cmd/go: prevent go work use panic when given a file

The current implementation fails to identify that an argument to go work
use is a file when expecting a directory, and panics when attempting to
access it as a directory. This change checks arguments are directories
and generates an error otherwise.

Fixes #51749

Change-Id: If8f69d233409e93fcf391a8774bace74c031c986
Reviewed-on: https://go-review.googlesource.com/c/go/+/393615
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/workcmd/use.go

@@ -85,13 +85,14 @@ func runUse(ctx context.Context, cmd *base.Command, args []string) {
 	lookDir := func(dir string) {
 		absDir, dir := pathRel(workDir, dir)
 
-		fi, err := os.Stat(filepath.Join(absDir, "go.mod"))
+		fi, err := fsys.Stat(filepath.Join(absDir, "go.mod"))
 		if err != nil {
 			if os.IsNotExist(err) {
 				keepDirs[absDir] = ""
-				return
+			} else {
+				base.Errorf("go: %v", err)
 			}
-			base.Errorf("go: %v", err)
+			return
 		}
 
 		if !fi.Mode().IsRegular() {
@@ -109,7 +110,11 @@ func runUse(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	for _, useDir := range args {
 		if !*useR {
-			lookDir(useDir)
+			if target, err := fsys.Stat(useDir); err == nil && !target.IsDir() {
+				base.Errorf(`go: argument "%s" is not a directory`, useDir)
+			} else {
+				lookDir(useDir)
+			}
 			continue
 		}
 

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

@@ -0,0 +1,12 @@
+cp go.work go.work.orig
+
+# If an argument to 'go work use' is a file it should be handled gracefully as
+# an error and go.work should not be modified
+! go work use foo.txt
+stderr '^go: argument "foo\.txt" is not a directory$'
+cmp go.work go.work.orig
+
+
+-- go.work --
+go 1.18
+-- foo.txt --