Prevent setting infinite loopback forwards

Some libraries, such as `libLAPACK` re-export their imported symbols,
presumably in an effort to make it easy for users to link against just
`libLAPACK` and get the BLAS symbols for free.  However, when LAPACK_jll
has linked against LBT as its backing BLAS library, we end up with an
infinite loop, as LBT then sets its BLAS symbols to its own trampoline.

This commit adds a special-case to our forwarding logic which explicitly
checks for such a loop, and silently refuses to perform such a forward.
This has the happy effect of filtering out direct loops, and makes it
much easier to mix and match libraries that themselves depend on LBT.

Author: staticfloat

src/libblastrampoline.c

@@ -355,7 +355,9 @@ LBT_DLLEXPORT int32_t lbt_forward(const char * libname, int32_t clear, int32_t v
         // Look up this symbol in the given library, if it is a valid symbol, set it!
         build_symbol_name(symbol_name, exported_func_names[symbol_idx], lib_suffix);
         void *addr = lookup_symbol(handle, symbol_name);
-        if (addr != NULL) {
+        void *self_symbol_addr = interface == LBT_INTERFACE_ILP64 ? exported_func64[symbol_idx] \
+                                                                  : exported_func32[symbol_idx];
+        if (addr != NULL && addr != self_symbol_addr) {
             set_forward_by_index(symbol_idx,  addr, interface, complex_retstyle, f2c, verbose);
             BITFIELD_SET(forwards, symbol_idx);
             nforwards++;
@@ -443,6 +445,17 @@ __attribute__((constructor)) void init(void) {
         default_func = lookup_self_symbol("lbt_default_func_print_error_and_exit");
     }
 
+    // Build our lists of self-symbol addresses
+    int32_t symbol_idx;
+    char symbol_name[MAX_SYMBOL_LEN];
+    for (symbol_idx=0; exported_func_names[symbol_idx] != NULL; ++symbol_idx) {
+        exported_func32[symbol_idx] = lookup_self_symbol(exported_func_names[symbol_idx]);
+
+        // Look up this symbol in the given library, if it is a valid symbol, set it!
+        build_symbol_name(symbol_name, exported_func_names[symbol_idx], "64_");
+        exported_func64[symbol_idx] = lookup_self_symbol(symbol_name);
+    }
+
     // LBT_DEFAULT_LIBS is a semicolon-separated list of paths that should be loaded as BLAS libraries.
     // Each library can have a `!suffix` tacked onto the end of it, providing a library-specific
     // suffix_hint.  Example:

src/libblastrampoline_trampdata.h

@@ -32,4 +32,20 @@ const void ** exported_func64_addrs[] = {
     NULL
 };
 #undef XX
-#undef XX_64
+#undef XX_64
+
+// Generate list of our own function addresses, so that we can filter
+// out libraries that link against us (such as LAPACK_jll) so that we
+// don't accidentally loop back to ourselves.
+#define XX(name)    NULL,
+#define XX_64(name) NULL,
+const void ** exported_func32[] = {
+    EXPORTED_FUNCS(XX)
+    NULL
+};
+const void ** exported_func64[] = {
+    EXPORTED_FUNCS(XX_64)
+    NULL
+};
+#undef XX
+#undef XX_64