Replace "$SAGE_LOCAL/bin" by more specific variables to make Sage easier to package, use in venvs

[tobihan]

Various scripts refer to "$SAGE_LOCAL/bin" in a way that conflates several different uses. We introduce new variables to give packagers more flexibility without having to patch, and to enable new installation schemes, including the use of Sage in users' venvs.

1a. Introduce $SAGE_BIN as the place where the binaries of all the SPKGs get installed to. Default is $SAGE_LOCAL/bin/.

1b. (Wishlist item:) Make it configurable through ./configure --bindir, but this would need major changes to spkg-configure scripts. (Supporting --bindir would be another step toward more autotools compliance, see Meta-ticket #21566.)

2a. Scripts such as src/bin/sage, src/bin/sage-eval are installed by the sagelib package using the scripts facility of distutils/setuptools. If sagelib is installed into a venv, these scripts are installed into the bin directory of the venv. For the default installation in sage-the-distribution --with-system-python, this venv is identical with SAGE_LOCAL. But if other venvs are created by the user, it will be unrelated to SAGE_LOCAL; in fact, SAGE_LOCAL may be undefined (in distribution packaging and when installing sagelib through pip). As proposed in #29013, we use the variable SAGE_VENV for this, so these scripts would be referred to as $SAGE_VENV/bin/sage-eval etc.

2b. Referring to the python3 used for sage. Likewise, this comes from the venv, not $SAGE_LOCAL/bin. So it should be referred to as $SAGE_VENV/bin/python3.

2c. The subset of the scripts in src/bin, currently installed by the sagelib package, that are "libexec"-like (not user-facing). Distribution packagers would like to hide these scripts. (In the old description of this ticket, it was proposed to use $SAGE_SCRIPTS_DIR for this, but a more specific name is needed: We will use $SAGE_VENV_SCRIPTS_DIR.)

In this ticket, we introduce SAGE_VENV for 2abc, ignoring the "libexec" aspect (using $SAGE_VENV/bin directly instead of going through SAGE_VENV_SCRIPTS_DIR). Items 1 and the libexec aspect of 2c can be done on follow-up tickets.

We also make sage-env-config optional from the viewpoint of the other scripts. This is preparation for #29850 / #29852.

See also:

• https://salsa.debian.org/science-team/sagemath/-/blob/master/debian/patches/u1-scripts-dir.patch
• src/bin/sage-env: Make sure $SAGE_VENV/bin is at the beginning of the PATH #30013 src/bin/sage-env: Make sure $SAGE_VENV/bin is at the beginning of the PATH
• src/bin/sage-env: Make SAGE_ROOT and SAGE_LOCAL optional #29951 src/bin/sage-env: Make SAGE_ROOT and SAGE_LOCAL optional
• Add src/requirements.txt for installation of sagelib in a venv #30578 Add src/requirements.txt for installation of sagelib in a venv

Depends on #29951

CC: @mkoeppe @jhpalmieri @orlitzky @tobiasdiez @isuruf @antonio-rojas @mwageringel

Component: porting

Author: Matthias Koeppe

Branch/Commit: 38eebc3

Reviewer: Tobias Diez, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/22731

[tobihan]

Branch: u/thansen/allow_installation_of_sage_scripts_to_an_arbotrary_location_by_using_sage_scripts_dir_

[tobihan]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
-
+Use SAGE_SCRIPTS_DIR in more places and define a fallback in env.py.
+    
+SAGE_SCRIPTS_DIR is already being defined in sage-env. With these changes Linux distros can install sage to /usr/bin and the other scripts somewhere else.

[tobihan]

Author: Tobias Hanen

[tobihan]

Commit: 6ae9bf1

[tobihan]

New commits:

6ae9bf1

Use SAGE_SCRIPTS_DIR in more places and define a fallback in env.py.

[tobihan]

Changed author from Tobias Hanen to Tobias Hansen

[jdemeyer]

comment:4

The name SAGE_SCRIPTS_DIR refers to the scripts of Sage itself. It is only meant to find sage-env and friends, nothing more.

Now, if you want a invent a new variable with the purpose of defining the bindir (to replace SAGE_LOCAL/bin), that's fine. Just don't call it SAGE_SCRIPTS_DIR.