Display messages when backtracking on a package

3 files changed, 83 insertions, 2 deletions

diff --git a/src/pip/_internal/resolution/resolvelib/reporter.py b/src/pip/_internal/resolution/resolvelib/reporter.py
new file mode 100644
index 000000000..56e805975
--- /dev/null
+++ b/src/pip/_internal/resolution/resolvelib/reporter.py
@@ -0,0 +1,35 @@
+from collections import defaultdict
+from logging import getLogger
+
+from pip._vendor.resolvelib.reporters import BaseReporter
+
+logger = getLogger(__name__)
+
+
+class PipReporter(BaseReporter):
+
+    def __init__(self):
+        self.backtracks_by_package = defaultdict(int)
+
+        self._messages_at_backtrack = {
+            8: (
+                "pip is looking at multiple versions of this package to determine "
+                "which version is compatible with other requirements. "
+                "This could take a while."
+            ),
+            13: (
+                "This is taking longer than usual. You might need to provide the "
+                "dependency resolver with stricter constraints to reduce runtime."
+                "If you want to abort this run, you can press Ctrl + C to do so."
+            )
+        }
+
+    def backtracking(self, candidate):
+        self.backtracks_by_package[candidate.name] += 1
+
+        count = self.backtracks_by_package[candidate.name]
+        if count not in self._messages_at_backtrack:
+            return
+
+        message = self._messages_at_backtrack[count]
+        logger.info(message)
diff --git a/src/pip/_internal/resolution/resolvelib/resolver.py b/src/pip/_internal/resolution/resolvelib/resolver.py
index cb7d1ae8a..acb7cfeda 100644
--- a/src/pip/_internal/resolution/resolvelib/resolver.py
+++ b/src/pip/_internal/resolution/resolvelib/resolver.py
@@ -3,7 +3,7 @@ import logging
 
 from pip._vendor import six
 from pip._vendor.packaging.utils import canonicalize_name
-from pip._vendor.resolvelib import BaseReporter, ResolutionImpossible
+from pip._vendor.resolvelib import ResolutionImpossible
 from pip._vendor.resolvelib import Resolver as RLResolver
 
 from pip._internal.exceptions import InstallationError
@@ -11,6 +11,7 @@ from pip._internal.req.req_install import check_invalid_constraint_type
 from pip._internal.req.req_set import RequirementSet
 from pip._internal.resolution.base import BaseResolver
 from pip._internal.resolution.resolvelib.provider import PipProvider
+from pip._internal.resolution.resolvelib.reporter import PipReporter
 from pip._internal.utils.misc import dist_is_editable
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
@@ -103,7 +104,7 @@ class Resolver(BaseResolver):
             upgrade_strategy=self.upgrade_strategy,
             user_requested=user_requested,
         )
-        reporter = BaseReporter()
+        reporter = PipReporter()
         resolver = RLResolver(provider, reporter)
 
         try:
diff --git a/tests/functional/test_new_resolver.py b/tests/functional/test_new_resolver.py
index 1718ab8a8..aa1744cc8 100644
--- a/tests/functional/test_new_resolver.py
+++ b/tests/functional/test_new_resolver.py
@@ -1046,3 +1046,48 @@ def test_new_resolver_prefers_installed_in_upgrade_if_latest(script):
         "pkg",
     )
     assert_installed(script, pkg="2")
+
+
+@pytest.mark.parametrize("N", [10, 20])
+def test_new_resolver_presents_messages_when_backtracking_a_lot(script, N):
+    # Generate a set of wheels that will definitely cause backtracking.
+    for index in range(1, N+1):
+        A_version = "{index}.0.0".format(index=index)
+        B_version = "{index}.0.0".format(index=index)
+        C_version = "{index_minus_one}.0.0".format(index_minus_one=index - 1)
+
+        depends = ["B == " + B_version]
+        if index != 1:
+            depends.append("C == " + C_version)
+
+        print("A", A_version, "B", B_version, "C", C_version)
+        create_basic_wheel_for_package(script, "A", A_version, depends=depends)
+
+    for index in range(1, N+1):
+        B_version = "{index}.0.0".format(index=index)
+        C_version = "{index}.0.0".format(index=index)
+        depends = ["C == " + C_version]
+
+        print("B", B_version, "C", C_version)
+        create_basic_wheel_for_package(script, "B", B_version, depends=depends)
+
+    for index in range(1, N+1):
+        C_version = "{index}.0.0".format(index=index)
+        print("C", C_version)
+        create_basic_wheel_for_package(script, "C", C_version)
+
+    # Install A
+    result = script.pip(
+        "install",
+        "--use-feature=2020-resolver",
+        "--no-cache-dir",
+        "--no-index",
+        "--find-links", script.scratch_path,
+        "A"
+    )
+
+    assert_installed(script, A="1.0.0", B="1.0.0", C="1.0.0")
+    if N >= 8:  # this number is hard-coded in the code too.
+        assert "This could take a while." in result.stdout
+    if N >= 13:  # this number is hard-coded in the code too.
+        assert "press Ctrl + C" in result.stdout