HRUN: mesh-compose reads its pipeline input from MESH_RUN_INPUT

The composer answers the assigner's composition step: the request is read from the file
named by MESH_RUN_INPUT and the verdict is the last line of standard output, exit 0; the
compose-job path stays.

Builds: design 47 G §354, §63, §22; lead answer A-167

Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

cmd/mesh-compose/main.go

@@ -11,9 +11,12 @@
 // checkout or `git archive` of the catalogue repository at a ref: the command fetches nothing). -node adds a machine to
 // compose; -declarations answers each machine's declared paths with their owners and modes.
 //
-// It opens no network connection, bus, store or file beyond its standard input, the -request file and the -catalogue
-// directory, and reads
-// no environment. Exit codes: 0 the verdict is written (a machine that does not compose is a verdict, not an error);
+// Run as a pipeline step it reads its input, a composer.Request at depth compose, from the file named by the environment
+// variable MESH_RUN_INPUT and answers the run's output as its composer.Verdict on the last line of standard output
+// (design 47 G A-167).
+//
+// It opens no network connection, bus, store or file beyond its standard input, the -request file, the file
+// MESH_RUN_INPUT names and the -catalogue directory, and reads no other environment. Exit codes: 0 the verdict is written (a machine that does not compose is a verdict, not an error);
 // 1 the request cannot be read or run, said on standard error; 2 the flags are wrong.
 package main
 
@@ -65,6 +68,11 @@ func run(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 	}
 	var raw []byte
 	var err error
+	// A pipeline step hands its input as a file named by MESH_RUN_INPUT (design 47 G A-167); -request, when given, wins.
+	runInput := os.Getenv("MESH_RUN_INPUT")
+	if *requestFile == "" && runInput != "" {
+		*requestFile = runInput
+	}
 	if *requestFile != "" {
 		raw, err = os.ReadFile(*requestFile)
 	} else {

cmd/mesh-compose/main_test.go

@@ -267,6 +267,31 @@ func TestTheRequestIsReadFromAFile(t *testing.T) {
 	}
 }
 
+// A pipeline step names its input file in MESH_RUN_INPUT: the command reads the request there and the verdict is the last
+// line of standard output (design 47 G A-167).
+func TestThePipelineStepReadsMESH_RUN_INPUT(t *testing.T) {
+	file := filepath.Join(t.TempDir(), "input.json")
+	r := composer.Request{V: composer.Version, Depth: composer.DepthCompose, Cases: []composer.Case{{Name: "only", Manifests: map[string]composer.Manifest{
+		"demo": {Raw: json.RawMessage(`{"module":"demo","version":"1"}`)}}}}}
+	if err := os.WriteFile(file, encode(t, r), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("MESH_RUN_INPUT", file)
+	var out, errOut bytes.Buffer
+	if code := run(nil, bytes.NewReader(nil), &out, &errOut); code != 0 {
+		t.Fatalf("exit %d: %s", code, errOut.String())
+	}
+	lines := strings.Split(strings.TrimRight(out.String(), "\n"), "\n")
+	var v composer.Verdict
+	if err := json.Unmarshal([]byte(lines[len(lines)-1]), &v); err != nil || len(v.Cases) != 1 || !v.Cases[0].Manifests["demo"].Accepted {
+		t.Fatalf("last line is no verdict: %v\n%s", err, out.String())
+	}
+	t.Setenv("MESH_RUN_INPUT", file+".gone")
+	if code := run(nil, bytes.NewReader(nil), &out, &errOut); code != 1 || !strings.Contains(errOut.String(), "cannot be read") {
+		t.Errorf("a missing input file: exit %d, %q", code, errOut.String())
+	}
+}
+
 // The exit codes: 1 for a request that cannot be read or run, 2 for wrong flags, each said on standard error.
 func TestTheExitCodesSayWhatFailed(t *testing.T) {
 	unfilled := encode(t, composer.Request{V: composer.Version, Cases: []composer.Case{{Name: "c", Manifests: map[string]composer.Manifest{"x": {Digest: "abc"}}}}})