app: clean up interactions and handle all application state and setup/teardown

There was an earlier attempt of cleaning up the app package however it ended up creating even more of a mess and the code structure largely still looked like Ego with state setup scattered everywhere and a bunch of ugly hacks had to be implemented to keep track of all of them. In this commit the entire app package is rewritten to track everything that has to do with an app in one thread safe value.

In anticipation of the client/server split also made changes:
- Console messages are cleaned up to be consistent
- State tracking is fully rewritten to be cleaner and usable for multiple process and client/server
- Encapsulate errors to easier identify type of action causing the error as well as additional info
- System-level setup operations is grouped in a way that can be collectively committed/reverted
  and gracefully handles errors returned by each operation
- Resource sharing is made more fine-grained with PID-scoped resources whenever possible,
  a few remnants (X11, Wayland, PulseAudio) will be addressed when a generic proxy is available
- Application setup takes a JSON-friendly config struct and deterministically generates system setup operations

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/share.pulse.go

@@ -0,0 +1,116 @@
+package app
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+
+	"git.ophivana.moe/cat/fortify/acl"
+	"git.ophivana.moe/cat/fortify/internal/state"
+)
+
+const (
+	pulseServer = "PULSE_SERVER"
+	pulseCookie = "PULSE_COOKIE"
+
+	home          = "HOME"
+	xdgConfigHome = "XDG_CONFIG_HOME"
+)
+
+var (
+	ErrPulseCookie = errors.New("pulse cookie not present")
+	ErrPulseSocket = errors.New("pulse socket not present")
+	ErrPulseMode   = errors.New("unexpected pulse socket mode")
+)
+
+type (
+	PulseCookieAccessError BaseError
+	PulseSocketAccessError BaseError
+)
+
+func (seal *appSeal) sharePulse() error {
+	if !seal.et.Has(state.EnablePulse) {
+		return nil
+	}
+
+	// ensure PulseAudio directory ACL (e.g. `/run/user/%d/pulse`)
+	pd := path.Join(seal.RuntimePath, "pulse")
+	ps := path.Join(pd, "native")
+	if _, err := os.Stat(pd); err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return (*PulseSocketAccessError)(wrapError(err,
+				fmt.Sprintf("cannot access PulseAudio directory '%s':", pd), err))
+		}
+		return (*PulseSocketAccessError)(wrapError(ErrPulseSocket,
+			fmt.Sprintf("PulseAudio directory '%s' not found", pd)))
+	}
+
+	seal.appendEnv(pulseServer, "unix:"+ps)
+	seal.sys.updatePerm(pd, acl.Execute)
+
+	// ensure PulseAudio socket permission (e.g. `/run/user/%d/pulse/native`)
+	if s, err := os.Stat(ps); err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return (*PulseSocketAccessError)(wrapError(err,
+				fmt.Sprintf("cannot access PulseAudio socket '%s':", ps), err))
+		}
+		return (*PulseSocketAccessError)(wrapError(ErrPulseSocket,
+			fmt.Sprintf("PulseAudio directory '%s' found but socket does not exist", pd)))
+	} else {
+		if m := s.Mode(); m&0o006 != 0o006 {
+			return (*PulseSocketAccessError)(wrapError(ErrPulseMode,
+				fmt.Sprintf("unexpected permissions on '%s':", ps), m))
+		}
+	}
+
+	// publish current user's pulse cookie for target user
+	if src, err := discoverPulseCookie(); err != nil {
+		return err
+	} else {
+		dst := path.Join(seal.share, "pulse-cookie")
+		seal.appendEnv(pulseCookie, dst)
+		seal.sys.copyFile(dst, src)
+	}
+
+	return nil
+}
+
+// discoverPulseCookie attempts various standard methods to discover the current user's PulseAudio authentication cookie
+func discoverPulseCookie() (string, error) {
+	if p, ok := os.LookupEnv(pulseCookie); ok {
+		return p, nil
+	}
+
+	// dotfile $HOME/.pulse-cookie
+	if p, ok := os.LookupEnv(home); ok {
+		p = path.Join(p, ".pulse-cookie")
+		if s, err := os.Stat(p); err != nil {
+			if !errors.Is(err, fs.ErrNotExist) {
+				return p, (*PulseCookieAccessError)(wrapError(err,
+					fmt.Sprintf("cannot access PulseAudio cookie '%s':", p), err))
+			}
+			// not found, try next method
+		} else if !s.IsDir() {
+			return p, nil
+		}
+	}
+
+	// $XDG_CONFIG_HOME/pulse/cookie
+	if p, ok := os.LookupEnv(xdgConfigHome); ok {
+		p = path.Join(p, "pulse", "cookie")
+		if s, err := os.Stat(p); err != nil {
+			if !errors.Is(err, fs.ErrNotExist) {
+				return p, (*PulseCookieAccessError)(wrapError(err, "cannot access PulseAudio cookie", p+":", err))
+			}
+			// not found, try next method
+		} else if !s.IsDir() {
+			return p, nil
+		}
+	}
+
+	return "", (*PulseCookieAccessError)(wrapError(ErrPulseCookie,
+		fmt.Sprintf("cannot locate PulseAudio cookie (tried $%s, $%s/pulse/cookie, $%s/.pulse-cookie)",
+			pulseCookie, xdgConfigHome, home)))
+}