Better incomplete download error message

src/pip/_internal/exceptions.py

@@ -775,3 +775,36 @@ def __init__(self, *, distribution: "BaseDistribution") -> None:
             ),
             hint_stmt=None,
         )
+
+class IncompleteDownloadError(DiagnosticPipError):
+    """Raised when the downloader receives fewer bytes than advertised
+    in the Content-Length header."""
+
+    reference = "incomplete-download-error"
+
+    def __init__(
+        self, link: str, resume_incomplete: bool, resume_attempts: int
+    ) -> None:
+        if resume_incomplete:
+            message = (
+                "Download failed after {} attempts because not enough bytes are"
+                " received. The incomplete file has been cleaned up."
+            ).format(resume_attempts)
+            hint = "Use --incomplete-download-retries to configure resume retry limit."
+        else:
+            message = (
+                "Download failed because not enough bytes are received."
+                " The incomplete file has been cleaned up."
+            )
+            hint = (
+                "Use --incomplete-downloads=resume to make pip retry failed download."
+            )
+
+        super().__init__(
+            message=message,
+            context="File: {}\n"
+            "Resume failed download: {}\n"
+            "Resume retry limit: {}".format(link, resume_incomplete, resume_attempts),
+            hint_stmt=hint,
+            note_stmt="This is an issue with network connectivity, not pip.",
+        )

src/pip/_internal/network/download.py

@@ -11,7 +11,7 @@
 from pip._vendor.requests.models import Response
 
 from pip._internal.cli.progress_bars import get_download_progress_renderer
-from pip._internal.exceptions import NetworkConnectionError
+from pip._internal.exceptions import IncompleteDownloadError, NetworkConnectionError
 from pip._internal.models.index import PyPI
 from pip._internal.models.link import Link
 from pip._internal.network.cache import is_from_cache
@@ -229,21 +229,10 @@ def __call__(self, link: Link, location: str) -> Tuple[str, str]:
                         content_file.write(chunk)
 
         if total_length is not None and bytes_received < total_length:
-            if self._resume_incomplete:
-                logger.critical(
-                    "Failed to download %s after %d resumption attempts.",
-                    link,
-                    self._resume_attempts,
-                )
-            else:
-                logger.critical(
-                    "Failed to download %s."
-                    " Set --incomplete-downloads=resume to automatically"
-                    "resume incomplete download.",
-                    link,
-                )
             os.remove(filepath)
-            raise RuntimeError("Incomplete download")
+            raise IncompleteDownloadError(
+                str(link), self._resume_incomplete, self._resume_attempts
+            )
 
         content_type = resp.headers.get("Content-Type", "")
         return filepath, content_type

tests/unit/test_network_download.py

@@ -6,6 +6,7 @@
 
 import pytest
 
+from pip._internal.exceptions import IncompleteDownloadError
 from pip._internal.models.link import Link
 from pip._internal.network.download import (
     Downloader,
@@ -349,7 +350,7 @@ def test_downloader(
         if expected_bytes is None:
             remove = MagicMock(return_value=None)
             with patch("os.remove", remove):
-                with pytest.raises(RuntimeError):
+                with pytest.raises(IncompleteDownloadError):
                     downloader(link, str(tmpdir))
             # Make sure the incomplete file is removed
             remove.assert_called_once()