A delivered host reads its version from where it sits

novox/hq ADR 0142, which decided this and was not implemented: "It is
unpacked into a directory named for its version, so it can read its own
version from its path. The stamp goes, and with it the need for a build to
know what it will be called."

The mesh's toolchain stamps no version, on purpose, so a delivered host
called itself "development build" and the mesh could not tell which host
any machine ran — which is the whole of what 087 added. A delivered host
lives at <libexec>/versions/<version>/<binary>, and that directory is the
answer.

A host placed by hand keeps its stamp, which is the honest answer for one
the mesh did not deliver, and is every machine until a delivery reaches
it. A binary sitting anywhere else is not read as a version at all.

The decision is split from the reading so a test can ask about a path
without being that binary.

cmd/mesh-host/main.go

@@ -51,6 +51,43 @@ var builtFor = ""
 
 var version = "development build"
 
+// runningVersion is this host's version: the directory it was delivered into, or the link-time stamp
+// for one placed by hand.
+//
+// **From where it sits, not from its linker** (novox/hq ADR 0142): "It is unpacked into a directory
+// named for its version, so it can read its own version from its path. The stamp goes, and with it the
+// need for a build to know what it will be called."
+//
+// The mesh's toolchain does not stamp a version, on purpose — a build does not know what it will be
+// called — so a delivered host read as "development build" and the mesh could not tell which host any
+// machine ran (novox/hq 04-ISSUES/161, and 087 for why that matters). The path knows: a delivered host
+// lives at `<libexec>/versions/<version>/<binary>`.
+//
+// A host placed by hand keeps its stamp, which is the honest answer for one the mesh did not deliver.
+func runningVersion() string {
+	self, err := os.Executable()
+	if err != nil {
+		return version
+	}
+	return versionAt(self, version)
+}
+
+// versionAt is runningVersion's decision, with the executable's path and the link-time stamp given —
+// so a test can ask it about a path without being that binary.
+func versionAt(self, stamped string) string {
+	// .../versions/<version>/<binary> — the parent is the version, and its parent is the versions
+	// directory. Checked rather than assumed, so a binary somewhere else does not read a directory
+	// name as a version.
+	dir := filepath.Dir(self)
+	if filepath.Base(filepath.Dir(dir)) != upgrade.VersionsDirName {
+		return stamped
+	}
+	if name := filepath.Base(dir); name != "" && name != "." && name != string(filepath.Separator) {
+		return name
+	}
+	return stamped
+}
+
 const usage = `mesh-host — the node host
 
   profile     what this machine can be asked to do
@@ -254,7 +291,7 @@ func run(ctx context.Context, command string, opts options) error {
 		return runLink(ctx, opts)
 
 	case "version":
-		fmt.Println(version)
+		fmt.Println(runningVersion())
 		return nil
 
 	case "", "help", "-h", "--help":
@@ -594,8 +631,8 @@ func runApply(ctx context.Context, opts options, d *declaration.Declaration, raw
 	//
 	// A failure to record is reported and does not fail the apply. The apply worked; what is
 	// lost is a rollback's ability to come back here, which is worse to hide than to say.
-	if version != "" {
+	if v := runningVersion(); v != "" {
-		if err := upgrade.RecordKnownGood(upgrade.KnownGoodPath(opts.state), version); err != nil {
+		if err := upgrade.RecordKnownGood(upgrade.KnownGoodPath(opts.state), v); err != nil {
 			fmt.Fprintf(os.Stderr,
 				"mesh-host: applied, but could not record %s as known-good: %v\n"+
 					"  a rollback would have nothing to return to.\n", version, err)
@@ -1369,7 +1406,7 @@ func applyAndKeep(ctx context.Context, opts options, raw []byte, signed *store.D
 		sched.Sync(declared, held)
 	}
 
-	report := link.Report{Carried: carriedPorts(updated), Declared: digestOf(raw), Host: version}
+	report := link.Report{Carried: carriedPorts(updated), Declared: digestOf(raw), Host: runningVersion()}
 	// Which of this machine's links face outside, for the filter the mesh writes around them
 	// (novox/hq ADR 0140). Reported whatever the node's mode: a converged node's filter needs it,
 	// and an adopted one becomes converged without a further round trip. A machine that cannot read

cmd/mesh-host/version_from_path_test.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// A component's version comes from where it sits, not from its linker (novox/hq ADR 0142). The mesh's
+// toolchain stamps no version — a build does not know what it will be called — so a delivered host
+// read as "development build" and the mesh could not tell which host a machine ran (04-ISSUES/161).
+
+func TestADeliveredHostReadsItsVersionFromItsPath(t *testing.T) {
+	// A delivered host lives at <libexec>/versions/<version>/<binary>.
+	dir := t.TempDir()
+	versioned := filepath.Join(dir, "versions", "637f65559d16")
+	if err := os.MkdirAll(versioned, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	self := filepath.Join(versioned, "nox-mesh-host")
+	if err := os.WriteFile(self, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if got := versionAt(self, "development build"); got != "637f65559d16" {
+		t.Fatalf("a delivered host read its version as %q", got)
+	}
+}
+
+func TestAHostPlacedByHandKeepsItsStamp(t *testing.T) {
+	// The honest answer for one the mesh did not deliver — and every machine is in that state until
+	// a delivery reaches it.
+	if got := versionAt("/usr/bin/nox-mesh-host", "04a27ca"); got != "04a27ca" {
+		t.Fatalf("a hand-placed host read its version as %q", got)
+	}
+}
+
+func TestADirectoryThatIsNotAVersionIsNotReadAsOne(t *testing.T) {
+	// A binary sitting anywhere else must not have its parent directory's name read as a version.
+	for _, path := range []string{
+		"/opt/somewhere/nox-mesh-host",
+		"/usr/lib/nox-mesh-host/launch",
+		"/home/someone/build/nox-mesh-host",
+	} {
+		if got := versionAt(path, "the stamp"); got != "the stamp" {
+			t.Fatalf("%s read its version as %q", path, got)
+		}
+	}
+}