Pre-read DWZ file in DWARF reader

While working on background reading of DWARF, I came across the
DWZ-reading code.  This code can query the user (via the debuginfod
support) -- something that cannot be done off the main thread.

Looking into it, I realized that this code can be run much earlier,
avoiding this problem.  Digging a bit deeper, I also found a
discrepancy here between how the DWARF reader works in "readnow" mode
as compared to the normal modes.

This patch cleans this up by trying to read the DWZ file earlier, and
also by having the DWARF reader convert any exception here into a
warning.  This unifies the various cases, but also makes it so that
errors do not prevent gdb from continuing on to the extent possible.

Regression tested on x86-64 Fedora 38.

gdb/dwarf2/dwz.c

@@ -29,6 +29,7 @@
 #include "gdbcore.h"
 #include "gdbsupport/pathstuff.h"
 #include "gdbsupport/scoped_fd.h"
+#include "run-on-main-thread.h"
 
 const char *
 dwz_file::read_string (struct objfile *objfile, LONGEST str_offset)
@@ -196,8 +197,20 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd, bool require)
   size_t buildid_len;
   bfd_byte *buildid;
 
-  if (per_bfd->dwz_file != NULL)
-    return per_bfd->dwz_file.get ();
+  if (per_bfd->dwz_file.has_value ())
+    {
+      dwz_file *result = per_bfd->dwz_file->get ();
+      if (require && result == nullptr)
+	error (_("could not read '.gnu_debugaltlink' section"));
+      return result;
+    }
+
+  /* This may query the user via the debuginfod support, so it may
+     only be run in the main thread.  */
+  gdb_assert (is_main_thread ());
+
+  /* Set this early, so that on error it remains NULL.  */
+  per_bfd->dwz_file.emplace (nullptr);
 
   bfd_set_error (bfd_error_no_error);
   gdb::unique_xmalloc_ptr<char> data
@@ -283,5 +296,5 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd, bool require)
 
   gdb_bfd_record_inclusion (per_bfd->obfd, result->dwz_bfd.get ());
   per_bfd->dwz_file = std::move (result);
-  return per_bfd->dwz_file.get ();
+  return per_bfd->dwz_file->get ();
 }

gdb/dwarf2/read.c

@@ -3213,6 +3213,17 @@ dwarf2_initialize_objfile (struct objfile *objfile)
 
   dwarf_read_debug_printf ("called");
 
+  /* Try to fetch any potential dwz file early, while still on the
+     main thread.  */
+  try
+    {
+      dwarf2_get_dwz_file (per_bfd);
+    }
+  catch (const gdb_exception_error &err)
+    {
+      warning (_("%s"), err.what ());
+    }
+
   /* If we're about to read full symbols, don't bother with the
      indices.  In this case we also don't care if some other debug
      format is making psymtabs, because they are all about to be
@@ -5120,16 +5131,7 @@ create_all_units (dwarf2_per_objfile *per_objfile)
 				  &per_objfile->per_bfd->abbrev, 0,
 				  types_htab, rcuh_kind::TYPE);
 
-  dwz_file *dwz;
-  try
-    {
-      dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
-    }
-  catch (const gdb_exception_error &)
-    {
-      per_objfile->per_bfd->all_units.clear ();
-      throw;
-    }
+  dwz_file *dwz = dwarf2_get_dwz_file (per_objfile->per_bfd);
   if (dwz != NULL)
     {
       /* Pre-read the sections we'll need to construct an index.  */

gdb/dwarf2/read.h

@@ -524,7 +524,7 @@ public:
 
   /* The shared '.dwz' file, if one exists.  This is used when the
      original data was compressed using 'dwz -m'.  */
-  std::unique_ptr<struct dwz_file> dwz_file;
+  gdb::optional<std::unique_ptr<struct dwz_file>> dwz_file;
 
   /* Whether copy relocations are supported by this object format.  */
   bool can_copy;

gdb/testsuite/gdb.dwarf2/dwzbuildid.exp

@@ -142,13 +142,6 @@ proc do_test {} {
 
     gdb_load ${::binfile}-${::testname}
 
-    if { $::testname == "mismatch" && [readnow] } {
-	# Main is found in the minimal symbols.  When using readnow, a
-	# failure to read the dwarf also causes the minimal symbols to be
-	# unavailable.
-	# Setup a kfail for "FAIL: gdb_breakpoint: set breakpoint at main".
-	setup_kfail "symtab/26797" *-*-*
-    }
     if {![runto_main]} {
 	return
     }

gdb/testsuite/gdb.dwarf2/no-gnu-debuglink.exp

@@ -37,8 +37,8 @@ if { [build_executable $testfile.exp $testfile [list $srcfile $asm_file]] } {
 
 clean_restart
 
-set msg "\r\ncould not find '\.gnu_debugaltlink' file for \[^\r\n\]*"
+set msg "\r\nwarning: could not find '\.gnu_debugaltlink' file for \[^\r\n\]*"
 gdb_test "file $binfile" "$msg" "file command"
 
 set question "Load new symbol table from .*\? .y or n. "
-gdb_test "file $binfile" "$msg" "file command, again" $question "y"
+gdb_test "file $binfile" "" "file command, again" $question "y"