[trivial] Move SIGUSR1/SIGUSR2 to SIGRT for GC

changelog/posix_gc_signals.dd

@@ -0,0 +1,8 @@
+Posix (excl. Darwin): Switch default GC signals from SIGUSR1/2 to SIGRTMIN/SIGRTMIN+1
+
+As the SIGUSR ones might be used by 'system' libraries (e.g., Android
+Dalvik VM or LLVM libFuzzer), while the SIGRT ones are reserved for
+user-defined purposes and less likely to collide.
+
+The used signals can still be customized with an early call to
+`core.thread.osthread.thread_setGCSignals()`.

src/core/memory.d

@@ -38,7 +38,7 @@
  *
  * Notes_to_implementors:
  * $(UL
- * $(LI On POSIX systems, the signals SIGUSR1 and SIGUSR2 are reserved
+ * $(LI On POSIX systems, the signals `SIGRTMIN` and `SIGRTMIN + 1` are reserved
  *   by this module for use in the garbage collector implementation.
  *   Typically, they will be used to stop and resume other threads
  *   when performing a collection, but an implementation may choose

src/core/thread/osthread.d

@@ -1185,7 +1185,7 @@ version (CoreDdoc)
 {
     /**
      * Instruct the thread module, when initialized, to use a different set of
-     * signals besides SIGUSR1 and SIGUSR2 for suspension and resumption of threads.
+     * signals besides SIGRTMIN and SIGRTMIN + 1 for suspension and resumption of threads.
      * This function should be called at most once, prior to thread_init().
      * This function is Posix-only.
      */
@@ -1215,8 +1215,8 @@ else version (Posix)
 
 version (Posix)
 {
-    private __gshared int suspendSignalNumber = SIGUSR1;
-    private __gshared int resumeSignalNumber  = SIGUSR2;
+    private __gshared int suspendSignalNumber;
+    private __gshared int resumeSignalNumber;
 }
 
 private extern (D) ThreadBase attachThread(ThreadBase _thisThread) @nogc nothrow
@@ -1937,11 +1937,6 @@ extern (C) void thread_init() @nogc
     initLowlevelThreads();
     Thread.initLocks();
 
-    // The Android VM runtime intercepts SIGUSR1 and apparently doesn't allow
-    // its signal handler to run, so swap the two signals on Android, since
-    // thread_resumeHandler does nothing.
-    version (Android) thread_setGCSignals(SIGUSR2, SIGUSR1);
-
     version (Darwin)
     {
         // thread id different in forked child process
@@ -1957,6 +1952,16 @@ extern (C) void thread_init() @nogc
     }
     else version (Posix)
     {
+        if ( suspendSignalNumber == 0 )
+        {
+            suspendSignalNumber = SIGRTMIN;
+        }
+
+        if ( resumeSignalNumber == 0 )
+        {
+            resumeSignalNumber = SIGRTMIN + 1;
+            assert(resumeSignalNumber <= SIGRTMAX);
+        }
         int         status;
         sigaction_t suspend = void;
         sigaction_t resume = void;