Add --no-provision flag

tox can now be invoked with a new --no-provision flag that prevents provision,
if requires or minversion are not satisfied, tox will fail;
if a path is specified as an argument to the flag
(e.g. as `tox --no-provision missing.json`) and provision is prevented,
provision metadata are written as JSON to that path.

Fixes #1921

docs/changelog/1921.feature.rst

@@ -0,0 +1,6 @@
+tox can now be invoked with a new ``--no-provision`` flag that prevents provision,
+if :conf:`requires` or :conf:`minversion` are not satisfied,
+tox will fail;
+if a path is specified as an argument to the flag
+(e.g. as ``tox --no-provision missing.json``) and provision is prevented,
+provision metadata are written as JSON to that path - by :user:`hroncok`

src/tox/config/__init__.py

@@ -2,6 +2,7 @@
 
 import argparse
 import itertools
+import json
 import os
 import random
 import re
@@ -572,6 +573,16 @@ def tox_addoption(parser):
         action="store_true",
         help="override alwayscopy setting to True in all envs",
     )
+    parser.add_argument(
+        "--no-provision",
+        action="store",
+        nargs="?",
+        default=False,
+        const=True,
+        metavar="REQUIRES_JSON",
+        help="do not perform provision, but fail and if a path was provided "
+        "write provision metadata as JSON to it",
+    )
 
     cli_skip_missing_interpreter(parser)
     parser.add_argument("--workdir", metavar="PATH", help="tox working directory")
@@ -1318,8 +1329,8 @@ def handle_provision(self, config, reader):
         # raise on unknown args
         self.config._parser.parse_cli(args=self.config.args, strict=True)
 
-    @staticmethod
-    def ensure_requires_satisfied(config, requires, min_version):
+    @classmethod
+    def ensure_requires_satisfied(cls, config, requires, min_version):
         missing_requirements = []
         failed_to_parse = False
         deps = []
@@ -1346,12 +1357,29 @@ def ensure_requires_satisfied(config, requires, min_version):
                 missing_requirements.append(str(requirements.Requirement(require)))
         if failed_to_parse:
             raise tox.exception.BadRequirement()
+        if config.option.no_provision and missing_requirements:
+            msg = "provisioning explicitly disabled within {}, but missing {}"
+            if config.option.no_provision is not True:  # it's a path
+                msg += " and wrote to {}"
+                cls.write_requires_to_json_file(config)
+            raise tox.exception.Error(
+                msg.format(sys.executable, missing_requirements, config.option.no_provision)
+            )
         if WITHIN_PROVISION and missing_requirements:
             msg = "break infinite loop provisioning within {} missing {}"
             raise tox.exception.Error(msg.format(sys.executable, missing_requirements))
         config.run_provision = bool(len(missing_requirements))
         return deps
 
+    @staticmethod
+    def write_requires_to_json_file(config):
+        requires_dict = {
+            "minversion": config.minversion,
+            "requires": config.requires,
+        }
+        with open(config.option.no_provision, "w", encoding="utf-8") as outfile:
+            json.dump(requires_dict, outfile, indent=4)
+
     def parse_build_isolation(self, config, reader):
         config.isolated_build = reader.getbool("isolated_build", False)
         config.isolated_build_env = reader.getstring("isolated_build_env", ".package")