[PATCH] libdwfl: Handle debugaltlink in dwfl_standard_find_debuginfo.

[Mark Wielaard <mark@klomp.org>]

When we fall back to the debuginfod client then we need to do the
same trick we do for local lookups in dwfl_build_id_find_debuginfo.
If the debug file (dw) is already set, then we must be looking for
the altfile. But we cannot use the actual file/path name given as
hint. We'll have to lookup the alt file "build-id". Because the
debuginfod client only handles build-ids.

Previously we would use the build-id of the main file which meant
the debuginfod client would give us another copy of the debug file,
which would then be set as its own altfile. This caused lots of
confusion...

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libdwfl/ChangeLog        |  5 +++++
 libdwfl/find-debuginfo.c | 23 +++++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index daedaed8..3f9cd665 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,8 @@
+2020-04-30  Mark Wielaard  <mark@klomp.org>
+
+	* find-debuginfo.c (dwfl_standard_find_debuginfo): When mod->dw
+	is already set then try fetching debugaltlink.
+
 2020-04-25  Mark Wielaard  <mark@klomp.org>
 
 	* gzip.c (open_stream): Return DWFL_E_NOMEM instead of calling
diff --git a/libdwfl/find-debuginfo.c b/libdwfl/find-debuginfo.c
index 2dd11c48..4cfd0b8b 100644
--- a/libdwfl/find-debuginfo.c
+++ b/libdwfl/find-debuginfo.c
@@ -398,8 +398,27 @@ dwfl_standard_find_debuginfo (Dwfl_Module *mod,
       free (canon);
     }
 
-  if (fd < 0 && bits_len > 0)
-    fd = __libdwfl_debuginfod_find_debuginfo (mod->dwfl, bits, bits_len);
+  /* Still nothing? Try if we can use the debuginfod client.
+     But note that we might be looking for the alt file.
+     We use the same trick as dwfl_build_id_find_debuginfo.
+     If the debug file (dw) is already set, then we must be
+     looking for the altfile. But we cannot use the actual
+     file/path name given as hint. We'll have to lookup the
+     alt file "build-id". Because the debuginfod client only
+     handles build-ids.  */
+  if (fd < 0)
+    {
+      if (mod->dw != NULL)
+	{
+	  const char *altname;
+	  bits_len = INTUSE(dwelf_dwarf_gnu_debugaltlink) (mod->dw, &altname,
+							   (const void **)
+							   &bits);
+	}
+
+      if (bits_len > 0)
+	fd = __libdwfl_debuginfod_find_debuginfo (mod->dwfl, bits, bits_len);
+    }
 
   return fd;
 }
-- 
2.18.2