fix: auto-detect valgrind to skip fork tests

tests/test_file.c

@@ -271,7 +271,7 @@ void test_file() {
   test_to_disk_basic();
   test_to_disk_creates_dirs();
   test_file_content_to_buffer();
-  if (!getenv("FASTSYNC_UNDER_VALGRIND")) {
+  if (!is_running_under_valgrind()) {
     // Fork tests are skipped under valgrind because the parent process runs
     // orders of magnitude slower than the child (parent is instrumented, child
     // is not), which causes pipe-based protocol handshake timeouts. The parent

tests/test_utils.h

@@ -2,9 +2,24 @@
 #define TEST_UTILS_H
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <stdbool.h>
 
+// Detect if running under valgrind by checking /proc/self/maps for vgpreload.
+// This is used to skip fork-based tests that are incompatible with valgrind
+// (the instrumented parent runs too slowly, causing pipe timeouts).
+static inline bool is_running_under_valgrind(void) {
+  FILE* f = fopen("/proc/self/maps", "r");
+  if (!f)
+    return false;
+  char buf[4096];
+  size_t n = fread(buf, 1, sizeof(buf) - 1, f);
+  fclose(f);
+  buf[n] = '\0';
+  return strstr(buf, "vgpreload") != NULL;
+}
+
 // Global test suite status
 extern int tests_run;
 extern int tests_failed;