cerbero: Prefer searching using vswhere if available

vswhere.exe is the preferred way of detecting Visual Studio
installations if it is available. We still fallback to searching the
default installation locations if it is not available.

cerbero/ide/vs/env.py

@@ -23,6 +23,7 @@ from functools import lru_cache
 
 from cerbero.enums import Architecture
 from cerbero.errors import FatalError
+from cerbero.utils.shell import check_output
 
 # We only support Visual Studio 2015 as of now
 VCVARSALLS = {
@@ -81,6 +82,31 @@ def _get_custom_vs_install(vs_version, vs_install_path):
         raise FatalError('Can\'t find vcvarsall.bat inside vs_install_path {!r}'.format(path))
     return path.as_posix(), vs_version
 
+def _get_vswhere_vs_install(vswhere, vs_versions):
+    import json
+    vswhere_exe = str(vswhere)
+    # Get a list of installation paths for all installed Visual Studio
+    # instances, from VS 2013 to the latest one, sorted from newest to
+    # oldest, and including preview releases.
+    # Will not include BuildTools installations.
+    out = check_output([vswhere_exe, '-legacy', '-prerelease', '-format',
+                        'json', '-utf8', '-sort'])
+    installs = json.loads(out)
+    program_files = get_program_files_dir()
+    for install in installs:
+        version = install['installationVersion']
+        vs_version = 'vs' + version.split('.', maxsplit=1)[0]
+        if vs_version not in vs_versions:
+            continue
+        prefix = install['installationPath']
+        suffix = VCVARSALLS[vs_version][1]
+        path = program_files / prefix / suffix
+        # Find the location of the Visual Studio installation
+        if path.is_file():
+            return path.as_posix(), vs_version
+    raise FatalError('vswhere.exe could not find Visual Studio installation(s). '
+                     'Looked for version(s): ' + ', '.join(vs_versions))
+
 def get_vcvarsall(vs_version, vs_install_path):
     known_vs_versions = sorted(VCVARSALLS.keys(), reverse=True)
     if vs_version:
@@ -99,6 +125,16 @@ def get_vcvarsall(vs_version, vs_install_path):
         # pick the newest one found.
         vs_versions = known_vs_versions
     program_files = get_program_files_dir()
+    # Try to find using vswhere.exe
+    # vswhere is installed by Visual Studio 2017 and newer into a fixed
+    # location, and can also be installed separately. For others:
+    # - Visual Studio 2013 (can be found by vswhere -legacy, but we don't use it)
+    # - Visual Studio 2015 (can be found by vswhere -legacy)
+    # - Visual Studio 2019 Build Tools (cannot be found by vswhere)
+    vswhere = program_files / 'Microsoft Visual Studio' / 'Installer' / 'vswhere.exe'
+    if vswhere.is_file():
+        return _get_vswhere_vs_install(vswhere, vs_versions)
+    # Look in the default locations if vswhere.exe is not available.
     for vs_version in vs_versions:
         prefixes, suffix = VCVARSALLS[vs_version]
         for prefix in prefixes: