internal/pipewire: generic Core::Error handling

This flushes message buffer before queueing the event expecting the error. Since this is quite useful and relatively complex, it is relocated to a method of Context.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/core.go

@@ -642,27 +642,21 @@ func (registry *Registry) destroy(id Int) error {
 }
 
 // Destroy tries to destroy the global object with id.
-func (registry *Registry) Destroy(id Int) error {
-	if err := registry.destroy(id); err != nil {
+func (registry *Registry) Destroy(id Int) (err error) {
+	asCoreError := registry.ctx.expectsCoreError(registry.ID, &err)
+	if err != nil {
+		return
+	}
+	if err = registry.destroy(id); err != nil {
 		return err
 	}
-	destroySeq := registry.ctx.currentSeq()
-
-	err := registry.ctx.GetCore().Sync()
-	if err == nil {
+	if err = registry.ctx.GetCore().Sync(); err == nil {
 		return nil
 	}
 
-	var coreError *CoreError
-	if proxyErrors, ok := err.(ProxyConsumeError); !ok ||
-		len(proxyErrors) != 1 ||
-		!errors.As(proxyErrors[0], &coreError) ||
-		coreError == nil ||
-		coreError.ID != registry.ID ||
-		coreError.Sequence != destroySeq {
-		return err
-	}
-
+	if coreError := asCoreError(); coreError == nil {
+		return
+	} else {
 		switch syscall.Errno(-coreError.Result) {
 		case syscall.EPERM:
 			return &PermissionError{registry.ID, coreError.Message}
@@ -671,6 +665,7 @@ func (registry *Registry) Destroy(id Int) error {
 			return coreError
 		}
 	}
+}
 
 // An UnsupportedObjectTypeError is the name of a type not known by the server [Registry].
 type UnsupportedObjectTypeError string

internal/pipewire/pipewire.go

@@ -16,6 +16,7 @@ package pipewire
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -786,6 +787,40 @@ func (ctx *Context) Close() (err error) {
 	}
 }
 
+// expectsCoreError returns a function that inspects an error value and
+// returns the address of a [CoreError] if it is the only error present
+// and targets the specified proxy and sequence.
+//
+// The behaviour of expectsCoreError is only correct for an empty buf
+// prior to calling. If buf is not empty, [Core.Sync] is called, with
+// its return value stored to the value pointed to by errP if not nil,
+// and the function is not populated.
+//
+// The caller must queue a message and call [Core.Sync] immediately
+// after calling expectsCoreError.
+func (ctx *Context) expectsCoreError(id Int, errP *error) (asCoreError func() (coreError *CoreError)) {
+	if len(ctx.buf) > 0 {
+		if err := ctx.GetCore().Sync(); err != nil {
+			*errP = err
+			return nil
+		}
+	}
+
+	sequence := ctx.sequence
+	return func() (coreError *CoreError) {
+		if proxyErrors, ok := (*errP).(ProxyConsumeError); !ok ||
+			len(proxyErrors) != 1 ||
+			!errors.As(proxyErrors[0], &coreError) ||
+			coreError == nil ||
+			coreError.ID != id ||
+			coreError.Sequence != sequence {
+			// do not return a non-matching CoreError
+			coreError = nil
+		}
+		return
+	}
+}
+
 // A PermissionError describes an error emitted by the server when trying to
 // perform an operation that the client has no permission for.
 type PermissionError struct {