Avoid issues related with pins persistence (#355)

conda_libmamba_solver/solver.py

@@ -270,7 +270,7 @@ def _solving_loop(
         for attempt in range(1, max_attempts):
             log.debug("Starting solver attempt %s", attempt)
             try:
-                solved = self._solve_attempt(in_state, out_state, index)
+                solved = self._solve_attempt(in_state, out_state, index, attempt=attempt)
                 if solved:
                     break
             except (UnsatisfiableError, PackagesNotFoundError):
@@ -345,10 +345,11 @@ def _solve_attempt(
         in_state: SolverInputState,
         out_state: SolverOutputState,
         index: LibMambaIndexHelper,
+        attempt: int = 1,
     ):
         self._setup_solver(index)
 
-        log.debug("New solver attempt")
+        log.debug("New solver attempt: #%d", attempt)
         log.debug("Current conflicts (including learnt ones): %s", out_state.conflicts)
 
         # ## First, we need to obtain the list of specs ###
@@ -365,12 +366,13 @@ def _solve_attempt(
         log.debug("Computed specs: %s", out_state.specs)
 
         # ## Convert to tasks
-        out_state.pins.clear()
         n_pins = 0
         tasks = self._specs_to_tasks(in_state, out_state)
         for (task_name, task_type), specs in tasks.items():
             log.debug("Adding task %s with specs %s", task_name, specs)
-            if task_name == "ADD_PIN":
+            if task_name == "ADD_PIN" and attempt == 1:
+                # pins only need to be added once; since they persist in the pool
+                # adding them more times results in issues like #354
                 for spec in specs:
                     n_pins += 1
                     self.solver.add_pin(spec)
@@ -393,7 +395,7 @@ def _solve_attempt(
         new_conflicts = self._maybe_raise_for_problems(
             problems, old_conflicts, out_state.pins, index._channels
         )
-        log.debug("Attempt failed with %s conflicts", len(new_conflicts))
+        log.debug("Attempt %d failed with %s conflicts", attempt, len(new_conflicts))
         out_state.conflicts.update(new_conflicts.items(), reason="New conflict found")
         return False
 

news/355-pins-persistence-issues

@@ -0,0 +1,19 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* Configure pinned specs just once to avoid solver bugs related with their persistence (i.e. inability to downgrade environments if pinned specs are present and a transient dependency needs to be removed). (#354 via #355)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>

tests/test_solvers.py

@@ -458,3 +458,41 @@ def test_python_update_should_not_uninstall_history():
                 "--dry-run",
                 no_capture=True,
             )
+
+
+def test_python_downgrade_with_pins_removes_truststore():
+    """
+    https://github.com/conda/conda-libmamba-solver/issues/354
+    """
+    channels = "--override-channels", "-c", "conda-forge"
+    solver = "--solver", "libmamba"
+    with make_temp_env("python=3.10", "conda", *channels, *solver) as prefix:
+        zstd_version = PrefixData(prefix).get("zstd").version
+        for pin in (None, "zstd", f"zstd={zstd_version}"):
+            env = os.environ.copy()
+            if pin:
+                env["CONDA_PINNED_PACKAGES"] = pin
+            p = conda_subprocess(
+                Commands.INSTALL,
+                "-p",
+                prefix,
+                *channels,
+                *solver,
+                "--dry-run",
+                "--json",
+                "python=3.9",
+                env=env,
+            )
+            assert p.returncode == 0
+            data = json.loads(p.stdout)
+            assert data.get("success")
+            assert data.get("dry_run")
+            assertions = 0
+            link_dict = {pkg["name"]: pkg for pkg in data["actions"]["LINK"]}
+            unlink_dict = {pkg["name"]: pkg for pkg in data["actions"]["UNLINK"]}
+            assert link_dict["python"]["version"].startswith("3.9.")
+            assert "truststore" in unlink_dict
+            if pin:
+                # shouldn't have changed!
+                assert "zstd" not in link_dict
+                assert "zstd" not in unlink_dict