gh-145177: Bump Emscripten to 6.0.0 (#151611)

Bumps Emscripten to 6.0.0, and fixes related issues in getentropy() and umask().

Python/emscripten_syscalls.c

@@ -23,24 +23,27 @@ int __syscall_getuid32(void) {
     return __syscall_getuid32_js();
 }
 
-EM_JS(int, __syscall_umask_js, (int mask), {
-    // If we're in node and we can, call native process.umask()
+// Emscripten's syscall layer tracks the umask in SYSCALLS.currentUmask and
+// applies it itself when creating files and directories. We mount the real
+// filesystem via NODEFS, which applies proces.umask() to everything as well. To
+// avoid masking the mode twice, read and zero out process umask at startup,
+// and store it as emscripten's umask.
+EM_JS(void, __syscall_init_umask_js, (void), {
     if (ENVIRONMENT_IS_NODE) {
         try {
-            return process.umask(mask);
-        } catch(e) {
-            // oops...
-            // NodeJS docs: "In Worker threads, process.umask(mask) will throw an exception."
-            // umask docs: "This system call always succeeds"
-            return 0;
+            // process.umask(0) returns the previous umask and sets it to 0.
+            SYSCALLS.currentUmask = process.umask(0);
+        } catch (e) {
+            // NodeJS docs: "In Worker threads, process.umask(mask) will throw an
+            // exception." In that case just keep emscripten's default umask.
         }
     }
-    // Fall back to the stub case of returning 0.
-    return 0;
 })
 
-int __syscall_umask(int mask) {
-    return __syscall_umask_js(mask);
+EM_JS_DEPS(__syscall_init_umask, "$SYSCALLS");
+
+__attribute__((constructor)) void __syscall_init_umask(void) {
+    __syscall_init_umask_js();
 }
 
 #include <wasi/api.h>
@@ -290,6 +293,26 @@ int __syscall_poll(intptr_t fds, int nfds, int timeout) {
     return __block_for_int(p);
 }
 
+
+// Workaround for an Emscripten bug: getentropy(buffer, 1) returns the single
+// byte of entropy as the return code. Fixed upstream by
+// emscripten-core/emscripten#27122
+int __real_getentropy(void*, size_t);
+
+int __wrap_getentropy(void *buffer, size_t len) {
+    if (len != 1) {
+        return __real_getentropy(buffer, len);
+    }
+    // Length is 1. Workaround is to get two bytes of entropy and write the
+    // first one into the original target buffer.
+    uint8_t tmp[2];
+    int ret = __real_getentropy(tmp, 2);
+    if (ret == 0) {
+        *(uint8_t *)buffer = tmp[0];
+    }
+    return ret;
+}
+
 #include <sys/ioctl.h>
 
 int syscall_ioctl_orig(int fd, int request, void* varargs)