avoid calling dlerror unless absolutely necessary (#40392)

Most platforms implement this sensibly, but POSIX does not require that,
so FreeBSD does not implement this sensibly. But we can mostly avoid the
problems on FreeBSD by avoiding calling it unless we absolutely must.

Fixes #39582

src/dlload.c

@@ -120,7 +120,6 @@ JL_DLLEXPORT void *jl_dlopen(const char *filename, unsigned flags) JL_NOTSAFEPOI
         needsSymRefreshModuleList = 1;
     return lib;
 #else
-    dlerror(); /* Reset error status. */
     return dlopen(filename,
                   (flags & JL_RTLD_NOW ? RTLD_NOW : RTLD_LAZY)
                   | JL_RTLD(flags, LOCAL)
@@ -144,11 +143,15 @@ JL_DLLEXPORT void *jl_dlopen(const char *filename, unsigned flags) JL_NOTSAFEPOI
 JL_DLLEXPORT int jl_dlclose(void *handle) JL_NOTSAFEPOINT
 {
 #ifdef _OS_WINDOWS_
-    if (!handle) return -1;
+    if (!handle) {
+        return -1;
+    }
     return !FreeLibrary((HMODULE) handle);
 #else
-    dlerror(); /* Reset error status. */
-    if (!handle) return -1;
+    if (!handle) {
+        dlerror(); /* Reset error status. */
+        return -1;
+    }
     return dlclose(handle);
 #endif
 }
@@ -291,19 +294,26 @@ JL_DLLEXPORT int jl_dlsym(void *handle, const char *symbol, void ** value, int t
 #ifdef _OS_WINDOWS_
     *value = GetProcAddress((HMODULE) handle, symbol);
 #else
-    dlerror(); /* Reset error status. */
     *value = dlsym(handle, symbol);
 #endif
 
-    /* Next, check for errors.  On Windows, a NULL pointer means the symbol
-     * was not found.  On everything else, we can have NULL symbols, so we check
-     * for non-NULL returns from dlerror().  Note that means we unconditionally
-     * call dlerror() on POSIX systems.*/
-#ifdef _OS_WINDOWS_
+    /* Next, check for errors. On Windows, a NULL pointer means the symbol was
+     * not found. On everything else, we can have NULL symbols, so we check for
+     * non-NULL returns from dlerror(). Since POSIX doesn't require `dlerror`
+     * to be implemented safely, FreeBSD doesn't (unlike everyone else, who
+     * realized decades ago that threads are here to stay), so we avoid calling
+     * `dlerror` unless we need to get the error message.
+     * https://github.com/freebsd/freebsd-src/blob/12db51d20823a5e3b9e5f8a2ea73156fe1cbfc28/libexec/rtld-elf/rtld.c#L198
+     */
     symbol_found = *value != NULL;
-#else
-    const char *err = dlerror();
-    symbol_found = err == NULL;
+#ifndef _OS_WINDOWS_
+    const char *err;
+    if (!symbol_found) {
+        dlerror(); /* Reset error status. */
+        *value = dlsym(handle, symbol);
+        err = dlerror();
+        symbol_found = *value != NULL || err == NULL;
+    }
 #endif
 
     if (!symbol_found && throw_err) {