internal/pkg: support explicit overlay mount

This removes all but the /work/ auto overlay behaviour and enables much greater flexibility. This also renames ExecContainerPath to ExecPath so it is easier to type.

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

internal/pkg/exec.go

@@ -20,17 +20,27 @@ import (
 // AbsWork is the container pathname [CureContext.GetWorkDir] is mounted on.
 var AbsWork = fhs.AbsRoot.Append("work/")
 
-// ExecContainerPath is an [Artifact] and the [check.Absolute] pathname to make
-// it available under in the container.
-type ExecContainerPath struct {
+// ExecPath is a slice of [Artifact] and the [check.Absolute] pathname to make
+// it available at under in the container.
+type ExecPath struct {
+	// Pathname in the container mount namespace.
 	P *check.Absolute
-	A Artifact
+	// Artifacts to mount on the pathname, must contain at least one [Artifact].
+	// If there are multiple entries or W is true, P is set up as an overlay
+	// mount, and entries of A must not implement [File].
+	A []Artifact
+	// Whether to make the mount point writable via an invisible tmpfs upperdir.
+	W bool
 }
 
-// MustPath returns [ExecContainerPath] for pathname and [Artifact] and panics
-// if pathname is not absolute.
-func MustPath(pathname string, a Artifact) ExecContainerPath {
-	return ExecContainerPath{check.MustAbs(pathname), a}
+// Path returns a populated [ExecPath].
+func Path(pathname *check.Absolute, writable bool, a ...Artifact) ExecPath {
+	return ExecPath{pathname, a, writable}
+}
+
+// MustPath is like [Path], but takes a string pathname via [check.MustAbs].
+func MustPath(pathname string, writable bool, a ...Artifact) ExecPath {
+	return ExecPath{check.MustAbs(pathname), a, writable}
 }
 
 // An execArtifact is an [Artifact] that produces output by running a program
@@ -41,8 +51,8 @@ func MustPath(pathname string, a Artifact) ExecContainerPath {
 type execArtifact struct {
 	// Caller-supplied context.
 	ctx context.Context
-	// Caller-supplied inner read-only bind mounts.
-	paths []ExecContainerPath
+	// Caller-supplied inner mount points.
+	paths []ExecPath
 	// Caller-supplied logging facility, passed through to [container] and used
 	// internally to produce verbose output.
 	msg message.Msg
@@ -95,20 +105,11 @@ func (a *execNetArtifact) Cure(c *CureContext) error {
 //
 // The working and temporary directories are both created and mounted writable
 // on [AbsWork] and [fhs.AbsTmp] respectively. If one or more paths target
-// [fhs.AbsTmp], the final entry is set up as a writable overlay mount on /tmp
-// backed by the host side temporary directory. If one or more paths target
 // [AbsWork], the final entry is set up as a writable overlay mount on /work for
 // which the upperdir is the host side work directory. In this configuration,
-// the program must avoid causing whiteout files to be created, cure fails if
-// upperdir ends up with anything other than directory, regular or symlink
-// entries.
-//
-// If the first path targets [fhs.AbsRoot], it is made writable via an overlay
-// mount with writes going to an ephemeral tmpfs bound to the lifetime of the
-// container. This is primarily to make it possible for [container] to set up
-// mount points targeting paths not available in the [Artifact] backing root,
-// and to accommodate poorly written programs that insist on writing to awkward
-// paths, it must not be used as scratch space.
+// the W field is ignored, and the program must avoid causing whiteout files to
+// be created. Cure fails if upperdir ends up with entries other than directory,
+// regular or symlink.
 //
 // If checksum is non-nil, the resulting [Artifact] implements [KnownChecksum]
 // and its container runs in the host net namespace.
@@ -126,7 +127,7 @@ func NewExec(
 	path *check.Absolute,
 	args []string,
 
-	paths ...ExecContainerPath,
+	paths ...ExecPath,
 ) Artifact {
 	a := execArtifact{ctx, paths, msg, cures, dir, env, path, args}
 	if checksum == nil {
@@ -142,17 +143,22 @@ func (a *execArtifact) Kind() Kind { return KindExec }
 func (a *execArtifact) Params() []byte {
 	var buf bytes.Buffer
 	for _, p := range a.paths {
+		if p.W {
+			buf.WriteByte(1)
+		} else {
+			buf.WriteByte(0)
+		}
 		if p.P != nil {
 			buf.WriteString(p.P.String())
 		} else {
 			buf.WriteString("invalid P\x00")
 		}
-		if p.A != nil {
-			id := Ident(p.A)
+		buf.WriteByte(0)
+		for _, d := range p.A {
+			id := Ident(d)
 			buf.Write(id[:])
-		} else {
-			buf.WriteString("invalid A\x00")
 		}
+		buf.WriteByte(0)
 	}
 	buf.WriteByte(0)
 	buf.WriteString(a.dir.String())
@@ -170,15 +176,13 @@ func (a *execArtifact) Params() []byte {
 }
 
