docs: document what configure does with virtual environments

Given the recent confusion around how QEMU detects the system
Meson installation, and/or decides to install its own, it is
time to fill in the "Python virtual environments and the QEMU
build system" section of the documentation.

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

docs/devel/build-system.rst

@@ -122,10 +122,78 @@ functioning.  These are performed using a few more helper functions:
    indicated by $TMPC.
 
 
-Python virtual environments and the QEMU build system
------------------------------------------------------
+Python virtual environments and the build process
+-------------------------------------------------
+
+An important step in ``configure`` is to create a Python virtual
+environment (venv) during the configuration phase.  The Python interpreter
+comes from the ``--python`` command line option, the ``$PYTHON`` variable
+from the environment, or the system PATH, in this order.  The venv resides
+in the ``pyvenv`` directory in the build tree, and provides consistency
+in how the build process runs Python code.
+
+At this stage, ``configure`` also queries the chosen Python interpreter
+about QEMU's build dependencies.  Note that the build process does  *not*
+look for ``meson``, ``sphinx-build`` or ``avocado`` binaries in the PATH;
+likewise, there are no options such as ``--meson`` or ``--sphinx-build``.
+This avoids a potential mismatch, where Meson and Sphinx binaries on the
+PATH might operate in a different Python environment than the one chosen
+by the user during the build process.  On the other hand, it introduces
+a potential source of confusion where the user installs a dependency but
+``configure`` is not able to find it.  When this happens, the dependency
+was installed in the ``site-packages`` directory of another interpreter,
+or with the wrong ``pip`` program.
+
+If a package is available for the chosen interpreter, ``configure``
+prepares a small script that invokes it from the venv itself[#distlib]_.
+If not, ``configure`` can also optionally install dependencies in the
+virtual environment with ``pip``, either from wheels in ``python/wheels``
+or by downloading the package with PyPI.  Downloading can be disabled with
+``--disable-download``; and anyway, it only happens when a ``configure``
+option (currently, only ``--enable-docs``) is explicitly enabled but
+the dependencies are not present[#pip]_.
+
+.. [#distlib] The scripts are created based on the package's metadata,
+              specifically the ``console_script`` entry points.  This is the
+              same mechanism that ``pip`` uses when installing a package.
+              Currently, in all cases it would be possible to use ``python -m``
+              instead of an entry point script, which makes this approach a
+              bit overkill.  On the other hand, creating the scripts is
+              future proof and it makes the contents of the ``pyvenv/bin``
+              directory more informative.  Portability is also not an issue,
+              because the Python Packaging Authority provides a package
+              ``distlib.scripts`` to perform this task.
+
+.. [#pip] ``pip`` might also be used when running ``make check-avocado``
+           if downloading is enabled, to ensure that Avocado is
+           available.
+
+The required versions of the packages are stored in a configuration file
+``pythondeps.toml``.  The format is custom to QEMU, but it is documented
+at the top of the file itself and it should be easy to understand.  The
+requirements should make it possible to use the version that is packaged
+that is provided by supported distros.
+
+When dependencies are downloaded, instead, ``configure`` uses a "known
+good" version that is also listed in ``pythondeps.toml``.  In this
+scenario, ``pythondeps.toml`` behaves like the "lock file" used by
+``cargo``, ``poetry`` or other dependency management systems.
+
+
+Bundled Python packages
+-----------------------
+
+Python packages that are **mandatory** dependencies to build QEMU,
+but are not available in all supported distros, are bundled with the
+QEMU sources.  Currently this includes Meson (outdated in CentOS 8
+and derivatives, Ubuntu 20.04 and 22.04, and openSUSE Leap) and tomli
+(absent in Ubuntu 20.04).
+
+If you need to update these, please do so by modifying and rerunning
+``python/scripts/vendor.py``.  This script embeds the sha256 hash of
+package sources and checks it.  The pypi.org web site provides an easy
+way to retrieve the sha256 hash of the sources.
 
-TBD
 
 Stage 2: Meson
 ==============
@@ -376,6 +444,15 @@ This is needed to obey the --python= option passed to the configure
 script, which may point to something other than the first python3
 binary on the path.
 
+By the time Meson runs, Python dependencies are available in the virtual
+environment and should be invoked through the scripts that ``configure``
+places under ``pyvenv``.  One way to do so is as follows, using Meson's
+``find_program`` function::
+
+  sphinx_build = find_program(
+       fs.parent(python.full_path()) / 'sphinx-build',
+       required: get_option('docs'))
+
 
 Stage 3: Make
 =============
@@ -434,6 +511,11 @@ number of dynamically created files listed later.
   executables.  Build rules for various subdirectories are included in
   other meson.build files spread throughout the QEMU source tree.
 
+``python/scripts/mkvenv.py``
+  A wrapper for the Python ``venv`` and ``distlib.scripts`` packages.
+  It handles creating the virtual environment, creating scripts in
+  ``pyvenv/bin``, and calling ``pip`` to install dependencies.
+
 ``tests/Makefile.include``
   Rules for external test harnesses. These include the TCG tests
   and the Avocado-based integration tests.

pythondeps.toml

@@ -10,7 +10,8 @@
 # - accepted: accepted versions when using a system package
 # - installed: fixed version to install in the virtual environment
 #              if a system package is not found; if not specified,
-#              the minimum and maximum
+#              defaults to the same as "accepted" or, if also missing,
+#              to the newest version available on PyPI.
 # - canary: if specified, use this program name to present more
 #           precise error diagnostics to the user.  For example,
 #           'sphinx-build' can be used as a bellwether for the