uniform scan-build detection process

Detect scan-build the same way when trying to launch it and when
generating the target.
The detection method is:
  1. look within SCANBUILD env variable
  2. shutil.which('scan-build')
  3. *on non-linux platforms only*: go through all the possible
     name candidates and test them individually.

The third step is added following this comment
https://github.com/mesonbuild/meson/pull/5857#issuecomment-528305788
However, going through a list of all the possible candidates is neither
easily maintainable nor performant, and is therefore skipped on
platforms that should not require such a step (currently, only Linux
platforms).

This is a follow-up to the issue raised by @lantw44 during PR:
https://github.com/mesonbuild/meson/pull/5857

mesonbuild/backend/ninjabackend.py

@@ -2646,8 +2646,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         self.create_target_alias('meson-dist')
 
     def generate_scanbuild(self):
-        import shutil
-        if shutil.which('scan-build') is None:
+        if not environment.detect_scanbuild():
             return
         cmd = self.environment.get_build_command() + \
             ['--internal', 'scanbuild', self.environment.source_dir, self.environment.build_dir] + \

mesonbuild/environment.py

@@ -163,6 +163,55 @@ def detect_ninja(version: str = '1.5', log: bool = False) -> str:
                 mlog.log('Found {}-{} at {}'.format(name, found, quote_arg(n)))
             return n
 
+def detect_scanbuild():
+    """ Look for scan-build binary on build platform
+
+    First, if a SCANBUILD env variable has been provided, give it precedence
+    on all platforms.
+
+    For most platforms, scan-build is found is the PATH contains a binary
+    named "scan-build". However, some distribution's package manager (FreeBSD)
+    don't. For those, loop through a list of candidates to see if one is
+    available.
+    Since this is a costly operation, limit it to the impacted platforms
+    (currently all non-linux platforms)
+
+    Return: a single-element list of the found scan-build binary ready to be
+        passed to Popen()
+    """
+    exelist = []
+    if 'SCANBUILD' in os.environ:
+        exelist = split_args(os.environ['SCANBUILD'])
+
+    elif shutil.which('scan-build') is not None:
+        exelist = [shutil.which('scan-build')]
+
+    elif platform.system() != 'Linux':
+        tools = [
+            'scan-build',  # base
+            'scan-build-8.0', 'scan-build80',
+            'scan-build-7.0', 'scan-build70',
+            'scan-build-6.0', 'scan-build60',
+            'scan-build-5.0', 'scan-build50',
+            'scan-build-4.0', 'scan-build40',
+            'scan-build-3.9', 'scan-build39',
+            'scan-build-3.8', 'scan-build38',
+            'scan-build-3.7', 'scan-build37',
+            'scan-build-3.6', 'scan-build36',
+            'scan-build-3.5', 'scan-build35',
+            'scan-build-9.0', 'scan-build-devel',  # development snapshot
+        ]
+        for tool in tools:
+            if shutil.which(tool) is not None:
+                exelist = [shutil.which(tool)]
+                break
+
+    if exelist:
+        tool = exelist[0]
+        if os.path.isfile(tool) and os.access(tool, os.X_OK):
+            return [tool]
+    return []
+
 def detect_native_windows_arch():
     """
     The architecture of Windows itself: x86, amd64 or arm64

mesonbuild/scripts/scanbuild.py

@@ -16,9 +16,10 @@ import os
 import subprocess
 import shutil
 import tempfile
-from ..environment import detect_ninja
+from ..environment import detect_ninja, detect_scanbuild
 from ..mesonlib import Popen_safe, split_args
 
+
 def scanbuild(exelist, srcdir, blddir, privdir, logdir, args):
     with tempfile.TemporaryDirectory(dir=privdir) as scandir:
         meson_cmd = exelist + args
@@ -28,6 +29,7 @@ def scanbuild(exelist, srcdir, blddir, privdir, logdir, args):
             return rc
         return subprocess.call(build_cmd)
 
+
 def run(args):
     srcdir = args[0]
     blddir = args[1]
@@ -35,40 +37,10 @@ def run(args):
     privdir = os.path.join(blddir, 'meson-private')
     logdir = os.path.join(blddir, 'meson-logs/scanbuild')
     shutil.rmtree(logdir, ignore_errors=True)
-    tools = [
-        'scan-build',  # base
-        'scan-build-8.0', 'scan-build80',
-        'scan-build-7.0', 'scan-build70',
-        'scan-build-6.0', 'scan-build60',
-        'scan-build-5.0', 'scan-build50',
-        'scan-build-4.0', 'scan-build40',
-        'scan-build-3.9', 'scan-build39',
-        'scan-build-3.8', 'scan-build38',
-        'scan-build-3.7', 'scan-build37',
-        'scan-build-3.6', 'scan-build36',
-        'scan-build-3.5', 'scan-build35',
-        'scan-build-9.0', 'scan-build-devel',  # development snapshot
-    ]
-    toolname = 'scan-build'
-    for tool in tools:
-        try:
-            p, out = Popen_safe([tool, '--help'])[:2]
-        except (FileNotFoundError, PermissionError):
-            continue
-        if p.returncode != 0:
-            continue
-        else:
-            toolname = tool
-            break
 
-    if 'SCANBUILD' in os.environ:
-        exelist = split_args(os.environ['SCANBUILD'])
-    else:
-        exelist = [toolname]
-
-    try:
-        Popen_safe(exelist + ['--help'])
-    except OSError:
+    exelist = detect_scanbuild()
+    if not exelist:
         print('Could not execute scan-build "%s"' % ' '.join(exelist))
         return 1
+
     return scanbuild(exelist, srcdir, blddir, privdir, logdir, meson_cmd)