ -1295,7 +1295,7 @@
 //
 // Usage:
 //
-// 	go mod vendor [-e] [-v]
+// 	go mod vendor [-e] [-v] [-o outdir]
 //
 // Vendor resets the main module's vendor directory to include all packages
 // needed to build and test all the main module's packages.
@@ -1307,6 +1307,11 @@
 // The -e flag causes vendor to attempt to proceed despite errors
 // encountered while loading packages.
 //
+// The -o flag causes vendor to create the vendor directory at the given
+// path instead of "vendor". The go command can only use a vendor directory
+// named "vendor" within the module root directory, so this flag is
+// primarily useful for other tools.
+//
 // See https://golang.org/ref/mod#go-mod-vendor for more about 'go mod vendor'.
 //
 //

src/cmd/go/internal/modcmd/vendor.go

@@ -31,7 +31,7 @@ import (
 )
 
 var cmdVendor = &base.Command{
-	UsageLine: "go mod vendor [-e] [-v]",
+	UsageLine: "go mod vendor [-e] [-v] [-o outdir]",
 	Short:     "make vendored copy of dependencies",
 	Long: `
 Vendor resets the main module's vendor directory to include all packages
@@ -44,16 +44,23 @@ modules and packages to standard error.
 The -e flag causes vendor to attempt to proceed despite errors
 encountered while loading packages.
 
+The -o flag causes vendor to create the vendor directory at the given
+path instead of "vendor". The go command can only use a vendor directory
+named "vendor" within the module root directory, so this flag is
+primarily useful for other tools.
+
 See https://golang.org/ref/mod#go-mod-vendor for more about 'go mod vendor'.
 	`,
 	Run: runVendor,
 }
 
-var vendorE bool // if true, report errors but proceed anyway
+var vendorE bool   // if true, report errors but proceed anyway
+var vendorO string // if set, overrides the default output directory
 
 func init() {
 	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
 	cmdVendor.Flag.BoolVar(&vendorE, "e", false, "")
+	cmdVendor.Flag.StringVar(&vendorO, "o", "", "")
 	base.AddModCommonFlags(&cmdVendor.Flag)
 }
 
@@ -74,7 +81,15 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 
-	vdir := filepath.Join(modload.VendorDir())
+	var vdir string
+	switch {
+	case filepath.IsAbs(vendorO):
+		vdir = vendorO
+	case vendorO != "":
+		vdir = filepath.Join(base.Cwd(), vendorO)
+	default:
+		vdir = filepath.Join(modload.VendorDir())
+	}
 	if err := os.RemoveAll(vdir); err != nil {
 		base.Fatalf("go: %v", err)
 	}

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

@@ -82,6 +82,48 @@ exists vendor/mysite/myname/mypkg/LICENSE.txt
 ! exists vendor/x/x2
 ! exists vendor/x/x2/LICENSE
 
+# 'go mod vendor' should work with an alternative vendor directory if the -o flag is provided.
+go mod vendor -v -o alternative-vendor-dir
+exists alternative-vendor-dir/modules.txt
+exists alternative-vendor-dir/a/foo/LICENSE
+
+# 'go mod vendor' should interpret paths relative to the current working directory when the -o flag is provided.
+mkdir dir1
+mkdir dir2
+
+cd dir1
+go mod vendor -v -o relative-vendor-dir
+
+go mod vendor -v -o ../dir2/relative-vendor-dir
+
+cd ..
+exists dir1/relative-vendor-dir/modules.txt
+exists dir1/relative-vendor-dir/a/foo/LICENSE
+exists dir2/relative-vendor-dir/modules.txt
+exists dir2/relative-vendor-dir/a/foo/LICENSE
+
+# 'go mod vendor' should fall back to the default 'vendor' directory when an empty argument is passed to the -o flag
+# the same behavior should be exhibited both on the module root directory, as well as nested subdirectories
+
+go mod vendor -v -o ''
+exists vendor/modules.txt
+
+env GOFLAGS=-o=foo
+go mod vendor -v -o ''
+exists vendor/modules.txt
+env GOFLAGS=''
+
+mkdir -p nested/dir
+cd nested/dir
+go mod vendor -v -o ''
+! exists vendor/
+exists ../../vendor/modules.txt
+cd ../..
+
+# 'go mod vendor' should work with absolute paths as well
+go mod vendor -v -o $WORK/tmp/absolute-vendor-dir
+exists $WORK/tmp/absolute-vendor-dir/modules.txt
+
 [short] stop
 
 # 'go build' and 'go test' using vendored packages should succeed.