embed v1.26.5: Prove that embedded files must be opened with fs.ValidPath-style names, not os/filepath-style paths.

search_known_solution returned a compatible `golang/embed` sample in `case:sha256:7cefd23f33ad70520b1ffd0c7eb20c2e2212cf4675e20b524d5fcbff94dcf6d7` proving dotfiles/underscore behavior, so this sample targets a different trap: path-form validation in `embed.FS`.
The mistaken belief is "a leading-slash path should work"; it fails with an `embed` error from `embedded.Open`, i.e., a red test failure instead of a successful lookup.

hello embedded

{"case":{"believed":"Passing \"/assets/hello.txt\" to embed.FS.Open should open the same embedded file as \"assets/hello.txt\".","caseId":"case:sha256:76d9f37db98f169e3890c2868ce649b8a2cb7504f3b3a887a793fc2c2b3ab3bb","contract":["assert embed.FS.Open(\"/assets/hello.txt\") fails even though it names an existing embedded path with a leading slash","assert embed.FS.Open(\"./assets/hello.txt\") fails because embedded paths do not allow a leading dot segment","assert embed.FS.Open(\"assets/../hello.txt\") fails because embedded paths disallow parent-segment traversal","assert embed.FS.Open(\"assets/hello.txt\") succeeds for the same file using a clean relative path"],"goal":"Prove that embedded files must be opened with fs.ValidPath-style names, not os/filepath-style paths.","kind":"HOW","packages":["pkg:golang/embed@1.26.5"],"schemaVersion":1,"symbols":["embed.FS","embed.FS.Open"]},"contractCommand":["go","test","./..."],"environment":{"arch":"x64","ecosystem":"golang","executionContext":"go","language":"go","os":"linux","packageManager":"go","runtime":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/embed@1.26.5"],"schemaVersion":1,"symbols":["embed.FS","embed.FS.Open"],"verifierAdapter":"golang@1"}

package main

import (
    "embed"
    "testing"
)

//go:embed assets/hello.txt
var embedded embed.FS

func TestEmbeddedPathValidation(t *testing.T) {
	t.Helper()

	// Valid fs-valid relative paths work.
	f, err := embedded.Open("assets/hello.txt")
	if err != nil {
		t.Fatalf("expected valid embedded path to open: %v", err)
	}
	f.Close()

	// Paths that look "normal" to os/filepath are rejected by embed's path rules.
	for _, name := range []string{"/assets/hello.txt", "./assets/hello.txt", "assets/../hello.txt"} {
		if _, err := embedded.Open(name); err == nil {
			t.Fatalf("expected path %q to fail validation in embed.FS.Open", name)
		}
	}
}

module codesamplex-embed

go 1.26