 // Dependencies returns a slice of all artifacts collected from caller-supplied
-// [ExecContainerPath].
+// [ExecPath].
 func (a *execArtifact) Dependencies() []Artifact {
-	artifacts := make([]Artifact, 0, len(a.paths))
+	artifacts := make([][]Artifact, 0, len(a.paths))
 	for _, p := range a.paths {
-		if p.A != nil {
-			artifacts = append(artifacts, p.A)
-		}
+		artifacts = append(artifacts, p.A)
 	}
-	return artifacts
+	return slices.Concat(artifacts...)
 }
 
 // Cure cures the [Artifact] by curing all its dependencies then running the
@@ -200,35 +204,42 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 		cures = runtime.NumCPU()
 	}
 
-	overlayTempIndex, overlayWorkIndex := -1, -1
-	paths := make([][2]*check.Absolute, len(a.paths))
+	overlayWorkIndex := -1
+	type curePath struct {
+		// Copied from ExecPath.P.
+		dst *check.Absolute
+		// Cured from ExecPath.A.
+		src []*check.Absolute
+	}
+	paths := make([]curePath, len(a.paths))
 	for i, p := range a.paths {
-		if p.P == nil || p.A == nil {
+		if p.P == nil || len(p.A) == 0 {
 			return os.ErrInvalid
 		}
-		if p.P.Is(fhs.AbsTmp) {
-			overlayTempIndex = i
-		} else if p.P.Is(AbsWork) {
+		if p.P.Is(AbsWork) {
 			overlayWorkIndex = i
 		}
-		paths[i][1] = p.P
+		paths[i].dst = p.P
+		paths[i].src = make([]*check.Absolute, len(p.A))
 	}
 
 	if len(paths) > 0 {
 		type cureArtifact struct {
 			// Index of pending Artifact in paths.
-			index int
+			index [2]int
 			// Pending artifact.
 			a Artifact
 		}
 		ac := make(chan cureArtifact, len(paths))
 		for i, p := range a.paths {
-			ac <- cureArtifact{i, p.A}
+			for j, d := range p.A {
+				ac <- cureArtifact{[2]int{i, j}, d}
+			}
 		}
 
 		type cureRes struct {
 			// Index of result in paths.
-			index int
+			index [2]int
 			// Cured pathname.
 			pathname *check.Absolute
 			// Error returned by c.
@@ -260,7 +271,7 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 			if cr.err != nil {
 				errs = append(errs, cr.err)
 			} else {
-				paths[cr.index][0] = cr.pathname
+				paths[cr.index[0]].src[cr.index[1]] = cr.pathname
 			}
 
 			if count == len(paths) {
@@ -293,32 +304,9 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 
 	z.Dir, z.Env, z.Path, z.Args = a.dir, a.env, a.path, a.args
 	z.Grow(len(paths) + 4)
-	if len(paths) > 0 && paths[0][1].Is(fhs.AbsRoot) {
-		z.OverlayEphemeral(fhs.AbsRoot, paths[0][0])
-		paths = paths[1:]
-		overlayTempIndex--
-		overlayWorkIndex--
-	}
 
 	temp, work := c.GetTempDir(), c.GetWorkDir()
 	for i, b := range paths {
-		if i == overlayTempIndex {
-			tempUpper := temp.Append("upper")
-			if err = os.MkdirAll(tempUpper.String(), 0700); err != nil {
-				return
-			}
-			tempWork := temp.Append("work")
-			if err = os.MkdirAll(tempWork.String(), 0700); err != nil {
-				return
-			}
-			z.Overlay(
-				fhs.AbsTmp,
-				tempUpper,
-				tempWork,
-				b[0],
-			)
-			continue
-		}
 		if i == overlayWorkIndex {
 			if err = os.MkdirAll(work.String(), 0700); err != nil {
 				return
@@ -331,11 +319,18 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 				AbsWork,
 				work,
 				tempWork,
-				b[0],
+				b.src...,
 			)
 			continue
 		}
-		z.Bind(b[0], b[1], 0)
+
+		if a.paths[i].W {
+			z.OverlayEphemeral(b.dst, b.src...)
+		} else if len(b.src) == 1 {
+			z.Bind(b.src[0], b.dst, 0)
+		} else {
+			z.OverlayReadonly(b.dst, b.src...)
+		}
 	}
 	if overlayWorkIndex < 0 {
 		z.Bind(
@@ -344,13 +339,11 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 			std.BindWritable|std.BindEnsure,
 		)
 	}
-	if overlayTempIndex < 0 {
-		z.Bind(
-			c.GetTempDir(),
-			fhs.AbsTmp,
-			std.BindWritable|std.BindEnsure,
-		)
-	}
+	z.Bind(
+		c.GetTempDir(),
+		fhs.AbsTmp,
+		std.BindWritable|std.BindEnsure,
+	)
 	z.Proc(fhs.AbsProc).Dev(fhs.AbsDev, true)
 
 	if err = z.Start(); err != nil {