fix: issue 795-download_url from PURL when not provided

Signed-off-by: Ruchit Agrawal <rragrawal16@gmail.com>

Author: RuchitAgrawal

packagedb/models.py

@@ -38,6 +38,7 @@
 from rest_framework.authtoken.models import Token
 
 from packagedb import schedules
+from packagedb.purl_url_utils import derive_download_url
 
 TRACE = False
 
@@ -574,6 +575,19 @@ class Meta:
     def __str__(self):
         return self.package_url
 
+    def save(self, *args, **kwargs):
+        """
+        Override save to auto-derive download_url from PURL if not provided.
+
+        Packages coming from federatedcode repos may not have a download_url.
+        We use purl2url to infer a real download URL when possible, and fall
+        back to a synthetic unique URL derived from the PURL components.
+        """
+        if not self.download_url and self.purl:
+            self.download_url = derive_download_url(self.purl)
+
+        super().save(*args, **kwargs)
+    
     @property
     def purl(self):
         return self.package_url

packagedb/purl_url_utils.py

@@ -0,0 +1,74 @@
+#
+# Copyright (c) nexB Inc. and others. All rights reserved.
+# purldb is a trademark of nexB Inc.
+# SPDX-License-Identifier: Apache-2.0
+# See http://www.apache.org/licenses/LICENSE-2.0 for the license text.
+# See https://github.com/aboutcode-org/purldb for support or download.
+# See https://aboutcode.org for more information about nexB OSS projects.
+#
+
+import logging
+
+from packageurl import PackageURL
+from packageurl.contrib import purl2url
+
+logger = logging.getLogger(__name__)
+
+
+def derive_download_url(purl_string, provided_download_url=None):
+    """
+    Return a download URL for the package identified by ``purl_string``.
+
+    If ``provided_download_url`` is given it is returned as-is. Otherwise
+    purl2url is used to infer a real download URL. When that also fails a
+    synthetic URL is built from the PURL components so that the unique
+    constraint on ``Package.download_url`` can still be satisfied.
+    """
+    if provided_download_url:
+        return provided_download_url
+
+    try:
+        download_url = purl2url.get_download_url(purl_string)
+        if download_url:
+            return download_url
+    except Exception:
+        pass
+
+    # Fall back to a synthetic URL so the uniqueness constraint is satisfied
+    # even when no real download URL is available (e.g. packages from
+    # federatedcode that only carry a PURL).
+    try:
+        purl = PackageURL.from_string(purl_string)
+        return generate_synthetic_download_url(purl)
+    except Exception as e:
+        logger.warning(f"Could not generate download URL for {purl_string!r}: {e}")
+        return f"purl:{purl_string}"
+
+
+def generate_synthetic_download_url(purl):
+    """
+    Return a synthetic download URL for ``purl`` in the form:
+        purl://<type>/<namespace>/<name>@<version>?<qualifiers>#<subpath>
+
+    All PURL components that affect identity are included so that two
+    packages which differ only by qualifier (e.g. Maven JARs with different
+    classifiers) still receive distinct synthetic URLs.
+    """
+    parts = ["purl://", purl.type]
+
+    if purl.namespace:
+        parts += ["/", purl.namespace]
+
+    parts += ["/", purl.name]
+
+    if purl.version:
+        parts += ["@", purl.version]
+
+    if purl.qualifiers:
+        qual_str = "&".join(f"{k}={v}" for k, v in sorted(purl.qualifiers.items()))
+        parts += ["?", qual_str]
+
+    if purl.subpath:
+        parts += ["#", purl.subpath]
+
+    return "".join(parts)

packagedb/tests/test_purl_download_url.py

@@ -0,0 +1,83 @@
+#
+# Copyright (c) nexB Inc. and others. All rights reserved.
+# purldb is a trademark of nexB Inc.
+# SPDX-License-Identifier: Apache-2.0
+# See http://www.apache.org/licenses/LICENSE-2.0 for the license text.
+# See https://github.com/aboutcode-org/purldb for support or download.
+# See https://aboutcode.org for more information about nexB OSS projects.
+#
+
+import unittest
+from unittest.mock import patch
+
+from packageurl import PackageURL
+
+from packagedb.purl_url_utils import derive_download_url
+from packagedb.purl_url_utils import generate_synthetic_download_url
+
+
+class TestDeriveDownloadURL(unittest.TestCase):
+    def test_provided_url_takes_precedence(self):
+        provided = "https://example.com/lodash-4.17.21.tgz"
+        result = derive_download_url("pkg:npm/lodash@4.17.21", provided)
+        self.assertEqual(result, provided)
+
+    @patch("packagedb.purl_url_utils.purl2url.get_download_url")
+    def test_infers_url_from_purl(self, mock_get_download):
+        expected = "https://rubygems.org/downloads/bundler-2.3.23.gem"
+        mock_get_download.return_value = expected
+
+        result = derive_download_url("pkg:gem/bundler@2.3.23")
+
+        mock_get_download.assert_called_once_with("pkg:gem/bundler@2.3.23")
+        self.assertEqual(result, expected)
+
+    @patch("packagedb.purl_url_utils.purl2url.get_download_url")
+    def test_falls_back_to_synthetic_url(self, mock_get_download):
+        mock_get_download.side_effect = Exception("cannot infer")
+
+        result = derive_download_url("pkg:generic/some-package@1.0.0")
+
+        self.assertTrue(result.startswith("purl://"))
+        self.assertIn("generic/some-package@1.0.0", result)
+
+    def test_invalid_purl_does_not_raise(self):
+        # Last-resort fallback: returns a purl:-prefixed string
+        result = derive_download_url("not-a-valid-purl")
+        self.assertIsNotNone(result)
+        self.assertIn("purl:", result)
+
+
+class TestGenerateSyntheticDownloadURL(unittest.TestCase):
+    def test_basic(self):
+        purl = PackageURL.from_string("pkg:npm/express@4.17.1")
+        self.assertEqual(generate_synthetic_download_url(purl), "purl://npm/express@4.17.1")
+
+    def test_includes_namespace(self):
+        purl = PackageURL.from_string("pkg:maven/org.apache.commons/commons-lang3@3.12.0")
+        url = generate_synthetic_download_url(purl)
+        self.assertTrue(url.startswith("purl://maven/org.apache.commons/"))
+        self.assertIn("commons-lang3@3.12.0", url)
+
+    def test_qualifiers_differentiate_packages(self):
+        # Maven JARs with different classifiers must produce different URLs
+        purl1 = PackageURL.from_string("pkg:maven/com.example/lib@1.0.0")
+        purl2 = PackageURL.from_string("pkg:maven/com.example/lib@1.0.0?classifier=sources")
+        self.assertNotEqual(
+            generate_synthetic_download_url(purl1),
+            generate_synthetic_download_url(purl2),
+        )
+
+    def test_no_version(self):
+        purl = PackageURL.from_string("pkg:npm/express")
+        url = generate_synthetic_download_url(purl)
+        self.assertNotIn("@", url)
+
+    def test_includes_subpath(self):
+        purl = PackageURL.from_string("pkg:github/user/repo@v1.0#path/to/file")
+        url = generate_synthetic_download_url(purl)
+        self.assertIn("#path/to/file", url)
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)