jemalloc-sys: don't enable background threads on unsupported targets

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Unreleased
 
+- tikv-jemalloc-sys: the `background_threads` feature no longer puts
+  `background_thread:true` into the default `malloc_conf` on targets that
+  don't support background threads, which now include Apple targets. jemalloc
+  rejected the option there, its initialization never completed, and every
+  allocation took a global lock.
 - tikv-jemalloc-sys: expose the jemalloc 5.4.0 extent-allocation hook flags
   `EXTENT_ALLOC_FLAG_PINNED` and `EXTENT_ALLOC_FLAG_MASK`, documenting the
   low-bit protocol of the pointer returned from `extent_alloc_t` hooks.

jemalloc-sys/README.md

@@ -94,7 +94,8 @@ This crate provides following cargo feature flags:
   process will be disabled regardless the state in parent process. See
   `stats.background_thread` for related stats. `opt.background_thread` can be
   used to set the default option. The background thread is only available on
-  selected pthread-based platforms.
+  selected pthread-based platforms; on targets where it is not supported (Apple
+  targets) this feature has no effect.
 
 * `unprefixed_malloc_on_supported_platforms`: when disabled, configure
   `jemalloc` with `--with-jemalloc-prefix=_rjem_`. Enabling this causes symbols

jemalloc-sys/build.rs

@@ -509,6 +509,18 @@ impl BackgroundThreadSupport {
                 "`background_threads_runtime_support` not supported for `{}`",
                 target
             );
+            if always_enabled {
+                // jemalloc rejects `background_thread:true` on these targets
+                // and its initialization never completes, so don't put it in
+                // the default `malloc_conf`.
+                warning!(
+                    "`background_threads` not supported for `{}`; not enabling background threads by default",
+                    target
+                );
+            }
+            return Some(Self {
+                always_enabled: false,
+            });
         }
 
         Some(Self { always_enabled })

jemalloc-sys/src/env.rs

@@ -11,8 +11,9 @@ pub static UNSUPPORTED_TARGETS: &[&str] = &[
 /// `jemalloc-sys` is not tested on these targets in CI:
 pub static UNTESTED_TARGETS: &[&str] = &["openbsd", "msvc"];
 
-/// `jemalloc`'s background_thread support is known not to work on these targets:
-pub static NO_BG_THREAD_TARGETS: &[&str] = &["musl"];
+/// `jemalloc`'s background_thread support is known not to work on these targets
+/// (jemalloc's `configure` never enables it on Apple targets):
+pub static NO_BG_THREAD_TARGETS: &[&str] = &["apple"];
 
 /// targets that don't support unprefixed `malloc`
 // “it was found that the `realpath` function in libc would allocate with libc malloc

jemallocator/tests/background_thread_defaults.rs

@@ -19,5 +19,10 @@ fn background_threads_runtime_defaults() {
         return;
     }
 
-    assert_eq!(background_threads(), cfg!(feature = "background_threads"));
+    // `background_threads` is ignored on targets in `NO_BG_THREAD_TARGETS`.
+    let supported = !cfg!(target_vendor = "apple");
+    assert_eq!(
+        background_threads(),
+        cfg!(feature = "background_threads") && supported
+    );
 }

jemallocator/tests/background_thread_enabled.rs

@@ -2,7 +2,6 @@
 //! library was compiled with background thread run-time support.
 #![cfg(feature = "background_threads_runtime_support")]
 #![cfg(not(feature = "unprefixed_malloc_on_supported_platforms"))]
-#![cfg(not(target_env = "musl"))]
 
 use tikv_jemallocator::Jemalloc;