internal/helper: relocate from helper

This package is ugly and is pending removal only kept alive by xdg-dbus-proxy.

Its exported symbols are made available until v0.4.0 where it will be removed for #24.

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

2 files changed, 272 insertions, 0 deletions

diff --git a/internal/helper/proc/files.go b/internal/helper/proc/files.go
new file mode 100644
index 00000000..0612718e
--- /dev/null
+++ b/internal/helper/proc/files.go
@@ -0,0 +1,162 @@
+package proc
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"sync/atomic"
+	"syscall"
+	"testing"
+	"time"
+)
+
+var FulfillmentTimeout = 2 * time.Second
+
+func init() {
+	if testing.Testing() {
+		FulfillmentTimeout *= 10
+	}
+}
+
+// A File is an extra file with deferred initialisation.
+type File interface {
+	// Init initialises File state. Init must not be called more than once.
+	Init(fd uintptr, v **os.File) uintptr
+	// Fd returns the fd value set on initialisation.
+	Fd() uintptr
+	// ErrCount returns count of error values emitted during fulfillment.
+	ErrCount() int
+	// Fulfill is called prior to process creation and must populate its corresponding file address.
+	// Calls to dispatchErr must match the return value of ErrCount.
+	// Fulfill must not be called more than once.
+	Fulfill(ctx context.Context, dispatchErr func(error)) error
+}
+
+// ExtraFilesPre is a linked list storing addresses of [os.File].
+type ExtraFilesPre struct {
+	n *ExtraFilesPre
+	v *os.File
+}
+
+// Append grows the list by one entry and returns an address of the address of [os.File] stored in the new entry.
+func (f *ExtraFilesPre) Append() (uintptr, **os.File) { return f.append(3) }
+
+// Files returns a slice pointing to a continuous segment of memory containing all addresses stored in f in order.
+func (f *ExtraFilesPre) Files() []*os.File { return f.copy(make([]*os.File, 0, f.len())) }
+
+func (f *ExtraFilesPre) append(i uintptr) (uintptr, **os.File) {
+	if f.n == nil {
+		f.n = new(ExtraFilesPre)
+		return i, &f.v
+	}
+	return f.n.append(i + 1)
+}
+func (f *ExtraFilesPre) len() uintptr {
+	if f == nil {
+		return 0
+	}
+	return f.n.len() + 1
+}
+func (f *ExtraFilesPre) copy(e []*os.File) []*os.File {
+	if f == nil {
+		// the public methods ensure the first call is never nil;
+		// the last element is unused, slice it off here
+		return e[:len(e)-1]
+	}
+	return f.n.copy(append(e, f.v))
+}
+
+// Fulfill calls the [File.Fulfill] method on all files, starts cmd and blocks until all fulfillment completes.
+func Fulfill(ctx context.Context,
+	v *[]*os.File, start func() error,
+	files []File, extraFiles *ExtraFilesPre,
+) (err error) {
+	var ecs int
+	for _, o := range files {
+		ecs += o.ErrCount()
+	}
+	ec := make(chan error, ecs)
+
+	c, cancel := context.WithTimeout(ctx, FulfillmentTimeout)
+	defer cancel()
+
+	for _, f := range files {
+		err = f.Fulfill(c, makeDispatchErr(f, ec))
+		if err != nil {
+			return
+		}
+	}
+
+	*v = extraFiles.Files()
+	if err = start(); err != nil {
+		return
+	}
+
+	for ecs > 0 {
+		select {
+		case err = <-ec:
+			ecs--
+			if err != nil {
+				break
+			}
+		case <-ctx.Done():
+			err = syscall.ECANCELED
+			break
+		}
+	}
+	return
+}
+
+// InitFile initialises f as part of the slice extraFiles points to,
+// and returns its final fd value.
+func InitFile(f File, extraFiles *ExtraFilesPre) (fd uintptr) { return f.Init(extraFiles.Append()) }
+
+// BaseFile implements the Init method of the File interface and provides indirect access to extra file state.
