Check pip-compile failed to resolve requirements (#482)

Author: reiyw

python/pip_install/pip_compile.py

@@ -74,17 +74,33 @@
         cli()
         print("cli() should exit", file=sys.stderr)
         sys.exit(1)
-    except SystemExit:
-        golden = open(requirements_txt).readlines()
-        out = open(requirements_out).readlines()
-        if golden != out:
-            import difflib
-
-            print(''.join(difflib.unified_diff(golden, out)), file=sys.stderr)
+    except SystemExit as e:
+        if e.code == 2:
             print(
-                "Lock file out of date. Run '"
-                + update_command
-                + "' to update.",
+                "pip-compile exited with code 2. This means that pip-compile found "
+                "incompatible requirements or could not find a version that matches "
+                f"the install requirement in {requirements_in}.",
                 file=sys.stderr,
             )
             sys.exit(1)
+        elif e.code == 0:
+            golden = open(requirements_txt).readlines()
+            out = open(requirements_out).readlines()
+            if golden != out:
+                import difflib
+
+                print(''.join(difflib.unified_diff(golden, out)), file=sys.stderr)
+                print(
+                    "Lock file out of date. Run '"
+                    + update_command
+                    + "' to update.",
+                    file=sys.stderr,
+                )
+                sys.exit(1)
+            sys.exit(0)
+        else:
+            print(
+                f"pip-compile unexpectedly exited with code {e.code}.",
+                file=sys.stderr
+            )
+            sys.exit(1)