Fall Back When a Replace Is Refused and Sweep Only Old Temporaries

Some mounts refuse to replace an existing file in one rename, and the
old remove-then-rename worked there where the atomic path now fails, so
rename_file() takes the two-step route when a one-step replace is
refused and write_file_atomically() writes in place whenever the rename
fails, not only for the Windows sharing case.

An instance that gave up waiting for the lock writes unlocked by design,
so a peer holding the lock could sweep its in-flight temporary and make
its rename fail; only temporaries older than ten minutes are removed now,
and the sweep uses the error-code overloads so an entry vanishing between
listing and stat cannot throw out of startup. The registry mutex is
leaked like the map it guards, so a save during static destruction does
not lock a destroyed mutex.

The physical printer loader is never called, so its guard is gone, while
the two delete paths that do run now hold the lock. The bundle metadata
loader is lock-free again, since the zip import reads it from a scratch
folder; the guard sits at the two scans that read the user's bundles.
Preset::save() builds what it writes before taking the lock, the cache
writer reserves its payload, and the retry test tolerates a slow runner.

tests/libslic3r/test_utils.cpp

@@ -8,6 +8,7 @@
 
 #include <algorithm>
 #include <cctype>
+#include <ctime>
 #include <fstream>
 #include <string>
 #include <system_error>
@@ -121,10 +122,15 @@ TEST_CASE("write_file_atomically writes through a symlink and keeps the target's
 }
 #endif
 
-TEST_CASE("remove_stale_temp_files removes only <name>.<pid>.tmp files", "[utils]") {
+TEST_CASE("remove_stale_temp_files removes only old <name>.<pid>.tmp files", "[utils]") {
     ScopedTemporaryDir dir;
-    for (const char *name : { "a.json.123.tmp", "b.info.4.tmp", "c.json", "d.tmp", "e.json.x.tmp", "f.json..tmp" })
+    for (const char *name : { "a.json.123.tmp", "b.info.4.tmp", "c.json", "d.tmp", "e.json.x.tmp", "f.json..tmp" }) {
         REQUIRE_FALSE(write_file_atomically((dir.path() / name).string(), "x"));
+        // An hour old: long past the age below which a temporary may still be in flight.
+        boost::filesystem::last_write_time(dir.path() / name, std::time(nullptr) - 3600);
+    }
+    // Just written: possibly another instance's in-flight save, so it stays.
+    REQUIRE_FALSE(write_file_atomically((dir.path() / "g.json.7.tmp").string(), "x"));
 
     SECTION("with a name prefix only matching names go") {
         REQUIRE(remove_stale_temp_files(dir.path(), "a.json") == 1);
@@ -138,7 +144,8 @@ TEST_CASE("remove_stale_temp_files removes only <name>.<pid>.tmp files", "[utils
             (void) entry;
             ++entries;
         }
-        REQUIRE(entries == 4);
+        REQUIRE(entries == 5);
+        REQUIRE(boost::filesystem::exists(dir.path() / "g.json.7.tmp"));
     }
 }