Handle __unset sentinel in Composer minified metadata

internal/handler/composer.go

@@ -16,6 +16,7 @@ import (
 const (
 	composerUpstream   = "https://packagist.org"
 	composerRepo       = "https://repo.packagist.org"
+	composerUnset      = "__unset"
 	vendorPackageParts = 2
 )
 
@@ -150,7 +151,8 @@ func (h *ComposerHandler) rewriteMetadata(body []byte) ([]byte, error) {
 
 // expandMinifiedVersions expands the Composer v2 minified format where each
 // version entry only contains fields that differ from the previous entry.
-// The "~dev" sentinel string resets the inheritance chain.
+// The "~dev" sentinel string resets the inheritance chain, and the "__unset"
+// value removes a field from the inherited state.
 func expandMinifiedVersions(versionList []any) []any {
 	expanded := make([]any, 0, len(versionList))
 	inherited := map[string]any{}
@@ -174,6 +176,10 @@ func expandMinifiedVersions(versionList []any) []any {
 			merged[k] = deepCopyValue(val)
 		}
 		for k, val := range vmap {
+			if val == composerUnset {
+				delete(merged, k)
+				continue
+			}
 			merged[k] = val
 		}
 

internal/handler/composer_test.go

@@ -177,6 +177,80 @@ func TestComposerRewriteMetadataMinifiedDevReset(t *testing.T) {
 	}
 }
 
+func TestComposerRewriteMetadataUnset(t *testing.T) {
+	h := &ComposerHandler{
+		proxy:    &Proxy{Logger: slog.Default()},
+		proxyURL: "http://localhost:8080",
+	}
+
+	// In the minified format, "__unset" removes a field from the inherited
+	// state. v1.29.0 has require-dev, v1.28.0 unsets it, v1.27.0 inherits the
+	// unset state. Composer rejects metadata where require-dev (or any link
+	// field) is the literal string "__unset" rather than an object.
+	input := `{
+		"minified": "composer/2.0",
+		"packages": {
+			"venturecraft/revisionable": [
+				{
+					"name": "venturecraft/revisionable",
+					"version": "1.29.0",
+					"require": {"php": ">=5.4"},
+					"require-dev": {"orchestra/testbench": "~3.0"},
+					"dist": {"url": "https://example.com/a.zip", "type": "zip"}
+				},
+				{
+					"version": "1.28.0",
+					"require-dev": "__unset"
+				},
+				{
+					"version": "1.27.0"
+				},
+				{
+					"version": "1.26.0",
+					"require-dev": {"foo/bar": "1.0"}
+				}
+			]
+		}
+	}`
+
+	output, err := h.rewriteMetadata([]byte(input))
+	if err != nil {
+		t.Fatalf("rewriteMetadata failed: %v", err)
+	}
+
+	var result map[string]any
+	if err := json.Unmarshal(output, &result); err != nil {
+		t.Fatalf("failed to parse output: %v", err)
+	}
+
+	versions := result["packages"].(map[string]any)["venturecraft/revisionable"].([]any)
+	if len(versions) != 4 {
+		t.Fatalf("expected 4 versions, got %d", len(versions))
+	}
+
+	byVersion := map[string]map[string]any{}
+	for _, v := range versions {
+		vmap := v.(map[string]any)
+		byVersion[vmap["version"].(string)] = vmap
+	}
+
+	if _, ok := byVersion["1.29.0"]["require-dev"].(map[string]any); !ok {
+		t.Errorf("1.29.0 require-dev should be an object, got %T", byVersion["1.29.0"]["require-dev"])
+	}
+	if rd, ok := byVersion["1.28.0"]["require-dev"]; ok {
+		t.Errorf("1.28.0 require-dev should be absent, got %v", rd)
+	}
+	if rd, ok := byVersion["1.27.0"]["require-dev"]; ok {
+		t.Errorf("1.27.0 require-dev should be absent (inherited unset), got %v", rd)
+	}
+	if _, ok := byVersion["1.26.0"]["require-dev"].(map[string]any); !ok {
+		t.Errorf("1.26.0 require-dev should be an object, got %T", byVersion["1.26.0"]["require-dev"])
+	}
+	if _, ok := byVersion["1.27.0"]["require"].(map[string]any); !ok {
+		t.Error("1.27.0 should still inherit require from 1.29.0")
+	}
+}
+
 func TestComposerRewriteMetadataCooldownPreservesNames(t *testing.T) {
 	now := time.Now()
 	old := now.Add(-10 * 24 * time.Hour).Format(time.RFC3339)