cmd/go/internal/modfetch: avoid duplicating path components in Git fetch errors

Fixes #51114

Change-Id: Iebfe65f826b7b583ff2b48b8bc9d2eb23f2726af
Reviewed-on: https://go-review.googlesource.com/c/go/+/386234
Reviewed-by: Valentin Deleplace <deleplace@google.com>
Reviewed-by: Nooras Saba‎ <saba@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

src/cmd/go/internal/modfetch/coderepo.go

@@ -792,7 +792,7 @@ func (r *codeRepo) findDir(version string) (rev, dir string, gomod []byte, err e
 	file1 := path.Join(r.codeDir, "go.mod")
 	gomod1, err1 := r.code.ReadFile(rev, file1, codehost.MaxGoMod)
 	if err1 != nil && !os.IsNotExist(err1) {
-		return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.pathPrefix, file1, rev, err1)
+		return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.codeRoot, file1, rev, err1)
 	}
 	mpath1 := modfile.ModulePath(gomod1)
 	found1 := err1 == nil && (isMajor(mpath1, r.pathMajor) || r.canReplaceMismatchedVersionDueToBug(mpath1))
@@ -810,7 +810,7 @@ func (r *codeRepo) findDir(version string) (rev, dir string, gomod []byte, err e
 		file2 = path.Join(dir2, "go.mod")
 		gomod2, err2 := r.code.ReadFile(rev, file2, codehost.MaxGoMod)
 		if err2 != nil && !os.IsNotExist(err2) {
-			return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.pathPrefix, file2, rev, err2)
+			return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.codeRoot, file2, rev, err2)
 		}
 		mpath2 := modfile.ModulePath(gomod2)
 		found2 := err2 == nil && isMajor(mpath2, r.pathMajor)
@@ -823,9 +823,9 @@ func (r *codeRepo) findDir(version string) (rev, dir string, gomod []byte, err e
 		}
 		if err2 == nil {
 			if mpath2 == "" {
-				return "", "", nil, fmt.Errorf("%s/%s is missing module path at revision %s", r.pathPrefix, file2, rev)
+				return "", "", nil, fmt.Errorf("%s/%s is missing module path at revision %s", r.codeRoot, file2, rev)
 			}
-			return "", "", nil, fmt.Errorf("%s/%s has non-...%s module path %q at revision %s", r.pathPrefix, file2, r.pathMajor, mpath2, rev)
+			return "", "", nil, fmt.Errorf("%s/%s has non-...%s module path %q at revision %s", r.codeRoot, file2, r.pathMajor, mpath2, rev)
 		}
 	}
 

src/cmd/go/testdata/script/mod_download_issue51114.txt

@@ -0,0 +1,21 @@
+[short] skip
+[!exec:git] skip
+[!net] skip
+[!linux] skip  # Uses XDG_CONFIG_HOME
+
+env GIT_CONFIG_GLOBAL=$WORK/.gitconfig
+env GOPROXY=direct
+
+! go mod download
+stderr '^go: github\.com/golang/notexist/subdir@v0.1.0: reading github\.com/golang/notexist/subdir/go\.mod at revision subdir/v0\.1\.0: '
+
+-- go.mod --
+module test
+
+go 1.18
+
+require github.com/golang/notexist/subdir v0.1.0
+
+-- $WORK/.gitconfig --
+[url "git@github.com:"]
+	insteadOf = https://github.com/

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

@@ -22,22 +22,18 @@ stderr '^If this is a private repository, see https://golang.org/doc/faq#git_htt
 ! stderr 'unknown revision'
 ! stdout .
 
-[!linux] stop  # Needs XDG_CONFIG_HOME.
 [!exec:false] stop
 
 # Test that Git clone errors will be shown to the user instead of a generic
 # "unknown revision" error. To do this we want to force git ls-remote to return
 # an error we don't already have special handling for. See golang/go#42751.
-#
-# Set XDG_CONFIG_HOME to tell Git where to look for the git config file listed
-# below, which turns on ssh.
-env XDG_CONFIG_HOME=$TMPDIR
+env HOME=$WORK${/}home${/}gopher
 env GIT_SSH_COMMAND=false
 ! go install github.com/golang/nonexist@master
 stderr 'fatal: Could not read from remote repository.'
 ! stderr 'unknown revision'
 ! stdout .
 
--- $TMPDIR/git/config --
+-- $WORK/home/gopher/.gitconfig --
 [url "git@github.com:"]
-    insteadOf = https://github.com/
+	insteadOf = https://github.com/