+type BaseFile struct {
+	fd uintptr
+	v  **os.File
+}
+
+func (f *BaseFile) Init(fd uintptr, v **os.File) uintptr {
+	if v == nil || fd < 3 {
+		panic("invalid extra file initial state")
+	}
+	if f.v != nil {
+		panic("extra file initialised twice")
+	}
+	f.fd, f.v = fd, v
+	return fd
+}
+
+func (f *BaseFile) Fd() uintptr {
+	if f.v == nil {
+		panic("use of uninitialised extra file")
+	}
+	return f.fd
+}
+
+func (f *BaseFile) Set(v *os.File) {
+	*f.v = v // runtime guards against use before init
+}
+
+func makeDispatchErr(f File, ec chan<- error) func(error) {
+	c := new(atomic.Int32)
+	c.Store(int32(f.ErrCount()))
+	return func(err error) {
+		if c.Add(-1) < 0 {
+			panic("unexpected error dispatches")
+		}
+		ec <- err
+	}
+}
+
+func ExtraFile(cmd *exec.Cmd, f *os.File) (fd uintptr) {
+	return ExtraFileSlice(&cmd.ExtraFiles, f)
+}
+
+func ExtraFileSlice(extraFiles *[]*os.File, f *os.File) (fd uintptr) {
+	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
+	fd = uintptr(3 + len(*extraFiles))
+	*extraFiles = append(*extraFiles, f)
+	return
+}
diff --git a/internal/helper/proc/pipe.go b/internal/helper/proc/pipe.go
new file mode 100644
index 00000000..838c4ae4
--- /dev/null
+++ b/internal/helper/proc/pipe.go
@@ -0,0 +1,110 @@
+package proc
+
+import (
+	"context"
+	"errors"
+	"io"
+	"os"
+	"runtime"
+)
+
+// NewWriterTo returns a [File] that receives content from wt on fulfillment.
+func NewWriterTo(wt io.WriterTo) File { return &writeToFile{wt: wt} }
+
+// writeToFile exports the read end of a pipe with data written by an [io.WriterTo].
+type writeToFile struct {
+	wt io.WriterTo
+	BaseFile
+}
+
+func (f *writeToFile) ErrCount() int { return 3 }
+func (f *writeToFile) Fulfill(ctx context.Context, dispatchErr func(error)) error {
+	r, w, err := os.Pipe()
+	if err != nil {
+		return err
+	}
+	f.Set(r)
+
+	done := make(chan struct{})
+	go func() {
+		_, err = f.wt.WriteTo(w)
+		dispatchErr(err)
+		dispatchErr(w.Close())
+		close(done)
+		runtime.KeepAlive(r)
+	}()
+	go func() {
+		select {
+		case <-done:
+			dispatchErr(nil)
+		case <-ctx.Done():
+			dispatchErr(w.Close()) // this aborts WriteTo with file already closed
+			runtime.KeepAlive(r)
+		}
+	}()
+
+	return nil
+}
+
+// NewStat returns a [File] implementing the behaviour
+// of the receiving end of xdg-dbus-proxy stat fd.
+func NewStat(s *io.Closer) File { return &statFile{s: s} }
+
+var (
+	ErrStatFault = errors.New("generic stat fd fault")
+	ErrStatRead  = errors.New("unexpected stat behaviour")
+)
+
+// statFile implements xdg-dbus-proxy stat fd behaviour.
+type statFile struct {
+	s *io.Closer
+	BaseFile
+}
+
+func (f *statFile) ErrCount() int { return 2 }
+func (f *statFile) Fulfill(ctx context.Context, dispatchErr func(error)) error {
+	r, w, err := os.Pipe()
+	if err != nil {
+		return err
+	}
+	f.Set(w)
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		var n int
+
+		n, err = r.Read(make([]byte, 1))
+		switch n {
+		case -1:
+			if err == nil {
+				err = ErrStatFault
+			}
+			dispatchErr(err)
+		case 0:
+			if err == nil {
+				err = ErrStatRead
+			}
+			dispatchErr(err)
+		case 1:
+			dispatchErr(err)
+		default:
+			panic("unreachable")
+		}
+		runtime.KeepAlive(w)
+	}()
+
+	go func() {
+		select {
+		case <-done:
+			dispatchErr(nil)
+		case <-ctx.Done():
+			dispatchErr(r.Close()) // this aborts Read with file already closed
+			runtime.KeepAlive(w)
+		}
+	}()
+
+	// this gets closed by the caller
+	*f.s = r
+	return nil
+}