ninja backend: Detect ninja only once and log it

Needed for the CI, but good to have in general too.

mesonbuild/backend/ninjabackend.py

@@ -177,6 +177,9 @@ int dummy;
 
     def generate(self, interp):
         self.interpreter = interp
+        self.ninja_command = environment.detect_ninja(log=True)
+        if self.ninja_command is None:
+            raise MesonException('Could not detect Ninja v1.5 or newer')
         outfilename = os.path.join(self.environment.get_build_dir(), self.ninja_filename)
         tempfilename = outfilename + '~'
         with open(tempfilename, 'w') as outfile:
@@ -210,10 +213,9 @@ int dummy;
 
     # http://clang.llvm.org/docs/JSONCompilationDatabase.html
     def generate_compdb(self):
-        ninja_exe = environment.detect_ninja()
         native_compilers = ['%s_COMPILER' % i for i in self.build.compilers]
         cross_compilers = ['%s_CROSS_COMPILER' % i for i in self.build.cross_compilers]
-        ninja_compdb = [ninja_exe, '-t', 'compdb'] + native_compilers + cross_compilers
+        ninja_compdb = [self.ninja_command, '-t', 'compdb'] + native_compilers + cross_compilers
         builddir = self.environment.get_build_dir()
         try:
             jsondb = subprocess.check_output(ninja_compdb, cwd=builddir)
@@ -2509,11 +2511,8 @@ rule FORTRAN_DEP_HACK
         default = 'default all\n\n'
         outfile.write(default)
 
-        ninja_command = environment.detect_ninja()
-        if ninja_command is None:
-            raise MesonException('Could not detect Ninja v1.6 or newer')
         elem = NinjaBuildElement(self.all_outputs, 'clean', 'CUSTOM_COMMAND', 'PHONY')
-        elem.add_item('COMMAND', [ninja_command, '-t', 'clean'])
+        elem.add_item('COMMAND', [self.ninja_command, '-t', 'clean'])
         elem.add_item('description', 'Cleaning.')
 
         # If we have custom targets in this project, add all their outputs to

mesonbuild/environment.py

@@ -89,16 +89,19 @@ def find_coverage_tools():
         genhtml_exe = None
     return gcovr_exe, lcov_exe, genhtml_exe
 
-def detect_ninja(version='1.5'):
+def detect_ninja(version='1.5', log=False):
     for n in ['ninja', 'ninja-build']:
         try:
             p, found = Popen_safe([n, '--version'])[0:2]
         except (FileNotFoundError, PermissionError):
             # Doesn't exist in PATH or isn't executable
             continue
+        found = found.strip()
         # Perhaps we should add a way for the caller to know the failure mode
         # (not found or too old)
         if p.returncode == 0 and mesonlib.version_compare(found, '>=' + version):
+            if log:
+                mlog.log('Found ninja-{} at {}'.format(found, shlex.quote(shutil.which(n))))
             return n
 
 def detect_native_windows_arch():