wrap: Lock subproject directory when downloading wraps

To avoid raceconditions, where one instance of meson currently downloads
a subproject defined in a wrapfile, while another either
  a. starts the download itself too
  b. attemts to evaluate the partially downloaded subproject
wraplock introduces a lockfile, which should prevent simultaneous access
of subprojects by wrap between different instances of meson.

.gitignore

@@ -22,6 +22,7 @@ __pycache__
 *~
 *.swp
 packagecache
+.wraplock
 /MANIFEST
 /build
 /dist

mesonbuild/wrap/wrap.py

@@ -30,7 +30,10 @@ from functools import lru_cache
 
 from . import WrapMode
 from .. import coredata
-from ..mesonlib import quiet_git, GIT, ProgressBar, MesonException, windows_proof_rmtree, Popen_safe
+from ..mesonlib import (
+    DirectoryLock, DirectoryLockAction, quiet_git, GIT, ProgressBar, MesonException,
+    windows_proof_rmtree, Popen_safe
+)
 from ..interpreterbase import FeatureNew
 from ..interpreterbase import SubProject
 from .. import mesonlib
@@ -449,7 +452,7 @@ class Resolver:
                 return wrap_name
         return None
 
-    def resolve(self, packagename: str, force_method: T.Optional[Method] = None) -> T.Tuple[str, Method]:
+    def _resolve(self, packagename: str, force_method: T.Optional[Method] = None) -> T.Tuple[str, Method]:
         wrap = self.wraps.get(packagename)
         if wrap is None:
             wrap = self.get_from_wrapdb(packagename)
@@ -547,6 +550,15 @@ class Resolver:
         self.wrap.update_hash_cache(self.dirname)
         return rel_path, method
 
+    def resolve(self, packagename: str, force_method: T.Optional[Method] = None) -> T.Tuple[str, Method]:
+        try:
+            with DirectoryLock(self.subdir_root, '.wraplock',
+                               DirectoryLockAction.WAIT,
+                               'Failed to lock subprojects directory'):
+                return self._resolve(packagename, force_method)
+        except FileNotFoundError:
+            raise WrapNotFoundException('Attempted to resolve subproject without subprojects directory present.')
+
     def check_can_download(self) -> None:
         # Don't download subproject data based on wrap file if requested.
         # Git submodules are ok (see above)!

test cases/failing/33 non-root subproject/test.json

@@ -1,7 +1,7 @@
 {
   "stdout": [
     {
-      "line": "test cases/failing/33 non-root subproject/some/meson.build:1:0: ERROR: Neither a subproject directory nor a someproj.wrap file was found."
+      "line": "test cases/failing/33 non-root subproject/some/meson.build:1:0: ERROR: Attempted to resolve subproject without subprojects directory present."
     }
   ]
 }

test cases/failing/73 subproj dependency not-found and required/test.json

@@ -1,7 +1,7 @@
 {
   "stdout": [
     {
-      "line": "test cases/failing/73 subproj dependency not-found and required/meson.build:2:10: ERROR: Neither a subproject directory nor a missing.wrap file was found."
+      "line": "test cases/failing/73 subproj dependency not-found and required/meson.build:2:10: ERROR: Attempted to resolve subproject without subprojects directory present."
     }
   ]
 }