[gdb] Fix missing symtab includes

[ The test-case requires commit c1a66c0629 "[gdb] Expand symbolless symtabs
using maint expand-symtabs". ]

Consider the debug info for the test-case included in this patch.  It consists
of a PU:
...
 <0><d2>: Abbrev Number: 2 (DW_TAG_partial_unit)
 <1><d3>: Abbrev Number: 0
...
imported by a CU:
...
 <0><df>: Abbrev Number: 2 (DW_TAG_compile_unit)
    <e0>   DW_AT_language    : 2        (non-ANSI C)
    <e1>   DW_AT_stmt_list   : 0xe9
 <1><e5>: Abbrev Number: 3 (DW_TAG_imported_unit)
    <e6>   DW_AT_import      : <0xd2>   [Abbrev Number: 2]
 <1><ea>: Abbrev Number: 0
...
and the CU has a dw2-symtab-includes.h file in the .debug_line file name
table:
...
 The Directory Table (offset 0x101):
  1     /data/gdb_versions/devel/src/gdb/testsuite/gdb.dwarf2

 The File Name Table (offset 0x138):
  Entry Dir     Time    Size    Name
  1     1       0       0       dw2-symtab-includes.h
...

After expanding all symtabs, we can see the CU listed in the user field of the
PU, and vice-versa the PU listed in the includes of the CU:
...
$ gdb.sh -batch \
  -iex "set language c" \
  outputs/gdb.dwarf2/dw2-symtab-includes/dw2-symtab-includes \
  -ex "maint expand-symtabs" \
  -ex "maint info symtabs"
  ...
  { ((struct compunit_symtab *) 0x394dd60)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x394dea0)
    user ((struct compunit_symtab *) 0x394dba0)
  }
  { ((struct compunit_symtab *) 0x394dba0)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x394dd10)
    user ((struct compunit_symtab *) (null))
    ( includes
      ((struct compunit_symtab *) 0x394dd60)
    )
  }
...

But if we instead only expand the symtab for the dw2-symtab-includes.h file,
the includes and user links are gone:
...
$ gdb -batch \
  -iex "set language c" \
  outputs/gdb.dwarf2/dw2-symtab-includes/dw2-symtab-includes \
  -ex "maint expand-symtabs dw2-symtab-includes.h" \
  -ex "maint info symtabs"
  ...
  { ((struct compunit_symtab *) 0x2728210)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x2728350)
    user ((struct compunit_symtab *) (null))
  }
  { ((struct compunit_symtab *) 0x2728050)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x27281c0)
    user ((struct compunit_symtab *) (null))
  }
...

The includes are calculated by process_cu_includes in gdb/dwarf2/read.c.

In the case of expanding all symtabs:
- the CU partial symtab is expanded using psymtab_to_symtab
- psymtab_to_symtab calls dwarf2_psymtab::read_symtab
- dwarf2_psymtab::read_symtab calls dwarf2_psymtab::expand_psymtab
- dwarf2_psymtab::read_symtab calls process_cu_includes, and we have the
  includes

In the case of expanding the symtab for dw2-symtab-includes.h:
- the dw2-symtab-includes.h partial symtab is expanded using psymtab_to_symtab
- psymtab_to_symtab calls dwarf2_include_psymtab::read_symtab
- dwarf2_include_psymtab::read_symtab calls
  dwarf2_include_psymtab::expand_psymtab
- dwarf2_include_psymtab::expand_psymtab calls
  partial_symtab::expand_dependencies
- partial_symtab::expand_dependencies calls dwarf2_psymtab::expand_psymtab
  for the CU partial symtab
- the CU partial symtab is expanded using dwarf2_psymtab::expand_psymtab
- process_cu_includes is never called

Fix this by making sure in dwarf2_include_psymtab::read_symtab that
read_symtab is called for the CU partial symtab.

Tested on x86_64-linux, with native, and target board cc-with-dwz and
cc-with-dwz-m.

In addition, tested test-case with target boards cc-with-gdb-index.exp,
cc-with-debug-names.exp and readnow.exp.

gdb/ChangeLog:

2020-04-14  Simon Marchi  <simon.marchi@polymtl.ca>
	    Tom de Vries  <tdevries@suse.de>

	PR symtab/25718
	* psympriv.h (struct partial_symtab::read_symtab)
	(struct partial_symtab::expand_psymtab)
	(struct partial_symtab::read_dependencies): Update comments.
	* dwarf2/read.c (struct dwarf2_include_psymtab::read_symtab): Call
	read_symtab for includer.
	(struct dwarf2_include_psymtab::expand_psymtab): Assert false.
	(struct dwarf2_include_psymtab::readin_p): Call readin_p () for includer.
	(struct dwarf2_include_psymtab::m_readin): Remove.
	(struct dwarf2_include_psymtab::includer): New member function.
	(dwarf2_psymtab::expand_psymtab): Assert !readin.

gdb/testsuite/ChangeLog:

2020-04-14  Tom de Vries  <tdevries@suse.de>

	PR symtab/25718
	* gdb.dwarf2/dw2-symtab-includes.exp: New file.

5 files changed, 141 insertions, 18 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 8c9fbe3..b054372 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,18 @@
+2020-04-14  Simon Marchi  <simon.marchi@polymtl.ca>
+	    Tom de Vries  <tdevries@suse.de>
+
+	PR symtab/25718
+	* psympriv.h (struct partial_symtab::read_symtab)
+	(struct partial_symtab::expand_psymtab)
+	(struct partial_symtab::read_dependencies): Update comments.
+	* dwarf2/read.c (struct dwarf2_include_psymtab::read_symtab): Call
+	read_symtab for includer.
+	(struct dwarf2_include_psymtab::expand_psymtab): Assert false.
+	(struct dwarf2_include_psymtab::readin_p): Call readin_p () for includer.
+	(struct dwarf2_include_psymtab::m_readin): Remove.
+	(struct dwarf2_include_psymtab::includer): New member function.
+	(dwarf2_psymtab::expand_psymtab): Assert !readin.
+
 2020-04-14  Tom de Vries  <tdevries@suse.de>
 
 	PR symtab/25720
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 9fa4970..4910c9b 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -5917,22 +5917,31 @@ struct dwarf2_include_psymtab : public partial_symtab
 
   void read_symtab (struct objfile *objfile) override
   {
-    expand_psymtab (objfile);
+    /* It's an include file, no symbols to read for it.
+       Everything is in the includer symtab.  */
+
+    /* The expansion of a dwarf2_include_psymtab is just a trigger for
+       expansion of the includer psymtab.  We use the dependencies[0] field to
+       model the includer.  But if we go the regular route of calling
+       expand_psymtab here, and having expand_psymtab call expand_dependencies
+       to expand the includer, we'll only use expand_psymtab on the includer
+       (making it a non-toplevel psymtab), while if we expand the includer via
+       another path, we'll use read_symtab (making it a toplevel psymtab).
+       So, don't pretend a dwarf2_include_psymtab is an actual toplevel
+       psymtab, and trigger read_symtab on the includer here directly.  */
+    includer ()->read_symtab (objfile);
   }
 
   void expand_psymtab (struct objfile *objfile) override
   {
-    if (m_readin)
-      return;
-    /* It's an include file, no symbols to read for it.
-       Everything is in the parent symtab.  */
-    expand_dependencies (objfile);
-    m_readin = true;
+    /* This is not called by read_symtab, and should not be called by any
+       expand_dependencies.  */
+    gdb_assert (false);
   }
 
   bool readin_p () const override
   {
-    return m_readin;
+    return includer ()->readin_p ();
   }
 
   struct compunit_symtab *get_compunit_symtab () const override
@@ -5941,8 +5950,13 @@ struct dwarf2_include_psymtab : public partial_symtab
   }
 
 private:
-
-  bool m_readin = false;
+  partial_symtab *includer () const
+  {
+    /* An include psymtab has exactly one dependency: the psymtab that
+       includes it.  */
+    gdb_assert (this->number_of_dependencies == 1);
+    return this->dependencies[0];
+  }
 };
 
