fix(spdx): write a name for the root package when the source has none (#5349)

* fix(spdx): write a name for the root package when the source has none

A source read from another SBOM (a purl list, a CycloneDX document
without metadata.component, an SPDX document without a DESCRIBES
relationship) can have no name. The SPDX encoder then wrote the document
root package with an empty PackageName, which SPDX requires. In
tag-value output the PackageName line is omitted, so the root package's
fields land in the previous section and syft cannot read its own output
back ("received unknown tag SPDXID in CreationInfo section").

Fall back to the document name for the root package, and make the
document name fall back to "unknown" for every source type rather than
only for unrecognized ones.

Signed-off-by: Tony <tjagodka@gmail.com>

* fix(spdx): drop syft's root package on decode for every SPDX version

- the root package name now comes from one fallback chain (source name, image repo name, source ID, document name), so the root purl no longer gets an empty name for a source decoded from CycloneDX
- decoding strips the root syft writes for an unknown source, so `syft convert` round trips stop adding an `unknown` package each pass
- for SPDX 2.1 and 2.2, which have no `primaryPackagePurpose`, the purpose is taken from the source type in syft's `DocumentRoot-<type>-` ID

**behavior change:** decoding a syft-written SPDX 2.1/2.2 document no longer returns the root as a package or its `CONTAINS` relationships, matching 2.3.

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Tony <tjagodka@gmail.com>
Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>
Co-authored-by: Alex Goodman <wagoodman@users.noreply.github.com>

syft/format/common/spdxhelpers/to_syft_model.go

@@ -147,12 +147,24 @@ func extractSource(spdxIDMap map[string]any, doc *spdx.Document) source.Descript
 	}
 
 	p := rootPackages[0]
+	rootType := syftRootType(p.PackageSPDXIdentifier)
 
-	switch p.PrimaryPackagePurpose {
+	purpose := p.PrimaryPackagePurpose
+	if purpose == "" {
+		// SPDX 2.1 and 2.2 have no primary package purpose, so fall back to the source type syft puts in the root's ID
+		purpose = syftRootPurpose(rootType)
+	}
+
+	switch purpose {
 	case spdxPrimaryPurposeContainer:
 		src = containerSource(p)
 	case spdxPrimaryPurposeFile:
 		src = fileSource(p)
+	case spdxPrimaryPurposeOther:
+		// syft writes a root package even for a source it knows nothing about, which is not a package in the SBOM
+		if rootType != prefixUnknown {
+			return src
+		}
 	default:
 		return src
 	}
@@ -165,6 +177,30 @@ func extractSource(spdxIDMap map[string]any, doc *spdx.Document) source.Descript
 	return src
 }
 
+// syftRootType returns the source type syft encodes in a root package ID ("DocumentRoot-<type>-<name>"), or "" for
+// any other ID.
+func syftRootType(id spdx.ElementID) string {
+	rest, ok := strings.CutPrefix(string(id), "DocumentRoot-")
+	if !ok {
+		return ""
+	}
+	rootType, _, _ := strings.Cut(rest, "-")
+	return rootType
+}
+
+// syftRootPurpose returns the primary package purpose syft writes for a root package of the given source type.
+func syftRootPurpose(rootType string) string {
+	switch rootType {
+	case prefixImage, prefixOCIModel, prefixSnap:
+		return spdxPrimaryPurposeContainer
+	case prefixDirectory, prefixFile:
+		return spdxPrimaryPurposeFile
+	case prefixUnknown:
+		return spdxPrimaryPurposeOther
+	}
+	return ""
+}
+
 func containerSource(p *spdx.Package) source.Description {
 	id := string(p.PackageSPDXIdentifier)