ccache for Python packages built in temporary directories (help wanted)

[mkoeppe]

Modern python packaging tools such as build build wheels in temporary directories.
This makes ccache ineffective for repeated builds.
A change like the one below helps partially, but only a small subset of the Cython modules give cache hits.

diff --git a/pkgs/sagemath-categories/setup.py b/pkgs/sagemath-categories/setup.py
index b2fbc8a6ac..d2f445e33c 100644
--- a/pkgs/sagemath-categories/setup.py
+++ b/pkgs/sagemath-categories/setup.py
@@ -20,6 +20,9 @@ sys.excepthook = excepthook
 from sage_setup.setenv import setenv
 setenv()
 
+os.environ['CCACHE_BASEDIR'] = sys.prefix
+os.environ['CCACHE_NOHASHDIR'] = '1'
+
 import sage.env
 sage.env.default_required_modules = sage.env.default_optional_modules = ()
 print(f'##################### {sage.env.SAGE_SRC=}')

[tornaria]

FWIW, I did SAGE_DEBUG=no SAGE_NUM_THREADS=8 PYTHONDONTWRITEBYTECODE=yes python -m build --no-isolation --wheel and I got 1 cache miss, 549 direct hits, 76 preprocessed hits.

For contrast, using SAGE_DEBUG=no SAGE_NUM_THREADS=8 PYTHONDONTWRITEBYTECODE=yes python setup.py build gives me 563 direct hits and 63 preprocessed hits (no misses).

(this is for sagemath-standard)

Maybe SAGE_DEBUG=no makes the difference ?

[mkoeppe]

--no-isolation for sure makes a difference...