Ask the running runtimes where they keep their data, and read every form a unit says it in

The third review of #64 (hq issue 339): an ExecStart continued over lines, a
quoted path, a variable from Environment= or EnvironmentFile= across the unit
and its drop-ins, dockerd's --config-file and podman's TOML literal strings
each hid a data root from the guard. The running runtimes are now asked first
(docker info, podman info) on a runner of the guard's own, and the files are
read beside them in all those forms.

internal/apply/runtime_roots_more_test.go

@@ -0,0 +1,62 @@
+package apply
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// The forms a runtime's data root is said in (novox/hq issue 339, the third review): an ExecStart continued over
+// lines, single quotes, a variable from Environment= or EnvironmentFile=, dockerd's --config-file, and TOML literal
+// strings in podman's storage.conf. And the running runtimes are asked first: they say where they really keep it.
+func TestEveryFormOfADataRootIsRead(t *testing.T) {
+	dir := t.TempDir()
+	write := func(name, content string) string {
+		t.Helper()
+		p := filepath.Join(dir, name)
+		if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(p, []byte(content), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		return p
+	}
+	envFile := write("docker.env", "DOCKER_ROOT=/data/from-env-file\n")
+	other := write("other-daemon.json", `{"data-root": "/data/from-config-file"}`)
+	unit := write("lib/docker.service", "[Service]\nEnvironment=\"ROOT=/data/from-env\"\nEnvironmentFile=-"+envFile+"\n"+
+		"ExecStart=/usr/bin/dockerd \\\n    -H fd:// \\\n    --data-root '/data/continued'\n")
+	dropIn := write("etc/docker.service.d/root.conf", "[Service]\nExecStart=\nExecStart=/usr/bin/dockerd --data-root=$ROOT "+
+		"--graph=${DOCKER_ROOT} --config-file "+other+"\n")
+	storage := write("containers/storage.conf", "[storage]\ngraphroot = '/data/podman-literal'\n")
+	before := runtimeConfigs
+	runtimeConfigs = runtimeFiles{daemonJSON: filepath.Join(dir, "none.json"), units: []string{unit},
+		dropInDirs: []string{filepath.Dir(dropIn)}, storageConf: storage}
+	t.Cleanup(func() { runtimeConfigs = before })
+
+	roots := strings.Join(readRuntimeRoots(context.Background(), nil), " ")
+	for _, want := range []string{"/data/continued", "/data/from-env", "/data/from-env-file", "/data/from-config-file",
+		"/data/podman-literal"} {
+		if !strings.Contains(roots+" ", want+" ") {
+			t.Errorf("%s was not read: %s", want, roots)
+		}
+	}
+
+	// Asked of the running runtimes first; what they answer counts beside the files.
+	asked := func(_ context.Context, name string, args ...string) (string, error) {
+		switch name {
+		case "docker":
+			return "/srv/docker-live\n", nil
+		case "podman":
+			return "", errors.New("podman: not installed")
+		}
+		return "", errors.New("unexpected")
+	}
+	roots = strings.Join(readRuntimeRoots(context.Background(), asked), " ")
+	if !strings.Contains(roots, "/srv/docker-live") || !strings.Contains(roots, "/data/continued") {
+		t.Errorf("the running runtime's answer, or the files beside it, are missing: %s", roots)
+	}
+}