[gdb/symtab] Return correct reader for top-level CU in cooked_indexer::ensure_cu_exists

With the test-case included in this patch, we run into:
...
$ gdb -q -batch $exec
Dwarf Error: Could not find abbrev number 3 in CU at offset 0xdb \
  [in module $exec]
...

The debug info consists of two CUs:
...
  Compilation Unit @ offset 0xb2:
   Length:        0x25 (32-bit)
   Version:       4
   Abbrev Offset: 0x6c
   Pointer Size:  8
 <0><bd>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <be>   DW_AT_language    : 2	(non-ANSI C)
 <1><bf>: Abbrev Number: 2 (DW_TAG_subprogram)
    <c0>   DW_AT_low_pc      : 0x4004a7
    <c8>   DW_AT_high_pc     : 0x4004b2
    <d0>   DW_AT_specification: <0xe8>
 <1><d4>: Abbrev Number: 3 (DW_TAG_subprogram)
    <d5>   DW_AT_name        : main
 <1><da>: Abbrev Number: 0
  Compilation Unit @ offset 0xdb:
   Length:        0xf (32-bit)
   Version:       4
   Abbrev Offset: 0x86
   Pointer Size:  8
 <0><e6>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <e7>   DW_AT_language    : 2	(non-ANSI C)
 <1><e8>: Abbrev Number: 2 (DW_TAG_subprogram)
    <e9>   DW_AT_specification: <0xd4>
 <1><ed>: Abbrev Number: 0
...
where:
- DIE 0xbf in CU@0xb2 contains an inter-CU reference to
- DIE 0xe8 in CU@0xdb, which contains an inter-CU reference to
- DIE 0xd4 back in CU@0xb2.

The dwarf error is caused by this bit of code in
cooked_indexer::ensure_cu_exists:
...
  if (per_cu == m_per_cu)
    return reader;
...

The dwarf error happens as follows:
- a cutu_reader A is created for CU@0xb2
- using cutu_reader A, the cooked index reader starts indexing dies, with
  m_per_cu set to CU@0xb2
- while indexing it scans the attributes of DIE 0xbf and encounters the
  inter-CU reference to DIE 0xe8
- it calls cooked_indexer::ensure_cu_exists, which creates a cutu_reader B for
  CU@0xdb and returns it
- using cutu_reader B, it continues scanning attributes of DIE 0xe8 and
  encounters the inter-CU reference to DIE 0xd4
- it calls cooked_indexer::ensure_cu_exists, the problematic bit is triggered
  and cutu_reader B is returned
- using cutu_reader B, it continues scanning attributes of DIE 0xd4
- this goes wrong because:
  - the attributes of the DIE are encoded using the abbreviation table at
    offset 0x6c, while
  - the decoding is done using cutu_reader B which uses the abbreviation table
    at offset 0x86.

Fix this by removing the problematic if clause.

Since cutu_reader A is not preserved in m_index_storage,
cooked_indexer::ensure_cu_exists cannot find it there and creates a duplicate
cutu_reader C for CU@0xb2.  Fix this in process_psymtab_comp_unit by preserving
the cutu_reader A as well in m_index_storage.

Tested on x86_64-linux and aarch64-linux.

PR symtab/32081
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32081

Approved-By: Tom Tromey <tom@tromey.com>
Reported-By: Andreas Schwab <schwab@linux-m68k.org>

2 files changed, 80 insertions, 15 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index f9f34fd..04a5398 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -4532,28 +4532,35 @@ process_psymtab_comp_unit (dwarf2_per_cu_data *this_cu,
 			   dwarf2_per_objfile *per_objfile,
 			   cooked_index_storage *storage)
 {
-  cutu_reader reader (this_cu, per_objfile, nullptr, nullptr, false,
-		      storage->get_abbrev_cache ());
+  cutu_reader *reader = storage->get_reader (this_cu);
+  if (reader == nullptr)
+    {
+      cutu_reader new_reader (this_cu, per_objfile, nullptr, nullptr, false,
+			      storage->get_abbrev_cache ());
 
-  if (reader.comp_unit_die == nullptr)
-    return;
+      if (new_reader.comp_unit_die == nullptr || new_reader.dummy_p)
+	return;
 
-  if (reader.dummy_p)
-    {
-      /* Nothing.  */
+      std::unique_ptr<cutu_reader> copy
+	(new cutu_reader (std::move (new_reader)));
+      reader = storage->preserve (std::move (copy));
     }
-  else if (this_cu->is_debug_types)
-    build_type_psymtabs_reader (&reader, storage);
-  else if (reader.comp_unit_die->tag != DW_TAG_partial_unit)
+
+  if (reader->comp_unit_die == nullptr || reader->dummy_p)
+    return;
+
+  if (this_cu->is_debug_types)
+    build_type_psymtabs_reader (reader, storage);
+  else if (reader->comp_unit_die->tag != DW_TAG_partial_unit)
     {
       bool nope = false;
       if (this_cu->scanned.compare_exchange_strong (nope, true))
 	{
-	  prepare_one_comp_unit (reader.cu, reader.comp_unit_die,
+	  prepare_one_comp_unit (reader->cu, reader->comp_unit_die,
 				 language_minimal);
 	  gdb_assert (storage != nullptr);
-	  cooked_indexer indexer (storage, this_cu, reader.cu->lang ());
-	  indexer.make_index (&reader);
+	  cooked_indexer indexer (storage, this_cu, reader->cu->lang ());
+	  indexer.make_index (reader);
 	}
     }
 }
@@ -16030,8 +16037,6 @@ cooked_indexer::ensure_cu_exists (cutu_reader *reader,
       if (!per_cu->scanned.compare_exchange_strong (nope, true))
 	return nullptr;
     }
-  if (per_cu == m_per_cu)
-    return reader;
 
   cutu_reader *result = m_index_storage->get_reader (per_cu);
   if (result == nullptr)
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-forth-and-back.exp b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-forth-and-back.exp
new file mode 100644
index 0000000..62674bd
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-forth-and-back.exp
@@ -0,0 +1,60 @@
+# Copyright 2024 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that the cooked index reader can handle inter-CU references:
+# - DIE1@CU1 -> DIE2@CU2
+# - DIE2@CU2 -> DIE3@CU1.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile main.c .S
+
+# Create the DWARF.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels label1 label2
+
+    cu {} {
+	compile_unit {{language @DW_LANG_C}} {
+	    subprogram {
+		{MACRO_AT_range { main }}
+		{DW_AT_specification %$label1}
+	    }
+
+	    label2: subprogram {
+		{DW_AT_name main}
+	    }
+	}
+    }
+
+    cu {} {
+	compile_unit {{language @DW_LANG_C}} {
+	    label1: subprogram {
+		{DW_AT_specification %$label2}
+	    }
+	}
+    }
+}
+
+if [prepare_for_testing "failed to prepare" $testfile \
+	[list $asm_file $srcfile] {nodebug}] {
+    return -1
+}
+
+# Regression test for PR32081.
+gdb_assert { ![regexp -nocase "error:" $gdb_file_cmd_msg] } "No Error message"