[musl] Memory leak issue in multi-threaded program

[Leesoo Ahn <yisooan@davolink.co.kr>]

Dear musl developers,

Hello!, it seems that musl currently has a memory leak issue in 
multi-threaded program. It occurs in the below situation of latest 
(v1.1.24) source. Also, not only in 32-bits[1], but also 64-bits[2] as 
well.

When a program create and run, at least, two threads or more with 
pthread APIs, VSZ of the program by ps command keeps increasing. But 
here is a weird thing that it is fine 'IF ONLY ONE' pthread is created 
and run.

To confirm the issue in your host machine, please follow the instructions,

0. Clone the musl git and get inside.
1. Build with these options for static build, ./configure 
--prefix=$(pwd)/_build_dir --disable-shared
2. Download the test code[3], then build with the command, 
./_build_dir/bin/musl-gcc ./test.c
3. Run this script, ./a.out &; while [ 1 ]; do { ps aux | grep [a].out | 
grep -v grep; sleep 1; } done

You may figure out that VSZ keeps increasing.

BUT, when I make it to try to allocate memory all the time by kernel 
mmap with this diff[4] as workaround, although it creates more pthreads 
than 2, the issue never happens.

It would be really thankful if you guys could confirm it and find out 
the way to fix the bug.

Thank you in advance and take care.

Best Regards,
Leesoo

----
[1] 32-bits env: https://pastebin.com/xR4PySaM
[2] 64-bits env: https://pastebin.com/stdVQXdE
[3] test code: https://pastebin.com/0s8nmdUv
[4] workaround patch:

diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index 9698259..3d39be7 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -288,7 +288,11 @@ void *malloc(size_t n)

  	if (adjust_size(&n) < 0) return 0;

+#if 1
+	if ( 1 ) {
+#else
  	if (n > MMAP_THRESHOLD) {
+#endif
  		size_t len = n + OVERHEAD + PAGE_SIZE - 1 & -PAGE_SIZE;
  		char *base = __mmap(0, len, PROT_READ|PROT_WRITE,
  			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);