[PATCH] Pre-read DWZ file in DWARF reader

[PATCH] Pre-read DWZ file in DWARF reader

[Tom Tromey]

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,
avoidig 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                              | 19 +++++++++++++---
 gdb/dwarf2/read.c                             | 22 ++++++++++---------
 gdb/dwarf2/read.h                             |  2 +-
 gdb/testsuite/gdb.dwarf2/dwzbuildid.exp       |  7 ------
 gdb/testsuite/gdb.dwarf2/no-gnu-debuglink.exp |  4 ++--
 5 files changed, 31 insertions(+), 23 deletions(-)

diff --git a/gdb/dwarf2/dwz.c b/gdb/dwarf2/dwz.c
index bd2bcb91d83..edd5b56d778 100644
--- a/gdb/dwarf2/dwz.c
+++ b/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 ();
 }
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index ea0b2328a3e..9a8a48fb016 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -3387,6 +3387,17 @@ 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
@@ -5294,16 +5305,7 @@ 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.  */
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index dc7abf23ba4..68c16ba3c6e 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -524,7 +524,7 @@ struct dwarf2_per_bfd
 
   /* 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;
diff --git a/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp b/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp
index 76e3a7d6e42..3814323926e 100644
--- a/gdb/testsuite/gdb.dwarf2/dwzbuildid.exp
+++ b/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
     }
diff --git a/gdb/testsuite/gdb.dwarf2/no-gnu-debuglink.exp b/gdb/testsuite/gdb.dwarf2/no-gnu-debuglink.exp
index 538004ec95a..a76136fdfd8 100644
--- a/gdb/testsuite/gdb.dwarf2/no-gnu-debuglink.exp
+++ b/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"
-- 
2.41.0

Re: [PATCH] Pre-read DWZ file in DWARF reader

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tom@tromey.com> writes:

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

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

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

I'm checking this in.

Tom