mounts: read a password only from the directory the configuration names, and say a domain goes in the username

A configuration without the passwords directory would have read a path
relative to the process's working directory; it is now refused as a fault.
Review of mesh-catalog#190 (issue 386, ADR 0283).

modules/mounts/README.md

@@ -69,7 +69,8 @@ All three are preferences whose default, `none`, names nothing (ADR 0262 allows
   For example `games=smb://192.0.2.76/Share@/mnt/games`.
 - `smb-users`: the username each SMB source logs in with, space-separated `name=username`, for example
   `games=root`. A username is not a secret, so it is a setting (novox/hq ADR 0283). It holds no space,
-  comma or `=`.
+  comma or `=`. A device that needs a domain takes it in the username, as `DOMAIN\user` or `user@domain`:
+  the credentials file has no `domain=` line.
 
 A mount point is an absolute, plainly written path, not a directory of the system's own (`/etc`, `/usr`,
 `/run`, …). An entry that cannot be read is said, and only that entry is left out.

modules/mounts/cmd/mounts/apply.go

@@ -714,10 +714,15 @@ func (a *Applier) login(cfg Config, source string) (SMBCredential, []health.Wait
 	if !hasUser {
 		waits = append(waits, health.Wait{Part: part, Setting: "smb-users", What: "the username of the source " + source})
 	}
+	// Never a relative path: without the directory the mesh places the passwords in, nothing is read.
+	if !filepath.IsAbs(cfg.Passwords) {
+		return SMBCredential{}, nil, "the module's configuration names no directory for its passwords, so " +
+			PasswordSecret(source) + " cannot be read"
+	}
 	var password string
 	raw, err := a.M.read(PasswordFile(cfg.Passwords, source))
 	switch {
-	case cfg.Passwords == "" || errors.Is(err, fs.ErrNotExist):
+	case errors.Is(err, fs.ErrNotExist):
 		waits = append(waits, health.Wait{Part: part, Secret: PasswordSecret(source), What: "the password of the source " + source})
 	case err != nil:
 		return SMBCredential{}, nil, "its password " + PasswordSecret(source) + " could not be read: " + bare(err)

modules/mounts/cmd/mounts/write_test.go

@@ -681,3 +681,36 @@ func TestAWaitBesideAFaultIsUnhealthy(t *testing.T) {
 		t.Errorf("a wait alone: %+v", h)
 	}
 }
+
+// A password that exists and cannot be read, and a configuration naming no directory for the passwords, are
+// refused as faults: said without the value and without the path, and nothing is read from a relative path.
+func TestAPasswordThatCannotBeReadIsAFaultSaidWithoutValueOrPath(t *testing.T) {
+	// A directory where the file should be: read fails, and not as "not there" (as root a mode would not stop it).
+	f := newSystem(t)
+	f.write(ConfigFile, "sources=games=smb://192.0.2.76/Share@/mnt/games\nsmb-users=games=root\npasswords=/var/lib/mesh/mounts\nstate=/var/lib/mesh/mounts\n")
+	f.write("/var/lib/mesh/mounts/smb-password-games.secret/inside", testPassword)
+	st := f.applier(time.Now()).Look(context.Background())
+	if len(st.Shares) != 1 || st.Shares[0].State != StateFailed || !strings.Contains(st.Shares[0].Why, "could not be read") ||
+		strings.Contains(st.Shares[0].Why, "/var/lib") || strings.Contains(st.Shares[0].Why, testPassword) {
+		t.Fatalf("an unreadable password: %+v", st.Shares)
+	}
+
+	// No passwords= and no state=: the source is not set up, and no relative path is read.
+	for _, cfg := range []string{
+		"sources=games=smb://192.0.2.76/Share@/mnt/games\nsmb-users=games=root\n",
+		"sources=games=smb://192.0.2.76/Share@/mnt/games\nsmb-users=games=root\npasswords=relative/dir\n",
+	} {
+		g := newSystem(t)
+		g.write(ConfigFile, cfg)
+		g.write("smb-password-games.secret", testPassword)
+		g.write("relative/dir/smb-password-games.secret", testPassword)
+		st := g.applier(time.Now()).Look(context.Background())
+		if len(st.Shares) != 1 || st.Shares[0].State != StateFailed || !strings.Contains(st.Shares[0].Why, "names no directory for its passwords") ||
+			strings.Contains(st.Shares[0].Why, testPassword) || strings.Contains(st.Shares[0].Why, "relative") {
+			t.Errorf("%q: %+v", cfg, st.Shares)
+		}
+		if _, err := os.Stat(filepath.Join(g.root, "/var/lib/mesh/mounts/smb-games.credentials")); !os.IsNotExist(err) {
+			t.Errorf("%q: a credentials file was written", cfg)
+		}
+	}
+}