python: mkvenv: introduce TOML-like representation of dependencies

We would like to place all Python dependencies in the same file, so that
we can add more information without having long and complex command lines.
The plan is to have a TOML file with one entry per package, for example

  [avocado]
  avocado-framework = {
    accepted = "(>=88.1, <93.0)",
    installed = "88.1",
    canary = "avocado"
  }

Each TOML section will thus be a dictionary of dictionaries.  Modify
mkvenv.py's workhorse function, _do_ensure, to already operate on such
a data structure.  The "ensure" subcommand is modified to separate the
depspec into a name and a version part, and use the result (plus the
--diagnose argument) to build a dictionary for each command line argument.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

python/scripts/mkvenv.py

@@ -46,6 +46,9 @@ options:
 
 """
 
+# The duplication between importlib and pkg_resources does not help
+# pylint: disable=too-many-lines
+
 # Copyright (C) 2022-2023 Red Hat, Inc.
 #
 # Authors:
@@ -69,6 +72,7 @@ import sysconfig
 from types import SimpleNamespace
 from typing import (
     Any,
+    Dict,
     Iterator,
     Optional,
     Sequence,
@@ -786,43 +790,67 @@ def pip_install(
     )
 
 
+def _make_version_constraint(info: Dict[str, str], install: bool) -> str:
+    """
+    Construct the version constraint part of a PEP 508 dependency
+    specification (for example '>=0.61.5') from the accepted and
+    installed keys of the provided dictionary.
+
+    :param info: A dictionary corresponding to a TOML key-value list.
+    :param install: True generates install constraints, False generates
+        presence constraints
+    """
+    if install and "installed" in info:
+        return "==" + info["installed"]
+
+    dep_spec = info.get("accepted", "")
+    dep_spec = dep_spec.strip()
+    # Double check that they didn't just use a version number
+    if dep_spec and dep_spec[0] not in "!~><=(":
+        raise Ouch(
+            "invalid dependency specifier " + dep_spec + " in dependency file"
+        )
+
+    return dep_spec
+
+
 def _do_ensure(
-    dep_specs: Sequence[str],
+    group: Dict[str, Dict[str, str]],
     online: bool = False,
     wheels_dir: Optional[Union[str, Path]] = None,
-    prog: Optional[str] = None,
 ) -> Optional[Tuple[str, bool]]:
     """
-    Use pip to ensure we have the package specified by @dep_specs.
+    Use pip to ensure we have the packages specified in @group.
 
-    If the package is already installed, do nothing. If online and
+    If the packages are already installed, do nothing. If online and
     wheels_dir are both provided, prefer packages found in wheels_dir
     first before connecting to PyPI.
 
-    :param dep_specs:
-        PEP 508 dependency specifications. e.g. ['meson>=0.61.5'].
+    :param group: A dictionary of dictionaries, corresponding to a
+        section in a pythondeps.toml file.
     :param online: If True, fall back to PyPI.
     :param wheels_dir: If specified, search this path for packages.
     """
     absent = []
     present = []
     canary = None
-    for spec in dep_specs:
-        matcher = distlib.version.LegacyMatcher(spec)
-        ver = _get_version(matcher.name)
+    for name, info in group.items():
+        constraint = _make_version_constraint(info, False)
+        matcher = distlib.version.LegacyMatcher(name + constraint)
+        ver = _get_version(name)
         if (
             ver is None
             # Always pass installed package to pip, so that they can be
             # updated if the requested version changes
-            or not _is_system_package(matcher.name)
+            or not _is_system_package(name)
             or not matcher.match(distlib.version.LegacyVersion(ver))
         ):
-            absent.append(spec)
-            if spec == dep_specs[0]:
-                canary = prog
+            absent.append(name + _make_version_constraint(info, True))
+            if len(absent) == 1:
+                canary = info.get("canary", None)
         else:
-            logger.info("found %s %s", matcher.name, ver)
-            present.append(matcher.name)
+            logger.info("found %s %s", name, ver)
+            present.append(name)
 
     if present:
         generate_console_scripts(present)
@@ -875,7 +903,24 @@ def ensure(
     if not HAVE_DISTLIB:
         raise Ouch("a usable distlib could not be found, please install it")
 
-    result = _do_ensure(dep_specs, online, wheels_dir, prog)
+    # Convert the depspecs to a dictionary, as if they came
+    # from a section in a pythondeps.toml file
+    group: Dict[str, Dict[str, str]] = {}
+    for spec in dep_specs:
+        name = distlib.version.LegacyMatcher(spec).name
+        group[name] = {}
+
+        spec = spec.strip()
+        pos = len(name)
+        ver = spec[pos:].strip()
+        if ver:
+            group[name]["accepted"] = ver
+
+        if prog:
+            group[name]["canary"] = prog
+            prog = None
+
+    result = _do_ensure(group, online, wheels_dir)
     if result:
         # Well, that's not good.
         if result[1]: