test cases: use best practices method to find the python3 program

We do not need the python module's find_installation() for this, as this
does various things to set up building and installing python modules
(pure python and C-API). This functionality is already tested in the
python tests.

Elsewhere, when we just need an interpreter capable of running python
scripts in order to guarantee a useful scripting language for custom
commands, it suffices to use find_program(), which does not run an
introspection script or do module imports, and is thus faster and
a bit cleaner.

Either way, both methods are guaranteed to find the python3 interpreter,
deferring to mesonlib.python_command for that guarantee.

test "71 summary" can sometimes return the python command with the
".exe" part all uppercased for mysterious Windows reasons. Smooth this
over with ExternalProgram.

13 files changed, 18 insertions, 15 deletions

diff --git a/test cases/common/257 generated header dep/meson.build b/test cases/common/257 generated header dep/meson.build
index 195d082..82db3b2 100644
--- a/test cases/common/257 generated header dep/meson.build
+++ b/test cases/common/257 generated header dep/meson.build
@@ -8,7 +8,7 @@ project('generated header dep', 'c')
 # dependency on the header file. This happened in GLib:
 # https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2917.
 
-python = import('python').find_installation()
+python = find_program('python3')
 header = custom_target(
   output: 'foo.h',
   capture: true,
diff --git a/test cases/common/44 pkgconfig-gen/meson.build b/test cases/common/44 pkgconfig-gen/meson.build
index 12a110e..e093e6f 100644
--- a/test cases/common/44 pkgconfig-gen/meson.build
+++ b/test cases/common/44 pkgconfig-gen/meson.build
@@ -18,7 +18,7 @@ if v.version_compare('<0.29')
   error('MESON_SKIP_TEST: pkg-config version \'' + v + '\' too old')
 endif
 
-python = import('python').find_installation()
+python = find_program('python3')
 fs = import('fs')
 pkgg = import('pkgconfig')
 
diff --git a/test cases/failing/115 run_target in test/meson.build b/test cases/failing/115 run_target in test/meson.build
index 1d448ec..117009e 100644
--- a/test cases/failing/115 run_target in test/meson.build
+++ b/test cases/failing/115 run_target in test/meson.build
@@ -1,7 +1,7 @@
 project('trivial test', 'c')
 
-py_inst = import('python').find_installation()
+python = find_program('python3')
 
 exe = executable('trivialprog', 'trivial.c')
-runt = run_target('invalid', command: [py_inst, '--version'])
+runt = run_target('invalid', command: [python, '--version'])
 test('runtest', exe, args: runt)
diff --git a/test cases/failing/116 run_target in add_install_script/meson.build b/test cases/failing/116 run_target in add_install_script/meson.build
index f8ae3a1..d3634bf 100644
--- a/test cases/failing/116 run_target in add_install_script/meson.build
+++ b/test cases/failing/116 run_target in add_install_script/meson.build
@@ -1,7 +1,7 @@
 project('trivial test', 'c')
 
-py_inst = import('python').find_installation()
+python = find_program('python3')
 
 exe = executable('trivialprog', 'trivial.c')
-runt = run_target('invalid', command: [py_inst, '--version'])
+runt = run_target('invalid', command: [python, '--version'])
 meson.add_install_script(exe, runt)
diff --git a/test cases/java/8 codegen custom target/com/mesonbuild/meson.build b/test cases/java/8 codegen custom target/com/mesonbuild/meson.build
index 0309941..5188f0a 100644
--- a/test cases/java/8 codegen custom target/com/mesonbuild/meson.build
+++ b/test cases/java/8 codegen custom target/com/mesonbuild/meson.build
@@ -1,4 +1,4 @@
-python = import('python').find_installation('python3')
+python = find_program('python3')
 
 config_file = custom_target('confgen',
                             input : 'Config.java.in',
diff --git a/test cases/unit/64 alias target/meson.build b/test cases/unit/64 alias target/meson.build
index bcd4005..197897b 100644
--- a/test cases/unit/64 alias target/meson.build
+++ b/test cases/unit/64 alias target/meson.build
@@ -1,6 +1,6 @@
 project('alias target', 'c')
 
-python3 = import('python').find_installation()
+python3 = find_program('python3')
 
 exe_target = executable('prog', 'main.c',
   build_by_default : false)
diff --git a/test cases/unit/70 cross test passed/meson.build b/test cases/unit/70 cross test passed/meson.build
index 4deb74b..3a09a77 100644
--- a/test cases/unit/70 cross test passed/meson.build
+++ b/test cases/unit/70 cross test passed/meson.build
@@ -6,7 +6,7 @@ project(
 
 e = executable('exec', 'src/main.c')
 
-py = import('python').find_installation()
+py = find_program('python3')
 
 test('root', e)
 test('main', py, args : [meson.current_source_dir() / 'script.py', e])
diff --git a/test cases/unit/71 summary/meson.build b/test cases/unit/71 summary/meson.build
index ce97fb3..76fc545 100644
--- a/test cases/unit/71 summary/meson.build
+++ b/test cases/unit/71 summary/meson.build
@@ -11,7 +11,7 @@ summary({'Some boolean': false,
          'enabled_opt': get_option('enabled_opt'),
         }, section: 'Configuration')
 summary({'missing prog': find_program('xyzzy', required: false),
-         'existing prog': import('python').find_installation(),
+         'existing prog': find_program(get_option('python')).full_path(),
          'missing dep': dependency('', required: false),
          'external dep': dependency('zlib', required: false),
          'internal dep': declare_dependency(),
diff --git a/test cases/unit/71 summary/meson_options.txt b/test cases/unit/71 summary/meson_options.txt
index 281c3b6..cf3f32c 100644
--- a/test cases/unit/71 summary/meson_options.txt
+++ b/test cases/unit/71 summary/meson_options.txt
@@ -1 +1,2 @@
 option('enabled_opt', type: 'feature', value: 'auto')
+option('python', type: 'string')
diff --git a/test cases/unit/73 dep files/meson.build b/test cases/unit/73 dep files/meson.build
index 4829f56..af7f6e4 100644
--- a/test cases/unit/73 dep files/meson.build
+++ b/test cases/unit/73 dep files/meson.build
@@ -1,6 +1,6 @@
 project('test', 'c')
 
-python = import('python').find_installation()
+python = find_program('python3')
 
 lib = library('foo', 'foo.c')
 
diff --git a/test cases/unit/99 custom target name/meson.build b/test cases/unit/99 custom target name/meson.build
index 8d148a8..8287614 100644
--- a/test cases/unit/99 custom target name/meson.build
+++ b/test cases/unit/99 custom target name/meson.build
@@ -1,6 +1,6 @@
 project('custom target name', 'c')
 
-python = import('python').find_installation()
+python = find_program('python3')
 
 # Name argument is optional and should default to 'file.txt'
 custom_target(
diff --git a/test cases/windows/13 test argument extra paths/test/meson.build b/test cases/windows/13 test argument extra paths/test/meson.build
index 2e608be..4daccae 100644
--- a/test cases/windows/13 test argument extra paths/test/meson.build
+++ b/test cases/windows/13 test argument extra paths/test/meson.build
@@ -1,3 +1,3 @@
-python3 = import('python').find_installation('')
+python3 = find_program('python3')
 
 test('run_exe', python3, args: [files('test_run_exe.py')[0], barexe])
diff --git a/unittests/allplatformstests.py b/unittests/allplatformstests.py
index a38b839..3dd8a32 100644
--- a/unittests/allplatformstests.py
+++ b/unittests/allplatformstests.py
@@ -44,6 +44,7 @@ from mesonbuild.mesonlib import (
     MesonException, EnvironmentException, OptionKey, ExecutableSerialisation, EnvironmentVariables,
     windows_proof_rm
 )
+from mesonbuild.programs import ExternalProgram
 
 from mesonbuild.compilers.mixins.clang import ClangCompiler
 from mesonbuild.compilers.mixins.gnu import GnuCompiler
@@ -3371,7 +3372,7 @@ class AllPlatformTests(BasePlatformTests):
 
     def test_summary(self):
         testdir = os.path.join(self.unit_test_dir, '71 summary')
-        out = self.init(testdir, extra_args=['-Denabled_opt=enabled'])
+        out = self.init(testdir, extra_args=['-Denabled_opt=enabled', f'-Dpython={sys.executable}'])
         expected = textwrap.dedent(r'''
             Some Subproject 2.0
 
@@ -3401,7 +3402,7 @@ class AllPlatformTests(BasePlatformTests):
 
               Stuff
                 missing prog   : NO
-                existing prog  : ''' + sys.executable + '''
+                existing prog  : ''' + ExternalProgram('python3', [sys.executable], silent=True).path + '''
                 missing dep    : NO
                 external dep   : YES 1.2.3
                 internal dep   : YES
@@ -3421,6 +3422,7 @@ class AllPlatformTests(BasePlatformTests):
                 libdir         : lib
                 prefix         : /usr
                 enabled_opt    : enabled
+                python         : ''' + sys.executable + '''
             ''')
         expected_lines = expected.split('\n')[1:]
         out_start = out.find(expected_lines[0])