Stop --dry-run from touching the filesystem

Summary:
`--dry-run` is documented as "Show build plan without building". It did two
things to disk before showing anything.

**`--clean` ran first.** `main()` handles `--clean` at the point where the
directory manager is created, which is above the `--dry-run` early return. So
`--dry-run --clean` — a natural thing to type when you want to see what a clean
rebuild would do — called `DirectoryManager.clean_all()` and `rmtree`d every
fetched source and every build tree, then printed the plan and exited without
rebuilding any of it. The next real build re-clones all 23 repositories.

**The directory manager created its tree in `__init__`.** Constructing it
mkdir'd `install/`, `source/` and `build/`, so a dry run in any directory left
an `ocean_3rdparty/` skeleton behind — including when the plan it printed was
for a `--output-dir` the user was only evaluating.

`--clean` is now skipped under `--dry-run` with a line saying what it would
have deleted, and `DirectoryManager` takes `create=False` so it resolves paths
without creating them.

Found by an exhaustive review of `build/python/`; tracked as `OB-038`.

___

Differential Revision: D116053695

fbshipit-source-id: 498aba375d891af9b1d312577c25c3e990bedc3f

Author: janherlingmeta

build/python/build_ocean_3rdparty.py

@@ -1899,16 +1899,22 @@ def main() -> int:  # noqa: C901
             source_dir,
             build_dir,
             for_external_integration=args.for_external_integration,
+            create=not args.dry_run,
         )
     except OSError as e:
         print(f"Error: cannot create the output directories under {base_dir}: {e}")
         return 1
     fetcher = SourceFetcher(dir_manager, manifest_dir=manifest_path.parent)
 
-    # Handle --clean
+    # Handle --clean. Deliberately not done under --dry-run: deleting every
+    # fetched source and build tree is the most destructive thing this script
+    # can do, and "show me what you would build" must not do it.
     if args.clean:
-        print("Cleaning cache...")
-        dir_manager.clean_all()
+        if args.dry_run:
+            print("Dry run: skipping --clean (would delete the source and build cache)")
+        else:
+            print("Cleaning cache...")
+            dir_manager.clean_all()
 
     # Determine targets
     if requested_platforms is not None:

build/python/lib/directories.py

@@ -111,6 +111,7 @@ def __init__(
         source_dir: Optional[Path] = None,
         build_dir: Optional[Path] = None,
         for_external_integration: bool = False,
+        create: bool = True,
     ):
         """Initialize directory manager.
 
@@ -125,6 +126,9 @@ def __init__(
                 (default), produce the standard CMake install layout per
                 target (one complete prefix per library per target), which
                 works directly with find_package(... CONFIG).
+            create: If False, resolve the paths but do not create anything on
+                disk. Used by --dry-run, which must not leave directories
+                behind for a build it did not run.
         """
         self.install_dir = install_dir.resolve()
         self.for_external_integration = for_external_integration
@@ -137,9 +141,10 @@ def __init__(
         self.builds_dir = (build_dir or default_cache / "builds").resolve()
 
         # Create base directories
-        self.sources_dir.mkdir(parents=True, exist_ok=True)
-        self.builds_dir.mkdir(parents=True, exist_ok=True)
-        self.install_dir.mkdir(parents=True, exist_ok=True)
+        if create:
+            self.sources_dir.mkdir(parents=True, exist_ok=True)
+            self.builds_dir.mkdir(parents=True, exist_ok=True)
+            self.install_dir.mkdir(parents=True, exist_ok=True)
 
     def get_source_dir(self, library: str, version: str) -> Path:
         """Get the source directory for a library (shared across all targets)."""