Look for build-id-based separate debug files under the sysroot

When looking for a separate debug file that matches a given build-id,
GDB only looks in the host's debug dir (typically /usr/lib/debug).  This
patch makes it look in the sysroot as well.  This is to match the
behavior of GDB when using debuglink-based separate debug files,
introduced in :

402d2bfec42 ("Look for separate debug files in debug directories under a sysroot.")

In the following example, my sysroot is "/tmp/sysroot" and I am trying
to load symbols for
/tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so.  This is
the current behavior:

    (gdb) file /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
    Reading symbols from /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so...

    Looking for separate debug info (build-id) for /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
      Trying /usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... no, unable to compute real path

    <snip>
    (No debugging symbols found in /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so)

With this patch:

    (gdb) file /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
    Reading symbols from /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so...

    Looking for separate debug info (build-id) for /tmp/sysroot/usr/lib/arm-linux-gnueabihf/gconv/EBCDIC-AT-DE.so
      Trying /usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... no, unable to compute real path
      Trying /tmp/sysroot/usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug... yes!
    Reading symbols from /tmp/sysroot/usr/lib/debug/.build-id/f3/d6594d2600e985812cd4ba2ad083ac2aceae22.debug...

In the original code, there is a suspicious "abfd.release ()" in
build_id_to_debug_bfd, that I don't understand.  If a file with the
right name exists but its build-id note doesn't match, we release (leak)
our reference, meaning the file will stay open?  I removed it in the new
code, so that the reference is dropped if we end up not using that file.
I tested briefly by corrupting a separate debug file to trigger this
code, nothing exploded.

gdb/ChangeLog:

	* build-id.c (build_id_to_debug_bfd_1): New function.
	(build_id_to_debug_bfd): Look for separate debug file in
	sysroot.

2 files changed, 78 insertions, 43 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1856f48..6a79744 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2019-02-22  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* build-id.c (build_id_to_debug_bfd_1): New function.
+	(build_id_to_debug_bfd): Look for separate debug file in
+	sysroot.
+
 2019-02-22  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdbarch.sh: Update the copyright year range that is placed into
diff --git a/gdb/build-id.c b/gdb/build-id.c
index e0c3557..27f29cd 100644
--- a/gdb/build-id.c
+++ b/gdb/build-id.c
@@ -65,13 +65,63 @@ build_id_verify (bfd *abfd, size_t check_len, const bfd_byte *check)
   return retval;
 }
 
+/* Helper for build_id_to_debug_bfd.  LINK is a path to a potential
+   build-id-based separate debug file, potentially a symlink to the real file.
+   If the file exists and matches BUILD_ID, return a BFD reference to it.  */
+
+static gdb_bfd_ref_ptr
+build_id_to_debug_bfd_1 (const std::string &link, size_t build_id_len,
+			 const bfd_byte *build_id)
+{
+  if (separate_debug_file_debug)
+    {
+      printf_unfiltered (_("  Trying %s..."), link.c_str ());
+      gdb_flush (gdb_stdout);
+    }
+
+  /* lrealpath() is expensive even for the usually non-existent files.  */
+  gdb::unique_xmalloc_ptr<char> filename;
+  if (access (link.c_str (), F_OK) == 0)
+    filename.reset (lrealpath (link.c_str ()));
+
+  if (filename == NULL)
+    {
+      if (separate_debug_file_debug)
+	printf_unfiltered (_(" no, unable to compute real path\n"));
+
+      return {};
+    }
+
+  /* We expect to be silent on the non-existing files.  */
+  gdb_bfd_ref_ptr debug_bfd = gdb_bfd_open (filename.get (), gnutarget, -1);
+
+  if (debug_bfd == NULL)
+    {
+      if (separate_debug_file_debug)
+	printf_unfiltered (_(" no, unable to open.\n"));
+
+      return {};
+    }
+
+  if (!build_id_verify (debug_bfd.get(), build_id_len, build_id))
+    {
+      if (separate_debug_file_debug)
+	printf_unfiltered (_(" no, build-id does not match.\n"));
+
+      return {};
+    }
+
+  if (separate_debug_file_debug)
+    printf_unfiltered (_(" yes!\n"));
+
+  return debug_bfd;
+}
+
 /* See build-id.h.  */
 
 gdb_bfd_ref_ptr
 build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
 {
-  gdb_bfd_ref_ptr abfd;
-
   /* Keep backward compatibility so that DEBUG_FILE_DIRECTORY being "" will
      cause "/.build-id/..." lookups.  */
 
@@ -83,6 +133,10 @@ build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
       const gdb_byte *data = build_id;
       size_t size = build_id_len;
 
+      /* Compute where the file named after the build-id would be.
+
+	 If debugdir is "/usr/lib/debug" and the build-id is abcdef, this will
+         give "/usr/lib/debug/.build-id/ab/cdef.debug".  */
       std::string link = debugdir.get ();
       link += "/.build-id/";
 
@@ -97,53 +151,28 @@ build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
 
       link += ".debug";
 
-      if (separate_debug_file_debug)
-	{
-	  printf_unfiltered (_("  Trying %s..."), link.c_str ());
-	  gdb_flush (gdb_stdout);
-	}
+      gdb_bfd_ref_ptr debug_bfd
+	= build_id_to_debug_bfd_1 (link, build_id_len, build_id);
+      if (debug_bfd != NULL)
+	return debug_bfd;
 
-      /* lrealpath() is expensive even for the usually non-existent files.  */
-      gdb::unique_xmalloc_ptr<char> filename;
-      if (access (link.c_str (), F_OK) == 0)
-	filename.reset (lrealpath (link.c_str ()));
+      /* Try to look under the sysroot as well.  If the sysroot is
+         "/the/sysroot", it will give
+         "/the/sysroot/usr/lib/debug/.build-id/ab/cdef.debug".
 
-      if (filename == NULL)
+         Don't do it if the sysroot is the target system ("target:").  It
+         could work in theory, but the lrealpath in build_id_to_debug_bfd_1
+         only works with local paths.  */
+      if (strcmp (gdb_sysroot, TARGET_SYSROOT_PREFIX) != 0)
 	{
-	  if (separate_debug_file_debug)
-	    printf_unfiltered (_(" no, unable to compute real path\n"));
-
-	  continue;
+	  link = gdb_sysroot + link;
+	  debug_bfd = build_id_to_debug_bfd_1 (link, build_id_len, build_id);
+	  if (debug_bfd != NULL)
+	    return debug_bfd;
 	}
-
-      /* We expect to be silent on the non-existing files.  */
-      abfd = gdb_bfd_open (filename.get (), gnutarget, -1);
-
-      if (abfd == NULL)
-	{
-	  if (separate_debug_file_debug)
-	    printf_unfiltered (_(" no, unable to open.\n"));
-
-	  continue;
-	}
-
-      if (build_id_verify (abfd.get(), build_id_len, build_id))
-	{
-	  if (separate_debug_file_debug)
-	    printf_unfiltered (_(" yes!\n"));
-
-	  break;
-	}
-      else
-	{
-	  if (separate_debug_file_debug)
-	    printf_unfiltered (_(" no, build-id does not match.\n"));
-	}
-
-      abfd.release ();
     }
 
-  return abfd;
+  return {};
 }
 
 /* See build-id.h.  */