 /* Allocate a new partial symtab for file named NAME and mark this new
@@ -8858,8 +8872,7 @@ process_queue (struct dwarf2_per_objfile *dwarf2_per_objfile)
 void
 dwarf2_psymtab::expand_psymtab (struct objfile *objfile)
 {
-  if (readin)
-    return;
+  gdb_assert (!readin);
 
   expand_dependencies (objfile);
 
diff --git a/gdb/psympriv.h b/gdb/psympriv.h
index 9bc960a..fdcee99 100644
--- a/gdb/psympriv.h
+++ b/gdb/psympriv.h
@@ -124,16 +124,26 @@ struct partial_symtab
   {
   }
 
-  /* Read the full symbol table corresponding to this partial symbol
-     table.  */
+  /* Psymtab expansion is done in two steps:
+     - a call to read_symtab
+     - while that call is in progress, calls to expand_psymtab can be made,
+       both for this psymtab, and its dependencies.
+     This makes a distinction between a toplevel psymtab (for which both
+     read_symtab and expand_psymtab will be called) and a non-toplevel
+     psymtab (for which only expand_psymtab will be called). The
+     distinction can be used f.i. to do things before and after all
+     dependencies of a top-level psymtab have been expanded.
+
+     Read the full symbol table corresponding to this partial symbol
+     table.  Typically calls expand_psymtab.  */
   virtual void read_symtab (struct objfile *) = 0;
 
-  /* Psymtab expansion is done in two steps.  The first step is a call
-     to read_symtab; but while that is in progress, calls to
-     expand_psymtab can be made.  */
+  /* Expand the full symbol table for this partial symbol table.  Typically
+     calls expand_dependencies.  */
   virtual void expand_psymtab (struct objfile *) = 0;
 
-  /* Ensure that all the dependencies are read in.  */
+  /* Ensure that all the dependencies are read in.  Calls
+     expand_psymtab for each non-shared dependency.  */
   void expand_dependencies (struct objfile *);
 
   /* Return true if the symtab corresponding to this psymtab has been
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 73931c5..7259e05 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-04-14  Tom de Vries  <tdevries@suse.de>
 
+	PR symtab/25718
+	* gdb.dwarf2/dw2-symtab-includes.exp: New file.
+
+2020-04-14  Tom de Vries  <tdevries@suse.de>
+
 	PR symtab/25720
 	* gdb.base/maint-expand-symbols-header-file.c: New test.
 	* gdb.base/maint-expand-symbols-header-file.exp: New file.
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-symtab-includes.exp b/gdb/testsuite/gdb.dwarf2/dw2-symtab-includes.exp
new file mode 100644
index 0000000..1eaaf4a
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-symtab-includes.exp
@@ -0,0 +1,80 @@
+# Copyright 2020 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 symtab user and includes are present after symtab expansion
+# triggered by an include file.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile main.c .S
+
+# Create the DWARF.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels partial_label lines_label
+    global srcdir subdir srcfile
+
+    extern main
+
+    cu {} {
+	partial_label: partial_unit {} {
+	}
+    }
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {stmt_list ${lines_label} DW_FORM_sec_offset}
+	} {
+	    imported_unit {
+		{import $partial_label ref_addr}
+	    }
+	}
+    }
+
+    lines {version 2} lines_label {
+	include_dir "${srcdir}/${subdir}"
+	file_name "dw2-symtab-includes.h" 1
+	program {
+	    {DW_LNS_advance_line 1}
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile \
+	  "${asm_file} ${srcfile}" {}] } {
+    return -1
+}
+
+# Check that no symtabs are expanded.
+set test "no symtabs expanded"
+if { [readnow] } {
+    unsupported $test
+    return -1
+}
+gdb_test_no_output "maint info symtabs" $test
+
+# Expand dw2-symtab-includes.h symtab
+gdb_test "maint expand-symtab dw2-symtab-includes.h"
+
+# Check that there are includes.
+gdb_test "maint info symtabs" \
+    "\r\n    \\( includes\r\n.*" \
+    "check symtab includes"