Merge pull request 'A host the mesh builds knows what it was built for' (#158) from fix/161-a-built-host-knows-what-it-was-built-for into main

internal/builder/builder.go

@@ -877,6 +877,14 @@ func compile(ctx context.Context, run Runner, tree string, chain Toolchain,
 		base,
 	}
 	invocation = append(invocation, chain.Compile...)
+	// What it was built for, linked in. The compile line carries `-ldflags` already; this appends a
+	// second one, which the Go linker accepts and merges. A host with no system refuses every
+	// declaration before it applies anything (novox/hq 04-ISSUES/161), and it is the artifact that
+	// knows — the target is a property of the artifact rather than of the recipe (ADR 0142).
+	if chain.SystemStamp != "" && strings.TrimSpace(a.System) != "" {
+		invocation = append(invocation, "-ldflags",
+			"-X "+chain.SystemStamp+"="+strings.TrimSpace(a.System))
+	}
 	if chain.OutputFlag != "" {
 		// A compiler pointed at a package is told the file to write, not the directory: the name a
 		// machine runs it by is not always the name of the package that built it. The host's command

internal/builder/system_stamp_test.go

@@ -0,0 +1,49 @@
+package builder
+
+import (
+	"strings"
+	"testing"
+)
+
+// A host built without knowing its system refuses every declaration before applying anything —
+// safely, totally, and with nothing reporting it. The mesh built one, delivered it, started it, and
+// it would have refused the first thing it was asked to do (novox/hq 04-ISSUES/161).
+
+func TestTheGoToolchainStampsTheArtifactsSystem(t *testing.T) {
+	chain, err := ToolchainFor("go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if chain.SystemStamp != "main.builtFor" {
+		t.Fatalf("the go toolchain fills %q", chain.SystemStamp)
+	}
+}
+
+func TestALanguageWithNoPinnedSystemStampsNothing(t *testing.T) {
+	// Interpreted output is not pinned to a system, and a manifest declaring one for it is already
+	// refused. Nothing to fill.
+	for _, language := range []string{"typescript", "python"} {
+		chain, err := ToolchainFor(language)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if chain.SystemStamp != "" {
+			t.Fatalf("%s fills %q, and its output is not pinned to a system",
+				language, chain.SystemStamp)
+		}
+	}
+}
+
+func TestTheStampIsTheOneThingTakenFromTheArtifact(t *testing.T) {
+	// The toolchain accepts nothing else from the module — anything it could override it would be
+	// writing a Dockerfile to override. The system is the stated exception, because a compiled
+	// binary is per system and the artifact is what declares one (ADR 0142).
+	chain, err := ToolchainFor("go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	joined := strings.Join(chain.Compile, " ")
+	if strings.Contains(joined, "${") || strings.Contains(joined, "%s") {
+		t.Fatalf("the compile line takes something from the module: %q", joined)
+	}
+}

internal/builder/toolchain.go

@@ -49,6 +49,18 @@ type Toolchain struct {
 	// is named as it will be FOUND, inside the unpacked bundle, so the source is the same path with
 	// the output directory taken off the front and this on the end.
 	SourceExt string
+	// SystemStamp is the variable this language's linker fills with the artifact's declared system,
+	// for a language whose binaries are pinned to one at link time (novox/hq ADR 0005).
+	//
+	// **The one thing a toolchain takes from the artifact, and 0142 says why**: the target is a
+	// property of the artifact rather than of the recipe, because a compiled binary is per system
+	// and a toolchain that accepted it from the module would be accepting a build instruction. This
+	// is the narrow exception, named here rather than inferred.
+	//
+	// Empty for a language that compiles to nothing pinned. A host built without it refuses every
+	// declaration before applying anything — safely, totally, and with nothing reporting it
+	// (novox/hq 04-ISSUES/161).
+	SystemStamp string
 }
 
 // What a toolchain is pointed at.
@@ -122,6 +134,9 @@ var toolchains = []Toolchain{
 		// directory holding one executable, which is what the delivery mechanism expects
 		// (novox/hq ADR 0141).
 		Unit: UnitPackage,
+		// The mesh's own Go components read the system they were built for from this variable, and
+		// refuse to touch a machine without one.
+		SystemStamp: "main.builtFor",
 	},
 	{
 		Language: "python",