restore mypy cache.db copy-back atomicity

docs/notes/2.32.x.md

@@ -53,6 +53,9 @@ Pants will attempt to only preserve the `mypy` cache when `mypy` returns a [prob
 
 The default version of mypy used by Pants has been updated to [1.19.1](https://mypy-lang.blogspot.com/2025/11/mypy-119-released.html).  This includes support for the new "Fixed‑Format" cache.
 
+Copying the `mypy` cache back to the ["named cache"](https://www.pantsbuild.org/2.32/reference/global-options#named_caches_dir) should now be atomic in all cases.
+
+
 #### Shell
 
 #### Javascript

src/python/pants/backend/python/typecheck/mypy/rules.py

@@ -271,11 +271,11 @@ async def mypy_typecheck_partition(
                             #
                             # If we can hardlink (this means the two sides of the link are on the
                             # same filesystem), then after mypy runs, we hardlink from the sandbox
-                            # back to the named cache.
+                            # to a temp file in the named cache, then atomically rename it into place.
                             #
-                            # If we can't hardlink, we resort to copying the result next to the
-                            # cache under a temporary name, and finally doing an atomic mv from the
-                            # tempfile to the real one.
+                            # If we can't hardlink, we resort to copying the result to a temp file
+                            # in the named cache, and finally doing an atomic mv from the tempfile
+                            # to the real one.
                             #
                             # In either case, the result is an atomic replacement of the "old" named
                             # cache db, such that old references (via opened file descriptors) are
@@ -312,10 +312,13 @@ async def mypy_typecheck_partition(
                             # left the cache in an inconsistent state.
                             # See https://github.com/python/mypy/issues/6003 for exit codes
                             if [ $EXIT_CODE -le 1 ]; then
-                                if ! {ln.path} "$SANDBOX_CACHE_DB" "$NAMED_CACHE_DB" > /dev/null 2>&1; then
-                                    TMP_CACHE=$({mktemp.path} "$SANDBOX_CACHE_DB.tmp.XXXXXX")
-                                    {cp.path} "$SANDBOX_CACHE_DB" "$TMP_CACHE" > /dev/null 2>&1
-                                    {mv.path} "$TMP_CACHE" "$NAMED_CACHE_DB" > /dev/null 2>&1
+                                if LN_TMP=$({mktemp.path} -u "$NAMED_CACHE_DB.tmp.XXXXXX") &&
+                                   {ln.path} "$SANDBOX_CACHE_DB" "$LN_TMP" > /dev/null 2>&1; then
+                                    {mv.path} "$LN_TMP" "$NAMED_CACHE_DB" > /dev/null 2>&1
+                                else
+                                    CP_TMP=$({mktemp.path} "$NAMED_CACHE_DB.tmp.XXXXXX") &&
+                                        {cp.path} "$SANDBOX_CACHE_DB" "$CP_TMP" > /dev/null 2>&1 &&
+                                        {mv.path} "$CP_TMP" "$NAMED_CACHE_DB" > /dev/null 2>&1
                                 fi
                             fi