Make --no-allow-shlib-undefined not report an error on circular dependencies

Fixes #1445

Author: rui314

src/passes.cc

@@ -1122,12 +1122,13 @@ void check_shlib_undefined(Context<E> &ctx) {
   for (std::unique_ptr<SharedFile<E>> &file : ctx.dso_pool)
     deps[file->soname] = file->get_dt_needed(ctx);
 
-  std::function<bool(std::string_view, i64)> can_check;
-  can_check = [&](std::string_view soname, i64 depth) -> bool {
-    if (depth == deps.size() + 1)
-      Fatal(ctx) << "--no-allow-shlib-defined: mutually-recursive .so detected: "
-                 << soname;
-
+  std::function<bool(std::string_view, i64)> can_check =
+    [&](std::string_view soname, i64 depth) -> bool {
+    if (depth == deps.size() + 1) {
+      // A library with a circular dependency is rare but does exist and
+      // can be loaded without issue, so we need to handle such a case.
+      return true;
+    }
     auto it = deps.find(soname);
     if (it == deps.end())
       return false;

test/no-allow-shlib-undefined-circular.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+cat <<EOF | $CC -B. -shared -fPIC -o $t/libfoo.so -xc -
+void foo() {}
+EOF
+
+cat <<EOF | $CC -B. -shared -fPIC -o $t/libbar.so -xc - -L$t -lfoo
+void foo();
+void bar() { foo(); }
+EOF
+
+cat <<EOF | $CC -B. -shared -fPIC -o $t/libfoo.so -xc - -L$t -lbar
+void baz();
+void foo() { baz(); }
+EOF
+
+cat <<EOF | $CC -c -o $t/a.o -c -xc -
+int bar();
+int main() { bar(); }
+EOF
+
+not $CC -B. -o $t/exe1 $t/a.o -Wl,--no-allow-shlib-undefined -L$t -lfoo -lbar |&
+  grep 'libfoo.so: --no-allow-shlib-undefined: undefined symbol: baz'