Introduce a SPACK_PYTHON environment variable (#21222)

The SPACK_PYTHON environment variable can be set to a python interpreter to be
used by the spack command.  This allows the spack command itself to use a
consistent and separate interpreter from whatever python might be used for package
building.

bin/spack

@@ -10,9 +10,12 @@
 # Following line is a shell no-op, and starts a multi-line Python comment.
 # See https://stackoverflow.com/a/47886254
 """:"
-# prefer python3, then python, then python2
-for cmd in python3 python python2; do
-   command -v > /dev/null $cmd && exec $cmd $0 "$@"
+# prefer SPACK_PYTHON environment variable, python3, python, then python2
+for cmd in "${SPACK_PYTHON:-}" python3 python python2; do
+    if command -v > /dev/null "$cmd"; then
+        export SPACK_PYTHON="$(command -v "$cmd")"
+        exec "${SPACK_PYTHON}" "$0" "$@"
+    fi
 done
 
 echo "==> Error: spack could not find a python interpreter!" >&2

lib/spack/docs/getting_started.rst

@@ -75,6 +75,14 @@ shell integration for :ref:`certain commands <packaging-shell-support>`,
 If you do not want to use Spack's shell support, you can always just run
 the ``spack`` command directly from ``spack/bin/spack``.
 
+When the ``spack`` command is executed it searches for an appropriate
+Python interpreter to use, which can be explicitly overridden by setting
+the ``SPACK_PYTHON`` environment variable.  When sourcing the appropriate shell
+setup script, ``SPACK_PYTHON`` will be set to the interpreter found at
+sourcing time, ensuring future invocations of the ``spack`` command will
+continue to use the same consistent python version regardless of changes in
+the environment.
+
 
 ^^^^^^^^^^^^^^^^^^
 Check Installation

share/spack/setup-env.csh

@@ -58,6 +58,18 @@ alias spack          'set _sp_args = (\!*); source $_spack_share_dir/csh/spack.c
 alias spacktivate    'spack env activate'
 alias _spack_pathadd 'set _pa_args = (\!*) && source $_spack_share_dir/csh/pathadd.csh'
 
+# Identify and lock the python interpreter
+if (! $?SPACK_PYTHON) then
+    setenv SPACK_PYTHON ""
+endif
+foreach cmd ("$SPACK_PYTHON" python3 python python2)
+    command -v "$cmd" >& /dev/null
+    if ($status == 0) then
+        setenv SPACK_PYTHON `command -v "$cmd"`
+        break
+    endif
+end
+
 # Set variables needed by this script
 _spack_pathadd PATH "$SPACK_ROOT/bin"
 eval `spack --print-shell-vars csh`

share/spack/setup-env.fish

@@ -253,7 +253,7 @@ function match_flag -d "checks all combinations of flags ocurring inside of a st
     set -l _a (string sub -s 2 (string trim "x$argv[1]"))
     set -l _b (string sub -s 2 (string trim "x$argv[2]"))
 
-    if test -z "$_a" || test -z "$_b"
+    if test -z "$_a" or test -z "$_b"
         return 0
     end
 
@@ -667,6 +667,19 @@ set -l sp_source_file (status -f)  # name of current file
 
 
 
+#
+# Identify and lock the python interpreter
+#
+for cmd in "$SPACK_PYTHON" python3 python python2
+    set -l _sp_python (command -s "$cmd")
+    if test $status -eq 0
+        set -x SPACK_PYTHON $_sp_python
+        break
+    end
+end
+
+
+
 #
 # Find root directory and add bin to path.
 #

share/spack/setup-env.sh

@@ -326,6 +326,14 @@ if [ "$_sp_shell" = bash ]; then
     export -f _spack_shell_wrapper
 fi
 
+# Identify and lock the python interpreter
+for cmd in "${SPACK_PYTHON:-}" python3 python python2; do
+    if command -v > /dev/null "$cmd"; then
+        export SPACK_PYTHON="$(command -v "$cmd")"
+        break
+    fi
+done
+
 #
 # make available environment-modules
 #