Make static buffers in numToString thread local (#4134)

Validation is performed on multiple threads at once and when there are multiple
validation failures, those threads can all end up in `numToString` at the same
time as they construct their respective error messages. Previously the threads
would race on their access to the snprintf buffers, sometimes leading to
segfaults. Fix the data races by making the buffers thread local.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/emscripten-optimizer/simple_ast.h b/src/emscripten-optimizer/simple_ast.h
index 70ecb2acd..7644daac3 100644
--- a/src/emscripten-optimizer/simple_ast.h
+++ b/src/emscripten-optimizer/simple_ast.h
@@ -947,17 +947,19 @@ struct JSPrinter {
     bool integer = wasm::isInteger(d);
 #define BUFFERSIZE 1000
     // f is normal, e is scientific for float, x for integer
-    static char full_storage_f[BUFFERSIZE], full_storage_e[BUFFERSIZE];
+    // These need to be thread-local because they are returned.
+    thread_local char full_storage_f[BUFFERSIZE];
+    thread_local char full_storage_e[BUFFERSIZE];
     // full has one more char, for a possible '-'
-    static char *storage_f = full_storage_f + 1,
-                *storage_e = full_storage_e + 1;
+    char* storage_f = full_storage_f + 1;
+    char* storage_e = full_storage_e + 1;
     auto err_f = std::numeric_limits<double>::quiet_NaN();
     auto err_e = std::numeric_limits<double>::quiet_NaN();
     for (int e = 0; e <= 1; e++) {
       char* buffer = e ? storage_e : storage_f;
       double temp;
       if (!integer) {
-        static char format[6];
+        char format[6];
         for (int i = 0; i <= 18; i++) {
           format[0] = '%';
           format[1] = '.';