Bug 1241398 - Add a dry-run mode to mach build-backend. r=gps

When doing build system changes affecting backend-generated files, I
often use `mach build-backend --diff`. But most of the time I end up
wanting to look at the full diff again when doing further changes, which
leads me to stash my changes away, run `mach build-backend` to get the
initial state again, unstash and rerun `mach build-backend --diff`.

This has been a time drain for long enough :)

python/mozbuild/mozbuild/backend/base.py

@@ -83,6 +83,8 @@ class BuildBackend(LoggingMixin):
         # Mapping of changed file paths to diffs of the changes.
         self.file_diffs = {}
 
+        self.dry_run = False
+
         self._init()
 
     def summary(self):
@@ -149,7 +151,8 @@ class BuildBackend(LoggingMixin):
             except IOError:
                 pass
             try:
-                os.unlink(full_path)
+                if not self.dry_run:
+                    os.unlink(full_path)
                 self._deleted_count += 1
             except OSError:
                 pass
@@ -196,7 +199,7 @@ class BuildBackend(LoggingMixin):
 
         if path is not None:
             assert fh is None
-            fh = FileAvoidWrite(path, capture_diff=True)
+            fh = FileAvoidWrite(path, capture_diff=True, dry_run=self.dry_run)
         else:
             assert fh is not None
 

python/mozbuild/mozbuild/config_status.py

@@ -116,6 +116,8 @@ def config_status(topobjdir='.', topsrcdir='.',
                         default=default_backends,
                         help='what backend to build (default: %s).' %
                         ' '.join(default_backends))
+    parser.add_argument('--dry-run', action='store_true',
+                        help='do everything except writing files out.')
     options = parser.parse_args()
 
     # Without -n, the current directory is meant to be the top object directory
@@ -137,6 +139,10 @@ def config_status(topobjdir='.', topsrcdir='.',
     # or what is in BUILD_BACKENDS.
     selected_backends = [get_backend_class(b)(env) for b in options.backend]
 
+    if options.dry_run:
+        for b in selected_backends:
+            b.dry_run = True
+
     reader = BuildReader(env)
     emitter = TreeMetadataEmitter(env)
     # This won't actually do anything because of the magic of generators.

python/mozbuild/mozbuild/mach_commands.py

@@ -607,7 +607,9 @@ class Build(MachCommandBase):
         help='Which backend to build.')
     @CommandArgument('-v', '--verbose', action='store_true',
         help='Verbose output.')
-    def build_backend(self, backend, diff=False, verbose=False):
+    @CommandArgument('-n', '--dry-run', action='store_true',
+        help='Do everything except writing files out.')
+    def build_backend(self, backend, diff=False, verbose=False, dry_run=False):
         python = self.virtualenv_manager.python_path
         config_status = os.path.join(self.topobjdir, 'config.status')
 
@@ -625,6 +627,8 @@ class Build(MachCommandBase):
             args.append('--diff')
         if verbose:
             args.append('--verbose')
+        if dry_run:
+            args.append('--dry-run')
 
         return self._run_command_in_objdir(args=args, pass_thru=True,
             ensure_exit_code=False)

python/mozbuild/mozbuild/util.py

@@ -139,11 +139,16 @@ class FileAvoidWrite(BytesIO):
     Instances can optionally capture diffs of file changes. This feature is not
     enabled by default because it a) doesn't make sense for binary files b)
     could add unwanted overhead to calls.
+
+    Additionally, there is dry run mode where the file is not actually written
+    out, but reports whether the file was existing and would have been updated
+    still occur, as well as diff capture if requested.
     """
-    def __init__(self, filename, capture_diff=False, mode='rU'):
+    def __init__(self, filename, capture_diff=False, dry_run=False, mode='rU'):
         BytesIO.__init__(self)
         self.name = filename
         self._capture_diff = capture_diff
+        self._dry_run = dry_run
         self.diff = None
         self.mode = mode
 
@@ -183,14 +188,15 @@ class FileAvoidWrite(BytesIO):
             finally:
                 existing.close()
 
-        ensureParentDir(self.name)
-        # Maintain 'b' if specified.  'U' only applies to modes starting with
-        # 'r', so it is dropped.
-        writemode = 'w'
-        if 'b' in self.mode:
-            writemode += 'b'
-        with open(self.name, writemode) as file:
-            file.write(buf)
+        if not self._dry_run:
+            ensureParentDir(self.name)
+            # Maintain 'b' if specified.  'U' only applies to modes starting with
+            # 'r', so it is dropped.
+            writemode = 'w'
+            if 'b' in self.mode:
+                writemode += 'b'
+            with open(self.name, writemode) as file:
+                file.write(buf)
 
         if self._capture_diff:
             try: