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>

1 files changed, 162 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
+}